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Advanceshttp://adr.sagepub.com/ in Dental Research The Origin of Periodontal Infections R.J. Genco, J.J. Zambon and L.A. Christersson ADR 1988 2: 245 DOI: 10.1177/08959374880020020901 The online version of this article can be found at: http://adr.sagepub.com/content/2/2/245 Published by: http://www.sagepublications.com On behalf of: International and American Associations for Dental Research Additional services and information for Advances in Dental Research can be found at: Email Alerts: http://adr.sagepub.com/cgi/alerts Subscriptions: http://adr.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav >> Version of Record - Nov 1, 1988 What is This? Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. THE ORIGIN OF PERIODONTAL INFECTIONS RJ. GENCO, J. J. ZAMBON, AND L. A. CHRISTERSSON Departments of Oral Biology and Periodontology and Periodontal Disease Clinical Research Center, State University of New York at Buffalo, Poster Hall Buffalo, New York 14214 Adv Dent Res 2(2) :245-259, Month, 1988 ABSTRACT P eriodontal diseases are recognized as bacterial infections, and some forms are associated with specific organisms, such as Actinobacillus actinomycetemcomitans in juvenile periodontitis, and Bacteroides gingi- valis and others in adult periodontitis. The source of the periodontal organisms, whether they are part of the indigenous or resident flora and overgrow to become opportunistic oral pathogens, or whether they are exogenous oral pathogens, is important to determine. The chain of periodontal infection, microbial agent(s) and their transmission, and host response are reviewed with respect to the role of A. actinomycetemcomitans in localized juvenile periodontitis and B. gingivalis in adult periodontitis. The present data lead us to hypothesize that some periodontal organisms may be exogenous pathogens. Prevention of periodontal diseases may be influenced by the knowledge of whether various forms are caused by opportunistic organisms or exogenous pathogens. If exogenous pathogens are responsible, prevention can be directed to intercepting transmission, thereby preventing colonization. On the other hand, if the organisms are opportunistic pathogens, prevention might be directed at interfering with initial acquisition of the flora earlier in life, as well as suppressing them to low levels consistent with health. For those exogenous periodontal infections, attempts at eradication and prevention of re-infection are likely to be effective. If the organisms are part of the indigenous flora, there is little hope of complete elimination of the organism. Criteria for distinguishing exogenous periodontal pathogens from opportunistic periodontal pathogens include the prediction that exogenous pathogens would be transient members of the oral flora associated with periodontal disease, likely to be comprised of one or a few clonal types, and intrinsically virulent. In contrast, opportunistic periodontal pathogens would likely be members of the indigenous flora and would overgrow. They would likely be comprised of many clonal types, and have an intrinsically low level of virulence. I. INTRODUCTION Most forms of periodontal disease are bacteria infections (Socransky, 1970,1977; Slots, 1979; Loesche, 1982; Slots and Genco, 1984). Understanding the relationships among the causative bacteria, their transmission, and important host reactions to infection is critical in the control of periodontal diseases. In this report, we will address the pathogenesis of the infectious process, with emphasis on the source of the pathogenic periodontal microbiota. Do these microPresented at the Sunstar Portside Symposium, November 14-15, 1986, Kobe, Japan This work was supported in part by USPHS Grants No. DE07034, DE04898, and DE07497. Dr. Zambon is the recipient of a Research Career Development Award from the National Institute of Dental Research. organisms exist as part of the resident oral microflora, or are they transported to subgingival sites from an external source? Furthermore, what are the microbial agents, what is their mode of transmission, and what is their interaction with the host? A brief historical overview of periodontal research, implicating specific bacteria as etiologic agents, will provide a background. Evidence that bacteria play a role in periodontal disease will be discussed under studies of periodontal syndromes in animals, and studies of human periodontal disease. The following definitions will be used to help clarify the discussions. The indigenous or resident oral flora consists of those organisms whose habitat is the oral cavity, regardless of their pathogenic potential. They need not be present in the oral cavity in all patients at all times. In contrast, exogenous oral pathogens are transient members of the oral flora asso245 Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. 246 GENCO et al. ciated with pathologic states. Exogenous pathogens exhibit various degrees of virulence and are transmitted from their natural habitat to non-infected oral sites, which then become diseased. Opportunistic oral pathogens are those organisms which overgrow because of changes in the oral environment, or because of loss of host resistance factors, and cause disease. They may be components of the indigenous oral flora, but can also be free-living saprophytic organisms. In general, opportunistic pathogens are normally nonpathogenic, but become important pathogens in immunocompromised patients or under conditions of increased organ susceptibility, e.g., subacute endocarditis with mitral valve damage. A. Studies of Periodontal Syndromes in Animals Periodontal disease does not occur spontaneously in many species of experimental animals, or occurs when they are very old. However, Keyes and Likins (1946) made the important observation that periodontal pathosis does occur spontaneously in young Syrian hamsters. This observation led to a series of experiments, with the Syrian hamster used as a model for periodontal disease, which demonstrated the following: First, the periodontal syndrome in the hamster was both infectious and transmissible. The disease could be transmitted from infected Golden hamsters to the non-infected Albino hamsters (in which the disease does not occur spontaneously) through dental plaque or feces (Keyes and Jordan, 1964). Second, in this model the transmissible microbial agent was Actinomyces viscosus, an aerobic Gram-positive filamentous bacterium (Jordan and Keyes, 1964; Ho well et al., 1965). Other organisms isolated from the hamster — including Streptococci, Sarcina, Gram-negative rods, and oval or spiral-shaped cells — failed to produce significant pathology when inoculated into uninfected Albino hamsters. Hence, the hamster periodontitis model was one of the first to show microbiological specificity. Third, filamentous bacteria isolated from human root-surface caries —including Rothia dentocariosa, and strains of Actinomyces resembling A. naeslundii, A. viscosus, and A. odontolyticus — induced the periodontal syndrome as well as cervical caries in the hamster model system. Subsequent studies, including those in the rice rat (Dick and Shaw, 1966) and the gnotobiotic rat (Socransky, 1977), have also demonstrated the essential role of specific bacteria in rodent periodontal syndromes. Longitudinal studies examining the progression from naturally-occurring gingivitis to ligature-induced periodontitis in monkeys show the presence of increased numbers of Gram-negative anaerobes, including black-pigmented Bacteroides (Slots and Hausmann, 1979; Kornman et al., 1981), in sites with periodontal loss. For example, B. macacae, a B. gingivalis-like monkey species (Slots and Genco, 1980), increased to as high as 66% of the total cultivable organisms, concomitant with a significant loss in alveolar bone mass (Slots and Hausmann, 1979). Sim- Adv Dent Res November 1988 ilar results have also been obtained using beagle dogs (Siegrist et al, 1980). These animal studies are consistent with the hypothesis that periodontal diseases result from specific and transmissible bacterial infections; they do not address the issue of whether they are exogenous or opportunistic infections. B. Studies of Human Periodontal Disease Epidemiologic studies of periodontal diseases carried out in the 1950's and 1960's were reviewed by Russell (1967). He concluded that there is a direct relationship between the amount of plaque, oral debris, and calculus, on the one hand, and the severity of periodontal disease on the other. Furthermore, the linear correlation between oral debris and the periodontal disease index values is so strong as to leave little variation in disease severity to be accounted for by factors other than age or oral hygiene. The strong positive correlation between oral debris and periodontal disease established an association, but not necessarily a cause-and-effect relationship, between these factors. A direct cause-and-effect relationship between dental plaque and gingivitis has been demonstrated by Loe et al. (1965) and Theilade et al. (1966). In these experiments, dental plaque was allowed to accumulate by cessation of oral hygiene. Clinically measurable gingivitis resulted in most subjects after two to three weeks of plaque accumulation. Furthermore, when oral hygiene was re-instituted, the crevicular plaque material was reduced and gingivitis resolved. Hence, these studies provide convincing evidence that local alterations in the environment, such as occur with cessation of oral hygiene, allow for the growth of periodontal micro-organisms and the development of gingivitis. The gingival flora not only increases in mass, but there is also a shift from predominantly Gram-positive aerobic or facultative flora, consisting primarily of Streptococcus sanguis and Actinomyces species, to a much greater proportion of anaerobic Gram-negative rods and motile organisms. Bacteroides intermedius, for example, is isolated from the crevicular flora in gingivitis (van Palenstein Helderman, 1981; Moore et al., 1982a). Since gingivitis can be induced in most if not all oral sites in humans by dental plaque accumulation, it is reasonable to postulate that gingivitis is an opportunistic infection caused by an overgrowth of organisms which are part of the indigenous flora. However, the appearance of organisms such as Bacteroides intermedius raises the question of whether there are exogenous organisms which can colonize oral sites only when there are large amounts of pre-existing plaque. Or put another way, "Are periodontal pathogens such as B. intermedius members of the indigenous oral flora?" Given the apparent heterogeneity of B. intermedius, it may be that there are virulent and relatively avirulent forms of this micro-organism. The virulent forms may colonize as an exogenous infection, or they may be present at extremely low levels Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. Vol. 2 No. 2 PER1ODONTAL INFECTIONS 247 in the indigenous flora and overgrow during the depate in active peridontal disease (Tanner et al, 1987). velopment of gingivitis. Similar variability in strain In a recent analysis of the predominant cultivable mipathogenicity may occur among strains of other gincrobiota in active destructive lesions of patients with givitis-associated organisms, such as spirochetes, Fuperiodontitis, Dzink et al (1988) found that subginsobacterium species, and Adinomyces. gival sites undergoing active loss were often inhabLongitudinal experiments, in humans, demonited by Bacteroides forsythus, B. gingivalis, strating a cause-and-effect relationship between the Peptostreptococcus micros, Adinobacillus actinomycetemperiodontal microbiota and the development of pericomitans, Wolinella recta, or B. intermedius, or combiodontitis have not been performed. However, crossnations of these organisms. sectional studies of adults (Schei et al, 1959) show Localized juvenile periodontitis (LJP) patients harthat alveolar bone loss is most severe in individuals bor a subgingival flora different from that found in judged to have poor oral hygiene, i.e., those who gingivitis or adult periodontitis. In LJP, Adinobacillus have the largest accumulation of bacterial plaque at adinomycetemcomitans (A.a.) is often a predominant the gingival margin. These and other studies carried subgingival organism (Slots, 1976; Newman and Soout over the past three decades have established that cransky, 1977; Slots et al, 1980; Slots and Rosling, various forms of periodontal disease are initiated by 1983; Zambon, 1985; Asikainen et al, 1987). Capnomicro-organisms through the establishment of a cytophaga (Newman and Socransky, 1977) and Eikesubgingival microbiota. nella corrodens in combination with A. adinomycetemcomitans have also been reported to be Recently, the microbial specificity of human periprominent in LJP (Mandell and Socransky, 1981; odontal infections has been appreciated. This conMandell, 1984). Others, however, did not find this ceptual breakthrough was achieved by application of association (Okuda et al, 1984; Moore et al, 1985). technical advances in anaerobic microbiology as well Capnocytophaga species and Actinobacillus have been as the accumulation of advanced information on speassociated with advanced periodontitis in juvenile dicies identification and taxonomy to studies of the abetics (Mashimo et al, 1983) and in granulocytosubgingival flora of man. penic and other immunocompromised hosts. In acute The concept that specific bacteria are essential etinecrotizing ulcerative gingivitis, Baderoides intermeologic factors in different forms of human periodontal dius and intermediate-sized spirochetes appear to be disease has received convincing scientific support in prominent (Loesche et al, 1982; Chung et al, 1983). recent years (Socransky, 1977; Slots, 1979; van Palenstein Helderman, 1981; Loesche, 1982; Socransky Hence, it appears — from microbiologic studies of et al, 1982; Zambon et al, 1983a). Clear differences periodontal syndromes in animals, and from microin the subgingival microbiota between health and disbiologic studies of human adult and juvenile periease have been well-documented. For example, healthy odontitis — that these diseases exhibit significant sites are associated with sparse plaque which consists microbial specificity. largely of Gram-positive cocci (Listgarten, 1976; SavFor gingivitis, indigenous organisms may likely play ett and Socransky, 1984). These cocci are predomian important role, although the contribution, if any, nantly Adinomyces and streptococci (Moore et al., of exogenous pathogens to gingivitis is not clear. For 1982b). Plaque associated with gingivitis appears to some forms of periodontitis, we hypothesize that excontain increased numbers of Adinomyces species and ogenous pathogens acquired from an external source reduced numbers of Streptococci (Loesche and Syed, are responsible for tissue destruction. Knowledge of 1978). Moore et al. (1982a) have implicated Adinothe source of the micro-organisms which cause perimyces odontolyticus, Adinomyces naeslundii, Fusobader- odontitis is important and has several clinically imiwn nucleatum, Ladobacillus strain type D-2, Streptococcusportant implications. If the causative agents are angiosus, Veillonella parvnla, and Treponema species as members of the indigenous flora, they would likely the most likely etiologic agents of experimental ginbe difficult to eradicate, since they have an ecologic givitis in humans. Plaque associated with gingivitis advantage in the oral cavity. If they are suppressed has also been shown to contain Baderoides intermedius indigenous organisms might be expected to recolo(Kornman and Loesche, 1980; Jensen et al., 1981). In nize oral sites easily. In contrast, exogenous pathosevere forms of adult periodontitis, the subgingival gens are likely to be more easily eliminated from oral microflora contains Baderoides gingivalis (Spiegel et al., sites, since they may not be adapted to thrive in the 1979; Tanner et al, 1979; Zambon et al, 1981; Slots, healthy oral cavity, where many host and microbiol1982; Slots and Genco, 1984; White and Mayrand, ogical factors act to suppress their colonization. 1981; Van Winkelhoff et al, 1986). Other organisms found as prominent members of the subgingival flora II. THE CHAIN OF PERIODONTAL in periodontitis include Fusobaderium nucleatum, Eubaderium timidum (Moore et al, 1982c), and Bacteroides INFECTION: THE MICROBIAL AGENT, ITS TRANSMISSION AND THE HOST capillus (Kornman and Holt, 1981). Organisms including Haemophilus species, Wolinella species, Bacteroides forsythus, Selenomonas sputigena, Over the last two and one-half decades, there has Eikenella corrodens, and spirochetes may also partici- been considerable progress in the understanding of Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. 248 Adv Dent Res November 1988 GENCO et al. the infectious etiology of periodontal diseases. The concepts of bacterial specificity which have emerged allow us to hypothesize that some forms of periodontal diseases are the result of exogenous pathogens and are, therefore, amenable to effective control and prevention as with other exogenous infectious diseases, such as tuberculosis and polio. Knowledge of the route of infection is a key factor in the prevention and treatment of such diseases. Understanding the entire chain of infectious disease begins with identification of the agent, followed by knowledge of the route of transmission of the agent and, finally, with characterization of the host response to the agent. The infectious disease chain as it relates to periodontal infections is described in Table 1. A. The Microbial Agent The infectious agent is the first link in the chain of infection. Identification of the micro-organism(s) associated with many forms of periodontal disease has proceeded rapidly in the last decade. The next step is to determine whether the infection is caused by opportunistic organisms, i.e., does the agent already reside in the indigenous oral flora and the disease develop due to changes in the local environment or host susceptibility which allow its emergence or overgrowth? Or does the agent infect the host from an external source — is it an exogenous infection? Association of an exogenous bacterium with a disease provides circumstantial evidence that this bacterium is causative rather than merely a secondary invader which has emerged from the resident flora. Full evidence of a causative role, however, requires a detailed understanding of the host bacterial interactions leading to disease. Understanding several important characteristics of the infecting agent may help in disease control. For example, pathogenicity, which is the ability of an agent to cause disease, may be high or low. For an agent with high pathogenicity, disease invariably follows infection of the host. An example of this is the smallpox virus. Bacteria such as Actinomyces vicosus may be considered of low pathogenicity, since they have a high oral colonization rate in man, and only when they increase to large numbers are they associated with diseases such as gingivitis, likely as opportunistic pathogens. Pathogenicity may be further characterized by virulence, which describes the ability of the organisms to cause disease of various degrees of severity; strains of exogenous pathogens may vary in virulence. Another feature of pathogenicity is invasiveness, or the ability of the agent to enter the host, move through tissues, and replicate in situ. Pathogenicity is further defined by the ability of the organism either to evade important defense mechanisms and/or to produce histiolytic enzymes and toxins that destroy host tissues and cells. Other characteristics of the infecting agent which TABLE 1 INFECTIOUS DISEASE CHAIN IN PERIODONTAL DISEASES: FACTORS OF IMPORTANCE IN CONTROL AND PREVENTION I. II. III. Identification of Infectious Agent(s) for Each Form of Periodontal Disease. A) Are they endogenous or exogenous agents? B) What are their characteristics? 1) pathogenicity; virulence and invasiveness 2) infective dose 3) host specificity 4) antigenic variation 5) physical characteristics (survival) 6) genetic factors (resistance transfer plasmids) C) What are their antimicrobial specificities? Transmission (if exogenous A) What is their route or mode of transmission? 1) Contact; either direct (person-to-person), indirect, or droplet (< 1 meter) 2) Common vehicle such as food, water, or contaminated instruments 3) Airborne by droplet nuclei or dust (> 1 meter) 4) Vector What is their reservoir, natural habitat, and source of immediate transmission to the host? B) Host What is the site of entrance of the agent? A) What is the site of colonization of the agent? B) What are the important host defense mechanisms? C) 1) non-specific such as saliva, and inflammation, or 2) specific such as antibodies or cellular immunity (either natural or artificial) For description of these factors, see Mandell et al., 1985. Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. Vol. 2 No. 2 PER1ODONTAL INFECTIONS are important include the infective dose — that is, the number of organisms necessary to cause an infection - and the host specificity, which is defined by the species susceptible to infection. Further important characteristics include antigenic variation over time, which may occur in a single species of microorganism and change its pathogenicity markedly; the agent's physical characteristics, which affect its ability to survive outside of the host; and genetic factors, such as resistance transfer plasmids or chromosomally based antibiotic resistance. A feature of prime clinical importance in the characterization of the organism is its spectrum of antimicrobial susceptibility, which allows for selection of chemotherapeutic or chemoprophylactic measures. B. Transmission of the Organism Knowledge of transmission is important for control of exogenous infections. The route of transmission may occur through one or more of four modes: contact, common vehicle, airborne, or vector. Transmission by contact may occur through direct person-toperson contact, by indirect contact through an inanimate object such as a shared toothbrush, or by droplets of saliva or other fluid which harbor the agent. Transmission by contact is functionally defined as direct, or if indirect, by transmission over distances of less than 1 meter, as would be expected to occur via droplets of body fluids in which the organism was contained. A second route of transmission is through a common vehicle such as food, water, or soil which contains the agent. The third route of transmission is airborne by droplet nuclei or dust particles which can be carried over distances greater than 1 meter. These airborne particles may carry the agent over distances of several miles. The fourth route of transmission is by vectors, which are often insects. C. Reservoir and Source of Infection Another major consideration in transmission of the organism is its reservoir, defined as the natural habitat or the location in which the organism normally becomes established and multiplies. The source of transmission is the location from which the organism is immediately transmitted to the host, either directly or indirectly through a vehicle. In some instances, the reservoir and the source may be the same. For example, it is conceivable that the oral cavity of a periodontally diseased individual is both the natural habitat of certain pathogenic periodontal bacteria and the source of transmission by direct contact or via droplets. The reservoir and source may also be different. For example, the reservoir of many pathogens is the soil, but the direct source of transmission may be food contaminated by soil. D. The Host - The Last Link in the Chain of Infection The last link in the cycle or chain of infection is the host. The response of the host to the infection agent 249 determines one of three outcomes: (1) whether the infected individual is a carrier — that is, the subject harbors the agent and is able to transmit the agent but has little or no immunologic or tissue-destructive response to the agent; (2) the subject maybe subclinically infected, in which case there is often an immunologic response to the agent, but no overt signs or symptoms of disease; or (3) the host is diseased, when the infecting organisms cause signs and symptoms of disease and, most often, there is a host immune response to this infection. The site of entrance of the agent is of critical importance for control. This may be through skin, or through a mucous membrane in the gastro-intestinal, genito-urinary, or respiratory tracts. The site of colonization is another important host consideration. An organism may colonize one site, not cause a disease, but result in a carrier state. However, when the same agent colonizes a target organ, it may cause disease. Hence, the site of colonization may be different for carriers as compared with those subclinically infected or diseased. Knowledge of the key host-response mechanisms against the infecting agents may allow for the control or prevention of infectious diseases, not only by eliminating the agent, but also by enhancing potential host defenses or by suppressing host-destructive effects with agents such as anti-inflammatory drugs. The natural history of the disease is, to a large extent, determined by the host response. For example, periods of exacerbation and remission are characteristic of periodontitis and may be associated with changes in the host's specific immune status — changes which may affect the growth rate of the organisms. Defense mechanisms are either non-specific, such as many of those operative in saliva, or the mucous secretions or serum, or specific, such as the antibody response or cell-mediated immunity. This immunity may be natural, i.e., present prior to the onset of colonization, or it may occur as a result of infection. Specific immunity may provide long-lasting protection, unless it declines rapidly or there are changes in the antigens of the organism. Artificial immunity induced by vaccination or immunoprophylaxis has been among the most effective means of controlling infections. Vaccinations also offer the possibility of eradicating the agent from the population, as has been accomplished for smallpox. This is the ultimate goal in infection control. The long-term control and prevention of periodontal diseases are likely to result from measures directed toward steps in the infectious chain which are most susceptible to intervention. Therefore, it is important that we understand the infectious disease chain as it operates in the various forms of periodontal disease. It is instructive to review the research on host-parasite interactions in periodontal disease to identify those steps in the infectious chain which are well-described and those steps where gaps in our knowledge occur. Two forms of periodontal disease — localized juvenile peri- Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. 250 Adv GENCO et al. odontitis and severe adult periodontitis — will be described, since we have some information about the specific candidate pathogens for these infections. III. THE INFECTIOUS DISEASE CHAIN FOR LOCALIZED JUVENILE PERIODONTITIS Actinobacillus (Haemophilus) actinomycetemcomitans is strongly associated with localized juvenile periodontitis (LJP), and the evidence for its role as a pathogen has been reviewed by Slots and Genco (1984), Zambon (1985), and Zambon et al. (1986a). This evidence can be summarized as follows: A. actinomycetemcomitans levels in the subgingival flora are high in localized juvenile periodontitis patients and were not detectable or were at low levels in normal humans. For example, of a total of 60 LJP patients studied in our laboratories (Genco et al., 1986a) suffering from the classic molar-incisor pattern of bone and attachment loss, 57 harbored high levels of A. actinomycetemcomitans in approximately two-thirds of subgingival sites adjacent to periodontal lesions. This prevalence of 95% of LJP patients infected with A. actinomycetemcomitans can be compared with 4-5-times-lower prevalence in healthy subjects and adult periodontitis patients. Of the 142 healthy subjects evaluated, 17% harbored low levels of A. actinomycetemcomitans, and in 134 adult periodontitis patients, 24% harbored A. actinomycetemcomitans (Zambon et al., 1983a). The second line of evidence is the pathogenicity of A. actinomycetemcomitans, which is reviewed by Slots and Genco (1984). Briefly, this organism produces a leukotoxin active against neutrophils, an endotoxin, collagenase, and other histiolytic enzymes which may be of importance in tissue destruction. Furthermore, A. actinomycetemcomitans is resistant to the bactericidal activity of human serum. In the absence of serum killing, phagocytes such as neutrophils become a very important line of defense. The third line of evidence is the specific antibody response seen to this organism (see review by Genco and Slots, 1984). A great majority of LJP patients (over 98%) have serum antibody to A. actinomycetemcomitans, whereas only 8-10% of normals have serum antibody reactive with this organism. The association of A. actinomycetemcomitans with localized juvenile periodontitis and the host immune response to A. actinomycetemcomitans infection suggest a role for this organism in the pathogenesis of LJP. The question can then be asked, is A. actinomycetemcomitans an indigenous or exogenous organism? In several studies comparing small numbers of nondiseased subjects to LJP patients, A. actinomycetemcomitans was found either in low levels or not at all in the oral flora of periodontally healthy humans (for review, see Slots and Genco, 1984). Van der Velden et al. (1986) found that before or after induction of experimental gingivitis in humans, no A. actinomycetemcomitans could be cultured on mucosal surfaces Dent Res November 1988 of the oral cavity or from subgingival plaque. For further assessment of the natural habitat of A. actinomycetemcomitans, 283 patients presenting to the Preventive Dentistry Clinic at the SUNY/AB School of Dental Medicine were evaluated for the presence of A. actinomycetemcomitans in subgingival dental plaque samples (Zambon et al., 1986b). This group of patients included some seeking service for dental restorations, and others seeking preventive dental services or routine tooth-cleaning. Most had healthy periodontal tissues or mild gingivitis, and others had mild periodontitis. However, none suffered from moderate or severe periodontitis, or localized juvenile periodontitis. Sixty-four percent of these subjects did not demonstrate detectable A. actinomycetemcomitans in subgingival sites examined by indirect immunofluorescence, with a sensitivity of 0.1% (Fig. 1). None of the remaining subjects who harbored the organism had levels greater than 5% of the total subgingival flora. The subjects in whom A. actinomycetemcomitans was detected may be carriers, subclinically infected or suffering from mild adult periodontitis. In a study comparing the cultivable levels of A. actinomycetemcomitans in LJP patients with carefully matched controls, Asikainen and co-workers (1987) found that 19 of 22 LJP patients harbored A. actinomycetemcomitans in subgingival sites, while none of the 22 controls had detectable levels of A. actinomycetemcomitans. An important aspect of these studies is that A. acti- Distribution of Actinobacillus actinomycetemcomitans in Preventive Dentistry Sample A. actinomycetemcomitans level (% total cell count in pooled sample) Fig. 1 —Distribution of Actinobacillus actinomycetcmcomitans in the University of Buffalo Preventive Dentistry Sample. Two hundred eighty-three patients presenting to a Preventive Dentistry Clinic mainly corresponding to patients with little or no periodontal disease, but requiring preventive and restorative procedures and belonging to the American Dental Association Class I and II were included. B. gingivalis and A. actinomycetemcomitans were assessed by the indirect immunofluorescence procedure from four subgingival plaque samples taken from the mesial of the four first molars. There were no patients with LJP or severe periodontitis in this patient group, (adapted from Zambon et al., 1986b) Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. Vol. 2 No. 2 PERIODONTAL INFECTIONS nomycetemcomitans is present either not at all or only in low numbers in the flora of most humans not suffering from periodontal disease, suggesting that this organism is not a member of the indigenous oral flora. It is reasonable to hypothesize that A. actinomycetemcomitans is an exogenous pathogen associated with localized juvenile periodontitis and possibly some forms of adult periodontitis, since it is found to be associated with disease. If A. actinomycetemcomitans is established to be an exogenous pathogen, the carrier rate and rate of subclinical infection will be important to establish. What are the characteristics of A. actinomycetemcomitans which influence its ability to cause disease? The production of a leukotoxin for neutrophils and direct histiolytic properties of A. actinomycetemcomitans point to this organism as being highly pathogenic, capable of subverting key host defenses and causing severe periodontal destruction (Slots and Genco, 1984; Zambon et al., 1986a). Furthermore, A. actinomycetemcomitans appears to be tissue-invasive and can be regularly found in the gingival connective tissue of LJP patients (Saglie et al., 1982; Gillett and Johnson, 1982; Christersson et al, 1987a,b). Additional evidence for its virulence comes from reports of its participation in extra-oral infections. A. actinomycetemcomitans has been isolated, together with Actinomyces israelii, from the "sulfur granules'' of cervicofacial actinomycosis (Klinger, 1912), and it can cause other severe extra-oral infections, including endocarditis (Page and King, 1966; Peters et al., 1983; Pierce et al., 1984), abscesses of the brain (Garner, 1979), abscesses of the face (Page and King, 1966), thyroid gland (Burgher et al., 1973), urinary tract infections (Townsend and Gillenwater, 1969), meningitis (Genco et al., 1980), and vertebral osteomyelitis (Muhle et al., 1979). Hence, A. actinomycetemcomitans is an exogenous pathogen in human extra-oral sites, capable of causing severe and often fatal infections. Knowledge of the A. actinomycetemcomitans infectious dose is lacking, as is information on host preference. It is clear that Actinobacillus species are widely distributed among primates. For example, many individuals of several species of monkeys harbor A. actinomycetemcomitans in the gingival flora. It is noteworthy that most of these monkeys do not suffer from periodontal disease, hence either the monkey strains of Actinobacillus are non-virulent, or the monkeys are not susceptible to periodontal disease. This finding in monkeys points to the pitfall of concluding that Actinobacillus is part of the indigenous flora based upon its presence in animals. A. Antigenic Variations of A. actinomycetemcomitans Evidence from our laboratories (Umemoto et al., 1986) suggests that A. actinomycetemcomitans can undergo changes in colony morphology from rough to smooth colonies after being subcultured on agar media. A. actinomycetemcomitans isolates demonstrate three types of colony morphology: rough, often seen 251 on primary culture; intermediate, seen during transformation; and mucoid or smooth, seen after transformation. The organism can also revert to the rough type by repeated animal passage. Electron microscopic examination of the A. actinomycetemcomitans cells revealed that the rough type are heavily piliated with type A fimbriae, whereas the rough-to-smooth transition results in loss of fimbriae. Further studies showed that the ultrastructural and surface protein profiles of A. actinomycetemcomitans, cultured aerobically, differ from those cells grown anaerobically (Scannapieco et al., 1987). The extent to which this variation in surface antigens occurs during natural infection is unknown, but points to the possibility of antigenic or phase variation of Actinobacillus strains. The physical characteristics of the organism affecting its survival outside the body are unknown, and survival forms such as spores have not been described. However, A. actinomycetemcomitans is a capnophilic, facultative anaerobe, not particularly oxygensensitive, and would be expected to be reasonably stable outside the host. B. Transmission of Infection in Actinobacillus The evidence pointing to A. actinomycetemcomitans as an exogenous pathogen in man also points to the importance of understanding its transmission. The route of transmission for Actinobacillus is not clear; however, there are several studies which shed some light on human transmission. In a study of Zambon et al. (1983b), intrafamilial transmission of A. actinomycetemcomitans is strongly suggested, based upon a determination of the serotypes (three possible) and biotypes (10 possible) of A. actinomycetemcomitans isolated from individual family members. Zambon and co-workers found that each individual is infected with only one of the three serotypes of Actinobacillus. In any one family where more than one sibling suffers from LJP, they are all infected with A. actinomycetemcomitans. Furthermore, in any one family, all infected members harbor A. actinomycetemcomitans of the same serotype and also of the same biotype. When the antibody response of these family members is assessed, they all respond to antigens of the same infecting serotype of A. actinomycetemcomitans (Genco et al., 1980). The above studies suggest that the transmission of A. actinomycetemcomitans among family members possibly occurs by contact. Further studies assessing clonal types among family members may provide further insight into intrafamilial transmission of A. actinomycetemcomitans. Whether the contact is direct, indirect, or by droplet is not known. Preus and Olsen (1988) found that the strains of A. actinomycetemcomitans infecting a child with periodontitis and his pet dog were the same clonal type, again suggesting transmission of A. act inonn/cetenicomi tans by close household contact. Investigation of the common vehicle mode of transmission of A. actinomycetemcomitans by use of contaminated instruments was carried out bv Christersson Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. 252 GENCO et al. et al. (1985a). They showed that this species of Actinobacillus can be transmitted within an individual patient from an infected periodontal lesion to a noninfected periodontal site. However, A. actinomycetemcomitans only transiently colonizes these previously uninfected gingival sites, since in 2-3 weeks A. actinomycetemcomitans is no longer demonstrable in the recipient sites. Furthermore, A. actinomycetemcomitans does not cause disease at the recipient site. This experiment argues against simple transmission by a single or a few inoculations with a common vehicle such as a contaminated instrument in an already-infected patient. These experiments suggest that repeated, direct contact with an A. cictinomycetemcomitcins-iniected person (or pet?) is necessary for its transmission. The reservoir of A. actinomycetemcomitans is not known. It can be hypothesized that the reservoir or the natural habitat of A. actinomycetemcomitans is the subgingival flora of patients suffering from Actinobacillary-associated periodontal disease (Slots et al., 1980; Preus et al., 1987). Other reservoirs, of course — such as the soil, animals (including pets), or even other sites in the human body — are possible. The immediate source of the infecting organism is not known; however, it may well be saliva from the oral cavities of infected individuals. C. Host Factors in Localized Juvenile Periodontitis The site of entrance of the agent(s) is not known. They most likely gain entrance through the mucous membranes of the oral cavity, but other sites (such as through the skin or the gastro-intestinal tract) are also possible. The site of initial colonization is also not known; however, it is clear that the site of colonization during infection is the subgingival area adjacent to the periodontal pocket and within the gingival tissue. This is particularly important in therapy directed to removing or eradicating A. actinomycetemcomitans, since its presence within the gingival connective tissue in over 80% of LJP patients makes it difficult to remove by debridement of the periodontal pocket (Slots and Rosling, 1983; Christersson et al., 1985b). The host-bacterial interactions have been well studied in juvenile periodontitis and are summarized by Genco and Slots (1984) and Genco et al. (1986b). Briefly, it has been found that: (1) some strains of A. actinomycetemcomitans produce a leukotoxin which kills neutrophils and, to some extent, macrophages; (2) A. actinomycetemcomitans also produces a factor which inhibits neutrophil chemotaxis; (3) A. actinomycetemcomitans produces a host of histiolytic and tissuedestructive toxins, such as collagenase and endotoxin; (4) 70% of patients with LJP have defective neutrophil chemotaxis; and (5) the defective neutrophil chemotaxis seen in LJP is familial and is related to reduced levels of receptors for chemotactic agents on the neutrophil surface. This reduced neutrophil function may explain, in part, the increased susceptibility of certain individuals and families to infection Adv Dent Res November 1988 by A. actinomycetemcomitans resulting in localized juvenile periodontitis. A. actinomycetemcomitans also induces a very marked antibody response characterized by specific antibodies to various Actinobacillus antigens, including the serotype antigens (Genco and Slots, 1984). In addition, serum antibodies in LJP patients inhibit leukotoxin and are often opsonic. The serum antibody response, therefore, may be protective. Further identification of the protective antibodies and design of strategies for inducing these protective antibodies (leading to vaccine) are certainly desirable goals for immunoprophylaxis of periodontitis associated with A. actinomycetemcomitans. Briefly then, A. actinomycetemcomitans is a candidate for an exogenous infectious agent in LJP (Table 2). Control or treatment of LJP has been successfully carried out with methods that result in reduced levels of A. actinomycetemcomitans from the subgingival microflora and the gingival tissues of periodontal patients (Christersson et al., 1985b). Further studies directed to preventing the spread of the organism in families and in other contacts of LJP patients can be rationally designed once we have more information about the route of A. actinomycetemcomitans transmission, its reservoir, and its source. Future directions also include intercepting the cycle of infection by enhancing the host response — for example, by reconstitution of neutrophil abnormalities in LJP (Genco et al., 1986b) and vaccination against relevant antigens. IV. ASSESSMENT OF THE INFECTIOUS DISEASE CYCLE IN ADULT PERIODONTITIS Identification of the Infectious Agent(s) In adult periodontitis there may be several infectious agents. At the present time, Bacteroides gingivalis, Bacteroides intermedius, Actinobacillus actinomycetemcomitans, Bacteroides, forsythus, Fusobacterium nucleatum, Eikenella corrodens, and Wolinella recta are candidate pathogens, since they have been associated with active lesions in patients with severe periodontitis (Slots and Genco, 1984; Tanner et al., 1987; Dzink et al, 1988). Adult periodontitis may represent a series of infections similar to the pattern seen in acute otitis media. In a tabulation of 12 reports involving 4,675 cases of acute otitis media from centers in the United States, Finland, and Sweden between 1952 and 1981 (Bluestone and Klein, 1983), it was found that Streptococcus pneumonia accounted for 33%, Haemophilus influenzae accounted for 21%, and other Streptococcus species for 8% of the cases. Together, these three organisms account for 62% of the cases of acute otitis media. Another series of bacteria, including Streptococcus aureus, Branhamella catarrhalis, Gram-negative enteric bacilli, and several other miscellaneous bacteria, each accounted for anywhere from 1-2% of the remaining infections. No pathogens could be demonstrated in Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. Vol. 2 No. 2 PER1ODONTAL INFECTIONS 253 TABLE 2 INFECTIOUS DISEASE CHAIN IN LOCALIZED JUVENILE PERIODONTITIS I. Factor or Process Findings Agent A. actinomycetemcomitans strongly associated with LJP. 95% of LJP patients harbor A. a. in lesions. AM. strongly associated with disease and not health, hence likely exogenous. A) Indigenous or Exogenous B) Characteristics of Organisms 1. Pathogenicity Virulence Invasiveness Toxicity 2. Infective Dose 3. Host Specificity 4. Antigenic Variation 5. Physical Characteristics 6. Genetic Factors C) Antimicrobial Susceptibility Transmission A) Route B) Reservoir and Source III. Host A) Site of Entrance B) Site of Colonization C) Host Defense Mechanisms A. a. can cause severe extra-oral infections. A. a. found in LJP. A. a. produces a leukotoxin, collagenase, LPS. Not known Found in normal flora of monkey and rice rat, pathogenic in horses, swine, and man. Fimbriation and other surface structures vary. Unknown Resistance transfer plasmids found. Susceptible to tetracycline, some strains resistant to penicillin, metronidazole, erythromycin. II. Not known, may be contact. Unknown Unknown Periodontal lesion sites, oral mucous membranes. Neutrophils and antibodies likely important. the SUNY at Buffalo School of Dental Medicine Preventive Dentistry Clinic, we assessed whether B. gingivalis was a member of the gingival flora of normal periodontally healthy humans (Zambon et al., 1986b). The results are summarized in Figs. 2 a and b. In this study, 283 subjects seen in the Preventive Dentistry Clinic with little or no periodontal disease were compared with 70 subjects with moderate to severe periodontitis who were referred for periodontal treatment (Johansson et al., 1987). Indirect fluorescence microscopy was used to detect subgingival B. gingivalis and A. actinomycetemcomitans. From four to ten gingival or Bacteroids forsythus ("fusiform" Bacteroides), B. gingisubgingival plaque samples were analyzed for each valis, B. intermedius, Wolinella recta, or Peptostreptococsubject or patient. Alveolar bone heights were also cus micros was elevated either singly or in combination, measured and correlated with the microbiologic findthe likelihood that the lesion was actively undergoing ings. In the adult periodontitis group (Fig. 2.b), 65/ attachment loss was increased. 70, or 93%, harbored B. gingivalis, while in the PreIn the last decade, evidence has accumulated ventive Dentistry group (Fig. 2a.), 37% of the subjects strongly implicating Bacteroides gingivalis as one of the harbored subgingival B. gingivalis. Statistically, this prominent members of the pathogenic microbiota asdifference was highly significant (p< 0.001). The Presociated with severe adult periodontitis (Slots and ventive Dentistry group included some patients with Genco, 1984; Spiegel et al, 1979; Tanner et al., 1984; mild periodontitis, and these were the patients in White and Mayrand, 1981; Loesche et al., 1985; Van whom B. gingivalis was most often detected. There Winkelhoff et al., 1986; Dzink et al., 1988). Often, B. was a direct correlation between the amount of bone gingivalis is a prominent subgingival black-pigmented loss in this group and the levels of B. gingivalis found. Bacteroides. B. intermedius is also present in the It is reasonable to hypothesize from these results, subgingival flora of adult periodontitis patients, as is and from previous studies showing that B. gingivalis A. actinomycetemcomitans. is not found at detectable levels in the subgingival The question of whether B. gingivalis is indigenous flora of most normal humans, that B. gingivalis is an or exogenous has begun to be evaluated. In a study specific pathogen found in the lesions of a large of B. gingivalis in a population of patients coming to 30% of the cases. Hence, the pattern in otitis media, which may be analogous to that in adult periodontitis, is that a few pathogens account for a large percentage of the cases, with a large number of other organisms accounting for the remaining cases. Another possibility for explaining the role of specific pathogens in adult periodontitis is that the pathogenic bacteria occur in combinations. For example, Dzink and co-workers (1988), in a comparison of the cultivable microbiota of active and inactive periodontal lesions, found that when A. actinomycetemcomitans, Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. Adv Dent Res November 1988 GENCO et al. 254 TABLE 3 INFECTIOUS DISEASE CHAIN IN ADULT PERIODONTITIS I. Factor or Process Findings Agent Bacteroides gingivalis strongly associated, likely others such as B. intermedius, F. nucleatum, W. recta, E. corrodens. A) Endogenous or Exogenous B. gingivalis found in AP, but in few if any normal humans, hence likely an exogenous pathogen. Not known for other suspected pathogens for adult periodontitis. B) Characteristics of Organisms 1. Pathogenicity Virulence Invasiveness Toxicity 2. Infective Dose 3. Host Specificity 4. Antigenic Variation 5. Physical Characteristics 6. Genetic Factors C) Antimicrobial Susceptibility Transmission A) Route B) Reservoir and Source III. Host A) Site of Entrance B) Site of Colonization B. gingivalis can cause severe extra-oral infections. Unknown, except for oral-facial abscesses. B. gingivalis produces collagenase, a host of proteolytic enzymes, and LPS; resists phagocytosis. Unknown. B. gingivalis-like organisms found in beagle dogs, cats, jaguars, and monkeys. May be part of indigenous flora in monkeys. Unknown. B. gingivalis sensitive to oxygen. Unknown. B. gingivalis susceptible to tetracyclines, penicillins, metronidazole, clindamycin and erythromycin. No p-lactamase. II. C) Host Defense Mechanisms proportion of periodontitis patients. Further experiments using more sensitive measures of detecting B. gingivalis in the subgingival flora, at sites other than in the oral pharynx, as well as at other sites in the body, will be necessary for further determination of B. gingivalis as a resident of the indigenous flora of man, or if it is an exogenous pathogen. The carrier rate and the subclinical infection rate for B. gingivalis are as yet not described. The pathogenicity of B. gingivalis has been studied (see review by Slots and Genco, 1984). Many strains of B. gingivalis are virulent and associated with destruction of both connective tissues and bone in periodontal disease. B. gingivalis can cause severe extraoral infections, including mediastinal, fascial plane, brain, and lung abscesses (see Slots and Genco, 1984). The invasiveness of B. gingivalis has been evaluated in a study by Petrovic and Fillery (1984), and the organism was found to be attached to the epithelium of the periodontal pocket. B. gingivalis produces collagenase and a large battery of proteolytic enzymes (see review by Slots and Genco, 1984). It also demonstrates resistance to Unknown. Unknown. Unknown. May be the oral cavity, subgingival flora, oral mucous membranes, tonsils. Humoral antibodies may be important in defense, otherwise unknown. phagocytosis. B. gingivalis has also been shown to resist serum bactericidal mechanisms (Sundqvist and Johansson, 1980), and some strains are lethal when injected subcutaneously into mice (Van Steenbergen et al., 1982). The infective dose has not been established, and a good animal model is still needed for B. gingivalis infection. A. Important Host Preferences A B. gingivalis-\ike organism is found in beagle dogs (Kornman et al., 1981) as well as in monkeys (Slots et al., 1980a). Of considerable interest is the finding from both animal models of a B. gingivalis-like species in the oral flora prior to the initiation of ligature-induced periodontitis. Hence, a B. gingivalis-like organism may be a member of the indigenous oral flora of beagles and monkeys. The host preference of B. gingivalis may be based upon specific adherence factors to epithelial tissues or oral organisms via ligands on the bacterial surface. These specific reactions may exhibit marked species differences, accounting for the differences in distribution of B. gingivalis among man, beagles, and monkeys. Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. Vol. 2 No. 2 PERIODONTAL INFECTIONS Distribution of Bacteroides gingivalis Levels in Preventive Dentistry Sample gingivalis', however, plasmids have been found in B. intermedius (Dickinson et al., 1987). (a) I 0.1-0.9 1.0-1.9 2.0-2.9 3.0-3.9 ft gingivalis level (% total cell count in pooled sample) Distribution of Bacteroides gingivalis Levels in Periodontitis Patients 2.0-2.9 3.0-3.9 4.0-4.9 255 2,5.0 B. gingivalis level (Average % of total cell counts/patient) Fig. 2 —(a) Distribution of Bacteroides gingivalis levels in Preventive Dentistry Sample. Two hundred eighty-three patients presenting to a Preventive Dentistry Clinic mainly corresponding to patients with little or no periodontal disease, but requiring preventive and restorative procedures and belonging to the American Dental Association Class I and II were included. B. gingivalis and A. actinomycetemcomitans were assessed by the indirect immunofluorescence procedure from four subgingival plaque samples taken from the mesial surfaces of the four first molars. There were no patients with LJP or severe periodontitis in this patient group, (adapted from Zambon et al., 1986b) (b) Distribution of Bacteroides gingivalis levels in periodontitis patients. Seventy subjects with moderate to severe adult periodontitis, ADA Class III and Class IV, referred for periodontal treatment were assessed for subgingival B. gingivalis by indirect immunofluorescence techniques. From four to ten subgingival samples were analyzed for each subject. Antigenic variation of B. gingivalis has not been reported. The ability of the organism to survive outside the subgingival site has not been fully evaluated; however, it can be expected that since B. gingivalis is highly anaerobic, it may not persist for long periods of time in an oxygen-rich atmosphere. No plasmids or phage have been reported in B. The antimicrobial susceptibility has been described and reviewed by Genco (1981). B. gingivalis is sensitive to a large number of commonly used antibiotics, including penicillin and its derivatives, tetracycline, erythromycin, metronidazole, and clindamycin, as well as the derivatives of tetracycline, including minocycline, doxycycline, and chlortetracycline. Beta lactamase production by black-pigmented Bacteroides appears not be to a property of Bacteroides gingivalis', however, some strains of other black-pigmented Bacteroides have been shown to produce p-lactamase (Slots, 1982). B. Route of Transmission B. gingivalis appears to be a periodontal pathogen in man, and its route of transmission is of considerable interest. However, our present knowledge of the transmission of B. gingivalis is very limited. We have some evidence of the reservoir or source of infection. Since the organism is found in subgingival flora, on the lateral borders of the tongue, the buccal mucous membranes, and the tonsils of patients with periodontitis (Zambon et al., 1983a), the human reservoir may be the subgingival flora and the oral pharynx of infected adult periodontitis patients. Further study is necessary for full understanding of the transmission and acquisition of B. gingivalis. There are at least two possibilities: One is that B. gingivalis is acquired by healthy individuals in adolescence or early adulthood and resides as a member of the indigenous flora. Critical changes occur in the environment or the host susceptibility such that B. gingivalis overgrows and causes disease as an opportunistic pathogen. Alternatively, B. gingivalis may be an exogenous pathogen acquired shortly before the development of periodontal disease. C. Host Response to B. gingivalis in Adult Periodontitis The time of acquisition as well as the site of initial entrance and colonization of B. gingivalis are unknown. Initial colonization is likely to be in the oral cavity; however, other sites of colonization of B. gingivalis in the body — including the nasal pharynx, various levels of the gastro-intestinal tract, and the genito-urinary tract and skin — remain to be fully investigated. Host defense against B. gingivalis has been studied, particularly the humoral antibody response and cellmediated immunity (see review by Genco and Slots, 1984). Briefly, patients infected with B. gingivalis produce high titers of serum antibody reactive with surface antigens of this organism. A year or so after treatment, the serum antibody response declines. Furthermore, early studies by Reed et al. (1980) showed that B. gingivalis has unique antigens which distinguish this species from other black-pigmented Bac- Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. 256 Adv Dent Res November 1988 GENCO et al. teroides, both oral and non-oral. Since (a) the antigens are unique, (b) there is a significant specific immune response in adult periodontitis patients, and (c) this antibody response to B. gingivalis declines after therapy, it is reasonable to hypothesize that the immune response seen to B. gingivalis in humans with periodontal disease is the result of periodontal infection. The ability of these antibodies to opsonize or otherwise protect against B. gingivalis infection is unknown, and the role, if any, of antibodies to B. gingivalis in the tissue destruction seen in periodontitis requires further study. Treatment of adult periodontitis which results in the suppression or elimination of the subgingival microbiota has proved to be effective (Christersson et al., 1988). In these studies, the clinical and microbiological effects of antimicrobial agents as adjuncts to mechanical periodontal debridement were assessed in 79 adult periodontitis patients. The relationships between subgingival B. gingivalis and changes in probing attachment levels 12 months after therapy were sought. In a total of 427 lesions studied, B. gingivalis was detected in 53.1% of the lesions showing probing attachment loss (>1.5 mm), but in only 4.7% of lesions showing attachment gain (>1.5 mm). These results strongly suggest that suppression of subgingival B. gingivalis in adult periodontitis can lead to clinical success or, at least, the absence of B. gingivalis correlates well with healing after therapy. Furthermore, persistence of B. gingivalis is consistent with further periodontal breakdown. It remains to be determined if persistence of B. gingivalis results from inadequate elimination, repopulation by the organism from the host, or re-infection from another source. Since we have very little information on the route of B. gingivalis transmission or of its reservoir or source, it is premature to approach the prevention of B. gingivalis-dLSsotiated peridontal disease by means of interfering with the infectious disease chain. However, if the oral cavity in infected individuals is the reservoir and source of B. gingivalis infection, then suppression of B. gingivalis may be achieved by the use of antibiotics such as penicillin and tetracycline, or through use of topical antiplaque or antimicrobial agents. The evaluation of other organisms and their role in adult periodontitis is important and will likely be directed to evaluation of the chain of infection involving pathogenic agents such as Bacteroides intermedius, Fusobacterium nucleatum, Wolinella recta, Eikenella corrodens, Bacteroides forsythus, Eubacterium species, and Peptostreptococcus micros. It will be necessary for researchers to investigate the role of each of these species alone and in combination, keeping in mind the infectious disease chain and designing experiments which are directed to identifying the infectious agent, and to determining whether it is an exogenous pathogen or a member of the indigenous flora which becomes an opportunistic pathogen. CRITERIA FOR DISTINGUISHING AN EXOGENOUS FROM AN OPPORTUNISTIC ORAL PATHOGEN The following criteria may be useful in distinguishing exogenous periodontal pathogens from bacteria which are opportunistic oral pathogens: (1) An exogenous periodontal pathogen is a transient member of the oral flora associated with periodontal disease. It may also be present in carriers and those subclinically infected. (2) An exogenous periodontal pathogen is likely to be comprised of one or a few clonal types. (3) An exogenous periodontal pathogen has the potential to be virulent, acting either singly or in combination with other members of the periodontal microflora. In contrast: (1) An opportunistic periodontal pathogen is a member of the indigenous oral flora, and overgrows or expresses its pathogenicity when a change in the environment or host occurs. (2) An opportunistic periodontal pathogen is likely to be comprised of many clonal types. (3) An opportunistic periodontal pathogen has a low level of intrinsic virulence, but can cause disease when it overgrows at the site of the lesion, or when the host's defense mechanisms are depressed. Studies addressing the issue of whether various periodontal organisms are exogenous or opportunistic pathogens will provide much-needed information on the infectious disease chain in various forms of periodontal disease, including juvenile periodontitis, adult periodontitis, generalized juvenile periodontitis, rapidly advancing forms of periodontitis, and periodontitis associated with various systemic diseases such as diabetes, Down's Syndrome, neutrophil abnormalities, and acquired immune deficiency syndrome. With this information, we are likely to be better prepared to prevent and manage periodontal disease by interference at a susceptible stage in the chain of infection. ACKNOWLEDGMENTS The authors thank Drs. Per Brandtzaeg, Boris Albini, Jeff Ebersole, Mogens Kilian, and Else Theilade for critical discussions leading to the definitions of exogenous and opportunistic pathogens, and the indigenous flora, and to establishing criteria for distinguishing exogenous from opportunistic pathogens. We thank the staff of the State University of New York at Buffalo Periodontal Disease Clinical Research Center for their role in the clinical studies. Special appreciation is expressed to Mrs. Rose Parkhill for her expert help in preparing this manuscript. Downloaded from adr.sagepub.com at PENNSYLVANIA STATE UNIV on February 23, 2013 For personal use only. No other uses without permission. Vol. 2 No. 2 PERIODONTAL INFECTIONS 257 Use and Interpretation of Microbiological Assays in Periodontal Diseases, Oral Microbiol Immunol 1: 73-79. ASIKAINEN, S.; JOUSIMIES-SOMER, H.; KANERVO, A.; and GILLETT, R. and JOHNSON, N.W. (1982): Bacterial Invasion of SUMMANEN, P. 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