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‫مايكرو نظري ‪ /‬ثالث اسنان كركوك‬
‫د‪ .‬زبيدة‬
is the aggregate of microorganism that reside on the surface and
in deep layers of skin, in the saliva and oral mucosa, in the
conjuctiva, and in the gastrointestinal tracts.’
Normal Flora absent in Sterile tissues
In a healthy human, the internal tissues such as:
• blood
• brain
• muscle
• cerbrospinal fluid (csf.)
are normally free of microorganism
oral microflora
The microflora existing in the oral cavity is called as oral
Both host and bacteria are thought to derive benefit from
each other, and the associations are, for the most part,
The normal flora derive from their host a steady supply of
nutrients, a stable environment, and protection and
The host obtains from the normal flora certain nutritional
and digestive benefits, stimulation of the development and
activity of immune system, and protection against
colonization and infection by pathogenic microbes
The normal flora protect us from disease by •
• The mouth is similar to other sites in the
body in having a natural microflora with a
characteristic composition and existing, for
the most part, in a harmonious relationship
with the host
The Composition of Normal flora
The makeup of the normal flora influenced
and diet of the individual.
his relationship can break down in the mouth and
disease can occur. This is usually associated with:
1- major changes to the biology of the mouth from
exogenous sources (examples include: antibiotic
treatment or the frequent intake of fermentable
carbohydrates in the diet) or from endogenous
changes such as alterations in the integrity of
the host defences following drug therapy, which
perturb the natural stability of the microflora
Bacteria with the potential to cause disease in this
way are termed ‘opportunistic pathogens’
disease in the mouth caused by imbalances in the
composition of their resident oral microflora.
The commonest clinical manifestations of such
imbalances are dental caries and periodontal
Dental caries:- is the dissolution of enamel or root
surfaces (demineralization) by acid produced
primarily from the metabolism of fermentable
carbohydrates in the diet by bacteria colonizing the
tooth surface (dental plaque).
Dental plaque is also associated with the etiology of
periodontal diseases in which the host mounts an
inappropriate inflammatory response to an increased
microbial load (due to plaque accumulation) around
the gingivae, resulting in damage to the supporting
tissues of the teeth
Most diseases of the mouth have a polymicrobial
(multiple species) etiology.
The ability of consortia of bacteria to cause disease
depends on the outcome of various interactions
both among the microbes themselves, and between
these microorganisms and the host
The composition and metabolism of bacteria at a site
will be influenced by the
1- flow rate and properties of saliva
2- the life-style of an individual (in particular, the
presence of a tobacco habit, the nature of the diet,
and exposure to medication)
3- the integrity of the host defences.
4- Oral microflora
The inter-relationships that influence the microbial ecology of
the mouth in health and disease. The predominant
in the mouth might alter due to changes in saliva flow, life-style
(e.g. tobacco habit, diet) or to changes in the integrity of the
host14 defences. These changes may predispose sites to diseas
HABITAT)Sites and factors)
1-Mucosal surfaces
The mouth is similar to other ecosystems in the
digestive tract in having mucosal surfaces for
microbial colonization. The microbial load is relatively
low on such surfaces due to desquamation.
The papillary structure of the dorsum of the tongue
provides refuge for many microorganisms which
would otherwise be removed by mastication and the
flow of saliva.
• The tongue can also have a low redox potential.
which enable obligately anaerobic bacteria to
• The tongue can act as a reservoir for some of
the Gram negative anaerobes that are implicated
in the etiology of periodontal diseases , which
may influence the intra-oral distribution of some
microorganism, which will in turn influence the
composition of the resident microbial community
at a site.
2- Teeth
Teeth (and dentures) allow the accumulation of large
masses of microorganism and their extracellular
products, termed dental plaque.
Plaque is an example of a biofilm, also associated
with dental caries and periodontal disease.
Teeth do not provide a uniform habitat but possess
several distinct surfaces due to the physical nature of
the particular surface and the resulting different
biological properties of the area.
e.g. The stagnant areas between adjacent teeth
( approximal) and in the gingival crevice afford most
protection to colonizing microorganisms from the
adverse conditions in the mouth. Both sites are also
anaerobic and, in addition, the gingival crevice region
is bathed in the nutritionally-rich gingival crevicular
fluid GCF
Smooth surfaces of teeth are more exposed to the
environment and can only be colonized by a limited
number of bacterial species adapted to such
extreme conditions.
The properties of a smooth surface will differ
according to whether it faces the cheek (buccal
surface) or the inside (lingual surface) of the mouth
Pits and fissures of the biting (occlusal) surfaces of
the teeth also offer protection from oral removal
forces such as saliva flow, and can contain impacted
food debris. Such protected areas are associated
with the largest microbial communities and, in
general, the most disease.
Saliva plays a major role in maintaining the integrity
of teeth by clearing food and by buffering the
potentially damaging acids produced by dental
plaque following the metabolism of dietary
carbohydrates. Bicarbonate is the major buffering
system in saliva, but phosphates, peptides and
proteins are also involved.
The mean pH of saliva is between pH 6.75 and 7.25,
the pH and buffering capacity will vary with the flow
rate. The slowest flow of saliva occurring during
temperature of mouth (35–36°C) which provides
stable conditions suitable for the growth of a wide
range of microorganisms., with active disease
(inflammation) have a higher temperature(up to 39°C)
compared with healthy sites.
Even such relatively small rises in temperature can
significantly alter bacterial gene expression in
periodontal pathogens, such as Porphyromonas
Oxygen tension :
 The oxygen concentrations at different locations in the oral
cavity vary widely.
 As may be expected, the dorsum of the tongue and the buccal
and palatal mucosa are in an essentially aerobic environment.
 The oxygen tension inside a periodontal pocket is very low, with
the species having a tendency to become reduced rather than
oxidised, explaining the survival of obligate anaerobe.
 Therefore obligate aerobic organisms (which require oxygen)
cannot survive, whereas obligate anaerobic organisms (which
cannot tolerate the presence of oxygen) are able to thrive.
Many microorganisms require a pH around
neutrality for growth, and are sensitive to extremes
of acid or alkali.
The pH of most surfaces of the mouth is regulated
by saliva (the mean pH for saliva is between 6.75
and 7.25)
(i) Endogenous nutrients
The main source of endogenous nutrients is
1- Saliva, which contains amino acids, peptides,
proteins and glycoproteins (which also act as a
source of sugars and amino-sugars), vitamins and
2- Delivering components of the host defences,
contains novel nutrients, such as albumin and
other host proteins and glycoproteins, including
haemecontaining molecules
(ii) Exogenous (dietary) nutrients
Ii-1 - fermentable carbohydrates are the only class of
compound that markedly influence the ecology of the
mouth(Increased the acidity )
The frequent consumption of dietary carbohydrates
is associated with a shift in the proportions of the
microflora of dental plaque.
e.g. The levels of acid-tolerating species, especially
mutans streptococci and lactobacilli, increase while
the growth of acid-sensitive species (for example,
some strains of Streptococcus sanguinis and S.
gordonii) is inhibited, and they decrease.
Ii-2- -Dairy products (milk, cheese)
The ingestion of milk or milk products can protect the teeth
against caries. This may be due to the buffering capacity of
milk proteins or due to decarboxylation of amino acids after
proteolysis since several bacterial
species can metabolise casein.
Milk proteins and casein can also adsorb on to the tooth
surface, in exchange for albumin in the enamel pellicle, and
reduce the adhesion of S. mutans
Cheese has been shown to increase salivary flow rates and
elevate plaque pH changes following a sucrose rinse
Ii-4-Nitrate in green
vegetables may influence the oral microflora.
Nitrate derived from the diet is concentrated by
salivary glands so that salivary concentrations are
higher than plasma.
This nitrate can be rapidly converted to nitrite by
bacterial nitrate reductases. At low pH, this acidified
nitrite can be inhibitory to the growth of bacteria
both caries and periodontal diseases.
Host defences
Host factors :
1) Saliva
2) Crevicular fluid
1. Saliva :
• Various salivary components interact with oral flora in
ways that can either enhance or inhibit ability of these
organisms to survive.
• IgG is seen in gingival inflammation contributed by GCF.
• These make it more difficult for bacteria to bind to oral
epithelium or hard tissue surfaces.
Non-specific factors:
• These are lysozyme, lactoferrin and lactoperoxidase.
• Lysozyme degrades bacterial peptidoglycan i.e the cell wall,
rendering bacteria susceptible to osmotic disruption and
• Lactoferrin binds to iron which is a growth limiting
substance in bacteria. Making iron unavailable to bacteria
lactoferrin limits bacterial growth.
• Lactoperoxidase catalyses the formation of hypothiocyanate
ion ,inactivating bacterial enzymes and finally death.
2. Crevicular fluid :
• It is an inflammatory exudate derived from plasma with
large amount of antibody and complement proteins.
• The predominant immunoglobulin is IgG, derived from
plasma cells located in periodontal tissues as well as
from circulating plasma.
• These antibodies keep the subgingival flora in check by
inhibiting colonization or activating complement system.
(ii) Adaptive immunity
Components of the specific host defences
(intraepithelial lymphocytes and immunoglobulins
IgG and IgA) are found on and within the mucosa
where they act as a barrier to penetrating antigens.
Important Oral Bacteria
I1. Gram
Positive organisms:
• Bulk of oral bacteria
• Rods (bacilli), cocci or irregular shape (pleomorphic)
• Oxygen tolerance varies from aerobes to strict anaerobes
• Most are fermentative
• Three important genera:
• Actinomyces, facultative anaerobe
• Lactobacillus, produce lactic acid, facultative anaerobe, role in
dentine caries rather than enamel caries
• Streptococcus facultative anaerobic cocci, produce lactic acid some
implicated in caries
• Associated with bacterial endocarditis
• Isolated from all sights of the mouth
• Large proportion of resident microflora
• Majority α-haemolytic
Strep mutans:
• Associated with caries
Strep salivarius:
• Colonise mucosal surfaces especially the tongue
Strep angiosus:
• Isolated dental plaque & mucosal surfaces
• Seen in maxillofacial infections, brain, liver etc
Strep mitis:
• Opportunistic pathogens e.g. endocarditis
• Short pleomorphic rods with branching
• Major proportion of plaque
• Increase in gingivitis
• Associated with root caries
Important Oral Bacteria
Gram Negative organisms
Many Gram-negative bacteria found in the mouth, especially
in established/subgingival plaque
Cocci, rods, filamantous rods, spindle shaped or spiral shaped
Range of oxygen tolerance but most important strict or
facultative anaerobes Some fermentative, produce acids
which other organisms use acids as an energy source, others
produce enzymes which break down tissue
Most important Gram negative
Porphyromonas: P. gingivalis major periodontal pathogen
• Prevotella: P. intermedia a periodontal pathogen
• Fusobacterium: F. nucleatum periodontal pathogen
• Actinobacillus/ associated with aggressive periodontitis
•Treponema: group important in acute periodontal conditions
• Neisseria
• Veillonella
Some definition for medical
Habitat: the site where a microorganism
Ecology: Study of relationship between
organisms and their environment.
Ecological niche: The functional position of an
organism in its environment comprising the
living space, periods of time during which it is
active there and resources it obtains there.
• Indigenous Flora(Resident):
It comprises those indigenous species that are almost
always present in high numbers, greater than 1 % of the
total viable count.
• Supplemental Flora:
The supplemental flora are those bacterial species that
are nearly always present, but in low numbers, less than
1 % of total viable count
• Transient Flora:
Transient flora comprise organisms "just passing through"
a host. At any given time a particular species may or may
not be represented in the flora.
• Pathogens
Microorganisms that have the potential to cause disease
are termed pathogens.
• Opportunistic Pathogens
Micro-organisms that cause disease only under
exceptional circumstances .
• True Pathogens
Micro-organisms that are consistently associated with a
particular disease .