Download An Intrinsic Pathogenicity Index for Microorganisms

MICROBIAL ECOLOGY IN HEALTH AND DISEASE
VOL.
3 151-157 (1990)
An Intrinsic Pathogenicity Index for Microorganisms
Causing Infection in a Neonatal Surgical Unit
E. M. LEONARD*?,H. K . F. VAN SAENET, C. P. STOUTENBEEKg?, J. WALKERS AND P. K. H. TAMS
Departments of tMedical Microbiology and 1Paediatric Surgery, Royal Liverpool Children’s Hospital, Alder Hey,
Liverpool L12 2AP, U K , $Intensive Care Unit, 0 - L - V Gasthuis, 1091 HA Amsterdam, The Netherlands.
Received 5 June 1989; revised 22 December 1989
The intrinsic pathogenicity index (IPI) for a particular microorganismis defined as the ratio of the number of patients
infected by the microorganismto the number of patients colonised in the digestive tract by identical microorganisms.A
microorganism with an IPI of 1 will practically always cause infectionfollowing colonisation (‘high’ pathogen),whilst a
microorganism with an IPI of 0 is unlikely to be involved in infection (‘low’ pathogen). IPIs were calculated for
microorganisms causing infection in 40 neonates admitted to a neonatal surgical unit. The indices of the predominant
organisms involved in infectious problems in this group, Pseudornonas and Candida species, were 0.38 and 0.31
respectively, whilst those for coagulase-negativestaphylococci and enterococci were respectively 0.03 and 0. Possible
applications of the IPI are in further assessment of the value of surveillance cultures and as the basis of antibiotic
policies in populations with a high risk of infection.
KEY
WORDS-colonisation; Infection; Pathogenicity index; Neonate; Surgical unit.
INTRODUCTION
Studies of neonates in intensive care units have
shown high rates of nosocomial infection with
aerobic Gram-negative bacilli, yeasts, Staphylococcus aureus and coagulase-negative staphyloC O C C ~ . ~ , Little
~ - ~ , ’ published
~
information is
available on nosocomial infection in surgical
neonates.
We recently carried out a prospective inventory
study in a neonatal surgical unit to determine rates
of colonisation and infection with aerobic bacteria
and fungi. High colonisation rates with aerobic
Gram-negative bacilli were found, in particular with
Klebsiella spp. (55 per cent), Pseudomonas sp. (53
per cent) Escherichia coli (48 per cent) and Enterobacter spp. ( 3 3 per cent). Candida spp. colonised
38 per cent of infants. Most infants were colonised
with coagulase-negative staphylococci, alphastreptococci and enterococci (each 98 per cent). The
infection rate in the population was 35 per cent. The
predominant organisms causing infection were
Pseudomonas and Candida spp., microorganisms
which colonised infants at rates lower than those for
organisms such as coagulase-negative staphylococci
*Author to whom correspondence should be addressed. Current
address: Department of Medical Microbiology, University
College Hospital, Gower Street, London WClE 6BT, UK.
089 l-O6OX/90/030151-07 $05.00
0
1990 by John Wiley & Sons, Ltd.
and alpha-streptococci which caused few cases of
infection.
The results of this study, suggesting apparent
differences in pathogenicity of microorganisms,
prompted us to investigate why infections in the
neonatal surgical population should be caused by
certain organisms. Two major factors relating to the
chance of infection by a particular microorganism
developing in a patient are (i) host-related factors
(defence mechanisms, risk factors), and (ii) pathogenicity of the m i c r o o r g a n i ~ m . ’ ~Our
’ ~ ~aim was to
study the microbiological factor and quantify the
intrinsic potential of an organism to cause infection
in this population, by developing an index.
PATIENTS AND METHODS
Patients
The neonatal surgical unit at the Royal Liverpool
Children’s Hospital, Alder Hey, admits infants for
elective and emergency surgical procedures. Forty
infants (26 males, 14 females) admitted to the unit
and staying for at least 5 d were studied prospectively over a 3mth period. The mean age on
admission was 1 1 d and the average duration of stay
30 d. The mean gestation was 38 wks and mean
birthweight was 2.9 kg. The most frequent reason
I52
for admission was digestive tract disorder requiring
surgery (60 per cent of infants). Other reasons for
admission included conditions such as neural tube
defect and urological disorders.
Specimens and microbiological methods
For each baby, swabs were taken from the throat
and rectum on admission (baseline cultures) and
twice a week thereafter (surveillance cultures).
Samples of faeces were collected twice weekly when
possible. Urine specimens were collected once a
week. Collection of these and other specimens were
also performed when considered necessary, for
clinical purposes, by surgical staff. All specimens
were processed by the microbiologists (E.M.L.,
H.K.F.v.S.) after collection. Infants were examined
daily for clinical evidence of infection.
A Gram’s stain preparation was made from each
wound swab and examined for the presence of
leucocytes and microorganisms. All swabs were
qualitatively and semiquantitatively cultured. Four
solid media were used: MacConkey agar (Gibco
Limited) for evaluation of Enterobacteriaceae,
Pseudomonadaceae and Acinetobacter spp., blood
agar (Gibco Ltd., Paisley, Scotland) for streptococci
and staphylococci, kanamycin aesculin azide agar
(Lab M, Salford, UK) for enterococci, and yeast
morphology agar (Merck, Darmstadt, FRG) for
yeasts. The media were inoculated using the four
quadrant method. After each swab had been
streaked onto the agar plates, the tip was broken off
into 5 ml of brain-heart infusion broth (Lab M). All
cultures were incubated aerobically at 37°C.
MacConkey and kanamycin agar plates were examined after one night and blood and yeast morphology agar plates after two nights. If the broth
was turbid after one night’s incubation, inoculation
onto the four media was performed. Semiquantitative estimation of bacterial concentrations was
made by grading growth density from the four
quadrant method on a scale of 1 + to 5 + : growth
only in brain-heart infusion broth = 1 + (comparable to 1-10 microorganisms/ml), growth in the first
quadrant of the solid plate=2+ (< lo3), in the
second quadrant 3 + ( < lo’), in the third quadrant
4+ (< lo7) and on the whole plate 5+ (> lo7).
Faeces, urine and nasopharyngeal aspirates were
qualitatively and quantitatively cultured. One millilitre or gram of specimen was suspended in 9 ml of
brain-heart infusion broth and ten-fold serial
dilutions made. Tubes were cultured overnight at
37°C and those showing turbidity were subcultured
E. M. LEONARD ETAL.
onto the same four solid media. Incubation and
examination of the agar plates were performed in
the same way as for swab specimens. Quantitative
estimation of bacterial concentrations was made
from dilution series results.
All morphologically distinct colonies were
isolated in pure culture. Microorganisms were
identified by standard biochemical and serological
methods and typing was performed if necessary, in
particular for Pseudomonadaceae, to ascertain
whether or not isolates from different patients or
sites were identical. In addition, antimicrobial sensitivity patterns were determined for microorganisms
using the controlled disc diffusion method.
Bacterial charts were made of results obtained
and were used to aid assessment of colonisation patterns (Figure 1). Oropharynx and rectum were considered to be part of the same organ system, the
digestive tract, and charts for these were used in
combination.”
Intrinsic pathogenicity index (IFI)
For a microorganism, y species, causing infection
in a specific population:
IPI =
number of patients infected by y
number of patients colonised in throat/rectum by y
Recognised definitions for colonisation and
infection were used in the s t ~ d y . ’ ~ ~ ’ ’
Colonisation: a microbiological event, defined as
isolation of identical microorganisms from two or
more consecutive samples (taken at least 3 d apart)
from the same site, in any concentration of c.f.u./ml
orgofspecimen, without orwithonlyasmallnumber
of leucocytes, aild without signs of infection.
Infection: a microbiologically proven clinical
event, defined as a clinical diagnosis with microbiological proof (many leucocytes and one type of or
predominant microorganism) from the clinical
specimen.
Endogenous infection: infection caused by potentially pathogenic microorganisms which have first
colonised the digestive tract (either part of ‘normal’
resident flora, or newly acquired microorganisms
which have colonised the digestive tract).
Respiratory tract infection: clinical picture compatible with respiratory tract infection or pneumonia, radiological evidence, positive culture of
nasopharyngeal aspirate.
153
A PATHOGENICITY INDEX FOR MICROORGANISMS
PPMs’
1
Day on NSU
[
301 3 I 1321 33 341 35 1361 37 1381 391 401 4 I 142 1431 441 45 ( 4 6
I
-
Oropharynx
Klebsiella sp.
-
lo9
10
7
Pseudomonas sp.
-
-
to9
to5
S.viridans
(07
to9
107
S.faecalis
10 I
lo7
to7
-
to’
Rectum/faeces
Klebsiella sp.
I
o5
Pseudomonae sp.
I
o7
S.faecalis
I o9
[
I
o9
Figure 1. Example of a bacteriological chart used for assessment of colonisation patterns. PPMS =potentially pathogenic
microorganisms
Meningitis: compatible clinical picture, positive
culture of cerebrospinal fluid.
Urinary tract infection: turbid urine, pyuria (> 10
leucocytes/microscopic field (400 x ), positive culture of urine with one type of organism (> lo5
organisms/mlof urine).
Septicaemia: compatible clinical picture, haematological abnormalities (leucocytcosis, leucopenia,
‘left shift’, toxic granulation of leucocytes) and
positive blood culture.
Woundlsurface infection: erythma and/or pus at
site, purulence on Gram’s stain, isolation of one/
predominant organism.
RESULTS
Most babies in the study group were colonised in the
digestive tract with coagulase-negative staphylococci (98 per cent), enterococci (95 per cent) and
alpha-streptococci (95 per cent), microorganisms
characteristic of the indigenous flora (Table 1).
Aerobic Gram-negative bacilli (AGNB) frequently
colonised the throat/rectum, predominant organisms being Pseudomonas spp. (53 per cent of babies
Table 1. Rates of colonisation and infection in surgical
neonates
Microorganism
Coagulase-negative
staphylococci
Enterococci
Alpha-streptococci
Pseudomonas spp.
Klebsiella spp.
Escherichia coli
Staphylococcus aureus
Candida spp.
Enterobacter spp.
Proteus spp.
Citrobacter spp.
Acinetobacter spp.
Serratia spp.
Morganella spp.
Patients
colonised in
throat/rectum
(no.)
39 (98)
38 (95)
38 (95)
21 (53)
19 (48)
19 (48)
17 (43)
13 (33)
12 (30)
605)
6 (15)
4(10)
2 (5)
1 (3)
Figures in parentheses indicate percentages
Patients
infected
(no.)
154
E. M. LEONARD ET AL.
E.coli
Klebsiella rpp.
colonised), Klebsiella spp., Escherichia coli (43 per
cent) and Enterobacter spp. (30 per cent). Other
AGNB such as Proteus, Acinetobacter and Serratia
spp. were found to colonise the digestive tract of 15
per cent or less of children. Forty-three per cent of
babies carried Staphylococcus aureus, and one-third
were colonised by Candida spp. The infection rate in
this population was 35 per cent; the predominant
microorganisms causing infection were Pseudomonus spp., with 8/40 (20 per cent) children developing
one or more infections. Candida spp. caused infection in 10 per cent of children. Other organisms
which caused infection were Enterobacter sp., S.
aureus, E. coli,Klebsiella sp. and coagulase-negative
staphylococci, each causing a single episode of
infection.
These results were applied in the formula previously described to calculate the intrinsic pathogenicity indices of various microorganisms isolated
from the study group (Figure 2). The range of the
index is 0-1. In the surgical neonates, the highest
indices were found for Pseudomonas and Candida
spp. (0.38 and 0-31 respectively). Much lower
indices were found for other microorganisms,
including S. aureus (0-06), coagulase-negative
staphylococci (0.03) and enterococci (0).
DISCUSSION
The derivation of the formula for an intrinsic pathogenicity index (IPI) is based on our observation that
the pathogenesis of nosocomial infection in a population of surgical neonates was endogeno~s.’~
The
denominator used, ‘number of patients colonised in
throat/rectum’, is crucial to the formula. Several
studies of compromised patient populations, such
as long-term ventilated ICU patients, have shown
that three distinct stages in the development of
endogenous infection can be identified: (i) colonisation of the oropharynx/gastrointestinaltract, (ii)
colonisation of major organ systems (respiratory
tract, urinary tract, skin), and (iii) infection by
identical microorganisms.’ 220,24 This sequence
was identified in the surgical neonatal population;
infection caused by microorganisms was always
preceded by colonisation of the digestive tract by
identical organisms, the average time interval
between identification of colonisation and subsequent infection being 1 week. Since digestive tract
colonisation was considered to be a crucial stage in
the development of infection in these populations,
rates for this rather than for colonisation of body
surfaces were used for the IPI formula. The term
‘colonisation of body surfaces’ includes colonisation of systems other than the digestive tract,’ and
use of this term in the IPI formula would encompass
both first and second stages in development of infection. In addition, colonisation of another system
occurred occasionally without previous colonisation of the digestive tract having been identified,
but never resulted in infection in the neonatal group;
use of ‘digestive tract colonisation’ in the formula
excludes these cases.
A PATHOGENICITY INDEX FOR MICROORGANISMS
The formula gives an indication of the capacity of
a microorganism which first colonises the digestive
tract of a patient to cause infection in that patient.
The index derived for a particular microorganismis
applicable to a specific patient population.
The range of the IPI is 0-1. An IPI of 1 denotes a
microorganism which, after colonising a patient,
will always cause infection. Such an organism may
be described as highly virulent.22No such organism
was identified in the surgical neonatal population,
but it might be antipicated that an organism such as
Salmonella typhimurium would have a high index.
Conversely, in the case of an organism with an IPI
close to 0, colonisation will seldom be followed by
infection. Such an organism is described as poorly
virulent; this index would be characteristic of the
indigenous flora. Examples from the surgical neonatal population are coagulase-negative staphylococci (IPI 0.03) and enterococci (IPI 0). An index of
0.5 implies that colonisation by the organism will be
followed by infection in half the cases. Organisms
with indices in this region are described as potential
pathogens. The indices of the predominant organisms causing infection in the neonatal surgical population, Pseudomonas and Candida spp., came into
this category at 0.38 and 0.31 respectively.
The accuracy of the applied IPI formula must
depend on the accuracy of detection of colonisation
and infection. (These terms themselves still need to
be clearly defined and agreed by clinical and microbiological workers.) Several techniques were used in
our study to increase the sensitivity and accuracy of
detection of colonisation.
Collection of baseline and surveillance cultures
from both oropharynx and rectum was found to be
of great use. These sites were considered to be part
of the same organ system and results were considered in combination, using the chart system previously mentioned. Use of the broth incubation
technique (‘enrichment’step)2 enabled us to detect
small numbers of microorganisms, which is particularly important for oropharyngeal samples. More
detailed colonisation patterns that might otherwise
have been missed were thus detected and, in
addition, insight was gained into the inadequacies of
considering samples from each site in isolation. For
example, in some cases microorganisms were never
isolated from the oropharynx or were found at
irregular intervals, but when results of rectal swab
or faecal cultures were also considered,it was apparent that the digestive tract had become colonised
with these organisms. An explanation for these findings is that Gram-negative bacilli may be present
155
only transiently in the oral cavity because of lack of
adherence to surfaces and the actions of chewing,
sucking and swallowing,’6 but may be detected
later, after overgrowth at intestinal level, in faecal
samples. A further factor is that the quality of
samples, particularly from the oropharynx, is likely
to vary in populations of infants and children.
We consider that the IPI concept needs to be
developed, but at this stage would like to suggest
possible applications. A practical application is in
predicting which of those organisms isolated from a
surveillance swab is most likely to cause infection.
The value of surveillance cultures, in particular
as predictors of infection, is still subject to
debate.1.3*9.18
For such cultures to be of clinical use,
detectable colonisation must either precede infection or not be followed by infection; the more
reliable the prediction, the greater the value. This
raises the crucial point that the basic stage of detection of colonisation is itself dependent on several
factors, such as sites chosen for surveillance,criteria
used for definition of colonisation, interpretation of
results and microbiological techniques used. These
factors have not been constant among different
investigators, and this may have influenced results.
Apart from technical aspects, it is well established
that if the infection rate in a population is low, the
positive predictive value of even an excellent test will
also be low.’ Results obtained by using standardised sensitive techniques could be applied in a
formula such as that described, relating to pathogenicity of microorganisms, to aid assessment of the
value of surveillancecultures in specific populations
at high risk of infection.
Another application for the IPI is in aiding construction of antibiotic policies, both prophylactic
and therapeutic. After calculation of IPI values for
microorganisms in different patient populations,
those most likely to cause infection in that population might be identified and appropriate antibiotic
usage recommended.
Apparent differences in pathogenicity of microorganisms have also been noted in adult neutropenic patients by various worker^.^^,^','^.^^ It has
been shown that, once the blood neutrophil count
has fallen below 100/mm3, Pseudomonas spp. are
the bacteria most likely to translocate from gut
lumen into the bloodstream. This illustrates how
development of infection depends on the balance
between intrinsic pathogenicity of microorganisms
and defencecapacity of the host. Our index envelops
both factors, but does not differentiate between
them. Host defences may be reduced by many
156
factors such a s underlying disease, surgical trauma,
medical intervention and use of chemotherapeutic
agents and antibiotics. When resistance to colonisation is lowered by such factors, colonisation of the
digestive tract by potentially pathogenic microorganisms may occur more readily. Both host
defence and microbial pathogenicity should be
taken into consideration, therefore, when predicting
the likelihood of infection in a particular patient.
We suggest that a separate index for host defence, a
‘host colonisation index’, might be developed to
advantage, to predict which patients in a given
population are most likely to become infected.
ACKNOWLEDGEMENTS
We are very grateful to Mr R. C. M. Cook, Mr R. E.
Cudmore and Mr A. M. K. Rickwood for allowing us to
study their patients, the nursing and medical staff of the
Neonatal Surgical Unit, the infants and their parents, Dr
J. Green for statistical advice, Mr J. Duitsch for drawing
the figures, Professors C. A. Hart and D. A. Lloyd for
critically reviewing the paper, and to Liverpool Health
Authority.
REFERENCES
1. Evans ME, SchaffnerW, Federspiel CF, Cotton RB,
McKee KT Jr, Stratton CW. (1988). Sensitivity,
specificity, and predictive value of body surface cultures in a neonatal intensive care unit. Journal of the
American Medical Association 259,248-252.
2. Flynn DM, Weinstein RA, Nathan C, Gaston MA,
Kabins SA. (1987). Patients’endogenous flora as the
source of ‘nosocomial’ Enterobacter in cardiac
surgery. Journal of Infectious Diseases 156,363-368.
3. Goldmann DA. (1988). The bacterial flora
of neonates in intensive care-monitoring and
manipulation. Journal of Hospital Infection 11,
34&351.
4. Goldmann DA, Durbin WA Jr, Freeman J. (1981).
Nosocomial infections in a neonatal intensive care
unit. Journal of Infectious Diseases 144,449459.
5. Griner PF, Mayewski RJ,Mushlin AI, Greenland P.
(1981). Selection and interpretation of diagnostic
tests and procedures: principles and applications.
Annals of Internal Medicine 94,553-600.
6. Hemming VG, Overall JC Jr, Britt MR. (1976).
Nosocomial infections in a newborn intensive-care
unit. New England Journal of Medicine 294,
13lO-13 16.
I. Hensey OJ, Hart CA, Cooke RWI. (1985). Serious
infection in a neonatal intensive care unit: a
two-year survey. Journal of Hygiene 95,289-297.
E. M. LEONARD ET AL.
8. Hoogkamp-Korstanj JAA, Cats B, Senders RCh,
van Ertbruggen I. (1 982). Analysis of bacterial infections in a neonatal intensive care unit. Journal of
Hospital Infection 3,275-284.
9. IsaacsD, WilkinsonAR, Moxon ER. (1987). Surveillance of colonization and late-onset septicaemia in
neonates. Journalof Hospital Infection 10, 114-1 19.
10. Ketchel SJ, Rodriguez V. (1978). Acute infections in
cancer patients. Seminars in Oncology 5, 167-1 79.
11. Kurrle E, Bhadjuri S , Krieger D, Gaus W, Heimpel
H, Pflieger H, Arnold R, Vanek E. (1981). Risk factors for infections of the oropharynx and the respiratory tract in patients with acute leukemia. Journal
of Infectious Diseases 144,128-136.
12. La Force M. (1981). Hospital acquired Gramnegative rod pneumonias: an overview. American
Journal of Medicine 70,664-669.
13. Leonard EM, van Saene HKF, Shears P, Walker J,
Tam PKH. (I 990). Pathogenesis of colonization and
infection in a neonatal surgical unit. Critical Care
Medicine 18,264-269.
14. MacGregor RR 111, Tunnessen WW Jr. (1973). The
incidence of pathogenic organisms in the normal
flora of the neonate’s external ear and nasopharynx.
Clinical Pediatrics 12,697-100.
15. Maguire GC, Nordin J, Myers MG, Koontz FP,
Hierholzer W, Nassif E. (198 1). Infections acquired
by young infants. American Journal of Disease in
Children 135,693-698.
16. Penn RG, Sanders WE Jr, Sanders CC. (1981).
.
,
Colonization of the oropha&nx with gram-negative
bacilli: a major antecedent to nosocomial pneumonia. American Journal of Infection Control 9,
25-34.
17. Schimpff SC, Young VM, Greene WH, Vermeulen
GD, Moody MR, Wiernik PH. (1972). Origin of
infection in acute nonlymphocytic leukemia. Annals
of Internal Medicine 77,707-714.
18. Sprunt K. (1985). Practical use of surveillance for
prevention of nosocomial infection. Seminars in
Perinatology 9,47-50.
19. Stoutenbeek CP, van Saene HKF, Miranda DR,
Zandstra DF. (1984). The effect of selective decontamination of the digestive tract on colonisation and
infection rate in multiple trauma patients. intensive
Care Medicine 10, 185-192.
20. Stoutenbeek CP, van Saene HKF, Miranda DR,
Zandstra DF, Langhehr D. (1987). The effect of
oropharyngeal decontamination using topical
nonabsorbable antibiotics on the incidence of nosocomial respiratory tract infections in multiple
trauma patients. Journal of Trauma 27,357-364.
21. Tancrede CH, Andremont AO. (1985). Bacterial
translocation and Gram-negative bacteremia in
patients with hematological malignancies. Journal
of Infectious Diseases 152,99-103.
22. van Saene HKF, Stoutenbeek CP, Miranda DR,
Zandstra DF. (1983). A novel approach to infection
A PATHOGENICITY INDEX FOR MICROORGANISMS
control in the intensive care unit. Acta Anaesthesiologiea Belgica 3, 193-208.
23. van Saene HKF, Stoutenbeek CP, Zandstra DF,
Gilbertson AA, Murray A, Hart CA. (1987).
Nosocomial infections in severely traumatized
patients: magnitude of problem, pathogenesis, prevention and therapy. Acta Anaesthesiologica Belgica
38,347-3.53.
157
24. van Uffelen R, van Saene HKF, Fidler V, Lowenberg
A. ( 1984). Oropharyngeal flora as a source of bacteria
colonizing the lower airways in patients on artificial
ventilation. Intensive Care Medicine 10,233-237.
25. Weinstein L, Brown RB. (1977). Colonization,
suprainfection and superinfection: major microbiologic and clinical problems. Mount Sinai Journal of
Medicine 44, 10&112.