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- MATERIALS AND METHODS
Objective: Recently a publication of Brenner et al. introduced 11 genetically distinct species within the genus Citrobacter. These newly recognized Citrobacter species can be classified by means of their biochemical characteristics. The aim of this study was to examine the distribution and susceptibility of Citrobacter isolates in our patient population.
Method: A total of 126 samples—containing a Citrobacter species—was collected from 116 hospitalized patients during a 6-month period. Organisms were identified according to standard procedures. Antimicrobial susceptibility testing was performed by agar dilution on Mueller-Hinton agar, and interpretation was based on NCCLS criteria.
Results: C. freundii was the most common organism isolated (n=59), followed by C. braakii (n=25) and C. koseri (n=23). The urinary tract and the respiratory tract were found to be the predominant sites of colonization or infection, accounting for 45% and 32% of all isolates respectively. It appeared that young children (<12 months old) and the elderly were most at risk of acquiring Citrobacter. Two-thirds of all specimens contained other organisms in addition to Citrobacter. Most Citrobacter isolates were related with a predisposing factor. Species-related differences were found in the susceptibility pattern.
Conclusions: These findings suggest that citrobacteria are important opportunistic pathogens contributing to colonization or infection in our hospital population.
The genus Citrobacter consists of Gram-negative, facultative anaerobic, motile bacilli with growth on Simmons citrate medium (hence its name). These bacilli are commonly found in water, soil, food and the intestinal tracts of animals and humans. A new publication of Brenner et al.  proposed the introduction of 11 genetically distinct species within the genus Citrobacter. The 11 species are: C. koseri (formerly described as C. diversus), C. amalonaticus, C. farmeri, C. freundii, C. youngae, C. braakii, C. werkmanii, C. sedlakii and the three unnamed groups (Citrobacter species 9, 10 and 11). The term ‘C. freundii complex’ covers the last eight Citrobacter species mentioned. Citrobacteria have been associated with a variety of infectious diseases. Amongst others, hospital-acquired bacteremias, endocarditis, urinary tract infections, neonatal meningitis and brain abscess have been reported [2–9]. The etiologic significance of citrobacteria in diarrheal illnesses has been put forward in some studies, but is still unclear [10,11]. We conducted this study to evaluate some aspects of Citrobacter isolates of patients staying at our university hospital.
- Top of page
- MATERIALS AND METHODS
A recent article from Brenner et al. introduced 11 genomospecies within the genus Citrobacter based on DNA-relatedness studies. This study deals with 126 Citrobacter isolates—collected during six successive months—from 116 patients staying at our university hospital. Identification of these Citrobacter strains was performed by means of conventional biochemical tests proposed by Brenner et al. . The reported biochemical characteristics of the isolated Citrobacter species in this study (Table 1) correspond well with the results of Brenner et al. . The most striking differences are the larger percentage of indole-negative C. freundii and C. braakii and the larger percentage of H2S-positive C. braakii and C. youngae. Within the C. freundii complex, C. freundii was the most common organism isolated, followed by C. braakii. This is in contrast to the results of Janda et al. ; in their studies C. youngae came second instead of C. braakii. No strains of C. amalonaticus or C. farmeri were found in this study, suggesting that these species are less pathogenic for humans. The urinary tract and the respiratory tract were predominant sites of isolation, accounting for 45.2% and 32.5% of all isolates respectively. Citrobacter infections were seldom encountered between the age of 1 and 40. More than 77% of all Citrobacter isolates occurred after the age of 40. One more group that appears to be at a higher risk comprises young children (<12 months old). The majority of isolates came from patients with previous surgery, who were admitted to an intensive care unit, or who had serious underlying diseases or malignancies. A striking experience in this study was the large amount of cultures containing other organisms in addition to Citrobacter. Two-thirds of all isolates occurred as part of a mixed infection. Our findings suggest that citrobacteria, like several other species of Enterobacteriaceae, are important opportunistic microorganisms contributing to colonization or infection. The opportunistic character of citrobacteria is suggested by the age of affected patients, the large proportion of mixed cultures, and the predisposing factors which were often present (e.g. intubation, mechanical ventilation, indwelling catheters, invasive procedures, serious compromising diseases).
As mentioned in previous reports, we also found marked differences in the susceptibility patterns of C. koseri and the members of the C. freundii complex [2,9]. C. koseri strains were generally sensitive to amoxiclavulanate and cefazolin, but resistant to ampicillin. Members of the C. freundii complex, on the other hand, were generally resistant to ampicillin, amoxiclavulanate and cefazolin. There is sometimes a discordance between the activity of ampicillin and amoxiclavulanate for the members of the C. freundii complex. This is probably due to the intrinsic higher activity of ampicillin resulting from the production of an inducible chromosomally encoded type I cephalosporinase by these citrobacteria, which is not inhibited by clavulanic acid. A worrisome fact is the isolation of several strains of C. freundii and C. braakii multiply resistant to the newer β-lactam antibiotics.