Clinical and molecular epidemiology of multidrug-resistant Citrobacter spp. infections in a French university hospital

Citrobacter spp. are aerobic, gram-negative bacilli that are frequently found in water, soil, food, and the intestines of animals and humans [1]. Even though their level of virulence is low and they rarely cause infections in humans, these pathogens have been associated with a wide spectrum of infections involving the central nervous system and the gastrointestinal, urinary, or respiratory tracts [1]. The fact that Citrobacter spp. are rather uncommon in clinical specimens explains why few studies of them appear in the medical literature [2, 3]. However, these organisms may be responsible for serious infections, especially in immunocompromised hosts. To evaluate the epidemiology of Citrobacter spp. in our 1,700-bed university hospital in southern France, we conducted a prospective study from 1 October 2003 to 30 April 2004. Hospitalized patients were screened for the presence of Citrobacter spp. and nonrepetitive clinical strains isolated consecutively from infection sites were included in the study. Antibiotic susceptibility testing was performed using the Vitek 2 GNS-F7 card (bioMerieux, Marcy l’Etoile, France) and an agar disk-diffusion assay on Mueller-Hinton agar. Strains were classified as susceptible, intermediately resistant or resistant to the other antibiotics tested according to the recommendations of the Antibiotic Susceptibility Testing Committee of the French Society for Microbiology [4]. The strains that were found to be resistant to extended-spectrum cephalosporins were additionally screened for extended-spectrum β-lactamase (ESBL) production using the double-disk synergy test. Isoelectric focusing was performed using polyacrylamide gels as described previously [5]. Macrorestriction analysis of chromosomal DNAwas performed using pulsed-field gel electrophoresis according to previously published procedures and analyzed with Gel compar computer software (Applied Math, Belgium) as described previously [5]. The qnrA, qnrB and qnrS genes were screened using PCR as described previously [5]. During the study period, information concerning patient age, sex and immune status (MacCabe score [6]), hospital ward, source of infection, mortality, comorbid diseases, antecedent of hospitalization (notably prior stay in an intensive care unit), prior antibiotic therapy, date of bacterial isolation, length of hospitalization, and the source of infection were collected to determine the risk factors for infections caused by extended-spectrum, cephalosporinresistant Citrobacter spp. Duplicate isolates from the same patient were excluded. Of the 1,531 Enterobacteriaceae we isolated, 45 strains were Citrobacter spp., i.e., 2.94% isolated from 45 patients. The characteristics of these 45 patients are presented in Table 1. The infections were of nosocomial origin in 62% of cases. Citrobacter koseri was the species isolated most frequently (57.7%), but this finding differs from previous Eur J Clin Microbiol Infect Dis (2007) 26:439–441 DOI 10.1007/s10096-007-0315-3