Low-Virulence
Citrobacter
Species Encode Resistance to Multiple Antimicrobials
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abstract
ABSTRACTCitrobacter
spp. are gram-negative commensal bacteria that infrequently cause serious nosocomial infections in compromised hosts. They are often resistant to cephalosporins due to overexpression of their chromosomal β-lactamase. During a recent study of multidrug-resistant
Enterobacteriaceae
(MDRE) in solid-organ transplant patients, we found that almost half of patients colonized with MDRE carried one or more cefpodoxime-resistant
Citrobacter freundii
,
Citrobacter braakii
, or
Citrobacter amalonaticus
strains. Pulsed-field gel electrophoresis showed that 36 unique strains of
Citrobacter
were present among 32 patients. Genetic and phenotypic analysis of the resistance mechanisms of these bacteria showed that the extended-spectrum β-lactamase (ESBL) SHV-5 or SHV-12 was encoded by 8 strains (26%) and expressed by 7 strains (19%). A number of strains were resistant to other drug classes, including aminoglycosides (28%), trimethoprim-sulfamethoxazole (31%), and fluoroquinolones (8%). PCR and DNA analysis of these multiresistant strains revealed the presence of class I integrons, including the first integrons reported for
C. braakii
and
C. amalonaticus
. The integrons encoded aminoglycoside resistance, trimethoprim resistance, or both. Despite the prevalence of MDR
Citrobacter
spp. in our solid-organ transplant patients, only a single infection with a colonizing strain was recorded over 18 months. Low-virulence
Citrobacter
spp., which can persist in the host for long periods, could influence pathogen evolution by accumulation of genes encoding resistance to multiple antimicrobial classes.
