| Summary: | In many countries including Japan, the status of emerging antimicrobial resistance among <i>Serratia</i> spp. and <i>Citrobacter</i> spp. in companion animals remains unknown because these genera are rarely isolated from animals. In this study, 30 <i>Serratia</i> spp. and 23 <i>Citrobacter</i> spp. isolates from companion animals underwent susceptibility testing for 10 antimicrobials. Phenotypic and genetic approaches were used to identify the mechanisms of extended-spectrum cephalosporins (ESC). Subsequently, ESC-resistant <i>Citrobacter</i> spp. strains underwent multilocus sequence typing and pulsed-field gel electrophoresis (PFGE). A significantly higher rate (34.8%) of ESC resistance was observed in <i>Citrobacter</i> spp. isolates than in <i>Serratia</i> spp. isolates (0%). ESC resistance was detected in five <i>C. freundii</i> strains, two <i>C. portucalensis</i> strains, and one <i>C. koseri</i> strain. All of the ESC-resistant <i>Citrobacter</i> spp. strains harbored CMY-type and/or DHA-type AmpC β-lactamases. Three <i>C.</i> <i>freundii</i> strains harbored the CTX-M-3-type extended-spectrum β-lactamases. Notably, the three <i>bla</i>CTX-3-producing and two <i>bla</i>CMY-117-bearing <i>C. freundii</i> strains (obtained from different patients in one hospital) had the same sequence type (ST156 and ST18, respectively) and similar PFGE profiles. We believe that ESC-resistant <i>Citrobacter</i> spp. are important nosocomial pathogens in veterinary medicine. Therefore, infection control in animal hospitals is essential to prevent dissemination of these resistant pathogens.
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