Antibiotic Resistance Profiles and Molecular Characteristics of -Carrying Serovar Enteritidis Isolates from Healthy and Diseased Chickens in Korea

Extended-spectrum β-lactamase (ESBL)-producing serovar Enteritidis has emerged as a public health concern. The main objectives of this study were therefore to determine the antimicrobial susceptibility profiles of Enteritidis and to investigate the molecular characteristics of identified ESBL-producing isolates. In the study, 237 Enteritidis isolates (232 isolates from chickens, 4 from cattle, and 1 from a pig) were recovered from carcasses and fecal samples of healthy and diseased food animals between 2010 and 2017. Ceftiofur resistance was noted only in chicken isolates (43%, 102/237), with the highest in healthy chickens and their carcasses (48.3%, 83/172) compared with that in diseased chickens (31.7%, 19/60). All of the ceftiofur-resistant isolates exhibited resistance to multiple antimicrobials. Indeed, a relatively higher percentage of ceftiofur-resistant isolates demonstrated resistance to the tested aminoglycosides and tetracycline compared with the ceftiofur-susceptible strains. In this study, was the only ESBL gene detected in all of the ceftiofur-resistant isolates. The -carrying isolates belonged to 11 different pulsotypes. The gene was transferred from 20.6% (21/102) of the -harboring isolates to a recipient J53. The coexistence of IncHI2/ST2 and IncFIIs/ST1 plasmids was noted in the majority (81.8%, 18/22) of the transconjugants. J53 transconjugants carrying gene showed distinct genetic environments, predominantly IS-- (15/21, 71.4%). This study demonstrated that healthy chickens and their carcasses act as reservoirs of -carrying Enteritidis that can potentially be transmitted to humans.