Genetic characterization of antimicrobial resistance, virulence, and plasmid-mediated transferability in Enterohaemorrhagic E. coli O157 isolates from healthy food handlers in Japan, 2006–2021

Abstract Aims The increasing prevalence of antibiotic-resistant foodborne pathogens is a serious public health concern worldwide. Enterohaemorrhagic Escherichia coli O157 (EHEC-O157) poses a high food contamination risk. This study aimed to investigate changes in the prevalence of EHEC-O157 among food handlers in Japan in 2006–2021. Methods and Results A total of 279 randomly selected isolates from 2006–2009, 2019, and 2021 were analysed for their antimicrobial resistance patterns and virulence genes. EHEC-O157 isolation rates in 2012–2022 were 0.0011–0.0037%, with an average isolation rate of 0.0024% (659/27,848,713). Antimicrobial susceptibility testing showed that EHEC-O157 isolates from all years were most resistant to streptomycin (>34%) and ampicillin (>7.7%), followed by tetracycline (<5.1%). EHEC-O157 isolates resistant to chloramphenicol (>9.4%) and kanamycin (>2.6%) have been isolated since 2019. Furthermore, the detection of Shiga toxin–related genes revealed that stx2 (57.4%) was more prevalent than stx1 (6%) in all sampling years. A whole-genome analysis revealed the presence of aph(6)-ld, aph(3'')-ld, and blaTEM-1B in the multidrug-resistant isolates, suggesting that these genes contribute to streptomycin and ampicillin resistance. Furthermore, one strain was an extended-spectrum β-lactamase producer harbouring the IncFII plasmid that encodes blaCTX-M-15. Moreover, EHEC acquired a plasmid encoding blaNDM-1 from carbapenem-resistant Citrobacter freundii. Conclusions We characterized the antimicrobial resistance and virulence factors of EHEC-O157 from food handlers. EHEC strains can easily acquire and spread antibiotic resistance. Notably, EHEC-O157 isolates from healthy individuals exhibited uninterrupted streptomycin and ampicillin resistance over the past decade. In recent years, there has been a discernible trend towards multidrug resistance of EHEC-O157.