| Summary: | Reports have documented antimicrobial usage in aquaculture, and the aquatic ecosystem can be considered a genetic storage site for antibiotic-resistant bacteria. This study assessed the prevalence of antimicrobial resistance (AMR) among Gram-negative bacteria recovered from retail seafood in Hiroshima, Japan. A total of 412 bacteria were isolated and screened for the presence of β-lactamases, acquired carbapenemases, and mobile colistin-resistance (<i>mcr</i>) genes. Forty-five (10.9%) isolates were dominated by <i>Morganella</i> (28%), <i>Proteus</i> (22%), <i>Aeromonas</i> (14%), <i>Citrobacter</i> (8%), and <i>Escherichia</i> (8%) and carried AMR genes. The identified AMR genes included those encoded in integrons (19), <i>aac(6՛)-Ib</i> (11), <i>bla</i><sub>TEM-1</sub> (7), <i>bla</i><sub>CTX-M-like</sub> (12), <i>bla</i><sub>CTX-M-65</sub> (2), <i>bla</i><sub>SHV-12</sub> (1), <i>bla</i><sub>SHV-27</sub> (1), <i>bla</i><sub>OXA-10</sub> (1), <i>bla</i><sub>OXA-2</sub> (1), and <i>mcr</i> (2). The most common clinical resistances were against ampicillin, colistin, sulfamethoxazole/trimethoprim, tetracycline, and ciprofloxacin. Multidrug resistance (MDR) occurred in 27 (60%) AMR isolates, and multiple antibiotic resistance indices ranged from 0.2 to 0.8. A conjugation experiment showed that 10 of the 11 selected MDR strains harbored conjugable plasmids, although PCR-based replicon typing described seven strains as untypable. IncF replicon was identified in MDR extended-spectrum β-lactamase-producing <i>Escherichia coli</i> of the pathogenic B2 phylogroup. Our findings suggest that retail seafood harbors MDR bacteria of human interest that require strict resistance surveillance in the seafood production continuum.
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