We investigated the antimicrobial susceptibilities and resistance mechanisms of cattle-adapted Salmonella enterica serovar Dublin (S. Dublin) isolated in Japan in the past 30 years. The introduction of nalidixic acid into the veterinary field in the mid-1980s was followed by the emergence of nalidixic acid-resistant isolates, which are now predominant. We found only a single gyrA mutation (Asp-87 → Tyr) among the nalidixic acid-resistant isolates. Although reduced susceptibility to fluoroquinolones was observed among the nalidixic acid-resistant isolates, none of the isolates were resistant to the fluoroquinolones used in this study. Results of the susceptibility test using gyrA revertants and acrAB mutants suggest that isolates with the gyrA mutation were selected by the use of nalidixic acid while the AcrAB-TolC system accounted for the decreased fluoroquinolone susceptibility. These data suggest that the introduction of nalidixic acid in veterinary medicine affected the susceptibilities of S. Dublin among the cattle population in Japan, while the influence of the introduction of fluoroquinolones on susceptibility is not as clear.
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