Investigation of antimicrobial susceptibility and molecular epidemiology in Salmonella Agona isolated at animal quarantine service

Microbial sensitivity tests were conducted on 36 strains of Salmonella Agona isolated from day-old chicks and fish meal for animal feed sampled under animal quarantine by the Animal Quarantine Service of Ministry of Agriculture, Forestry and Fisheries of Japan. Plasmid profile analysis and pulsed-field gel electrophoresis (PFGE) were also applied to perform a molecular epidemiological study in parallel. The results of these analyses revealed the state of antimicrobial resistance in each isolated strain from day-old chicks and fish meal: 11% (4/36) in ampicillin, 56% (20/36) in dihydrostreptomycin, 14% (5/36) in kanamycin, 19% (7/36) in oxytetracycline, 3% (1/36) in chloramphenicol, and 17% (6/36) in nalidixic acid. In particular, it was verified that 33% (12/36) were multiply resistant strains. The present study indicated that three isolates from fish meal were resistant to nalidixic acid and enrofloxacin. Furthermore, taking into account the outcome from the analyses on genetic diversity such as plasmid profiles and electrophoretic patterns of strains isolated in each imported commodity group using the above analysis methods, it was estimated that these groups might have been contaminated by antimicrobials derived from a single source or multiple microbial sources in the exporting countries. Based on the data on isolated strains relevant to the latter sources, these could not be differentiated by treatment with restriction enzyme XbaI that is generally used in PFGE as a discriminatory typing technique. However, the PFGE patterns demonstrated that the strains manifested genetic diversity following BlnI. Therefore, this study indicates that BlnI treatment could yield higher discriminatory ability than XbaI in a molecular epidemiological survey of Salmonella Agona. Antimicrobial resistance has been a vital issue domestically and internationally. Therefore, surveillance and monitoring for following trends in antimicrobial resistance will grow more and more important for the future from the viewpoints of animal and public health.