Characterization of Antibiotic Resistance and the Emergence of AmpC-Producing Salmonella Infantis from Pigs

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During the period of 2007-2008, a total of 270 pig fecal samples were collected from a meat processing plant located in southern Japan and examined for Salmonella species. A total of 44 Salmonella isolates were recovered, and antimicrobial resistance was detected in serotypes Typhimurium (n=9), Infantis and Choleraesuis (n=2), and Derby, Miyazaki and Schwarzengrund (n=1). Multidrug resistance was seen in serotypes Typhimurium (n=8) and Infantis (n=2). The most commonly observed resistance phenotypes were against streptomycin, oxytetracycline and sulfamethoxazole (100%), ampicillin (90%), chloramphenicol (50%), cephalothin (30%) and cefoxitin, ceftazidime and kanamycin (each 20%). Extended-spectrum cephalosporin-resistant Salmonella Infantis isolates producing plasmid-mediated, bla(CMY-2) gene were detected. These AmpC-producing isolates showed resistance to ampicillin and cephems (cephalothin, cefoxitin and ceftazidime). Resistance transfer experiments showed that transconjugants and transformants coexpressed resistance phenotypes similar to the donor isolates. To the best of our knowledge, this is the first report worldwide describing serovar Infantis from pigs capable of producing AmpC β-lactamase. Then, we detected the pentadrug-resistance phenotype in Salmonella Typhimurium isolates, which yielded class 1 integron amplicons of 1.0 and 1.2 kb. Genetic fingerprinting analysis by pulsed-field gel electrophoresis and an assay by polymerase chain reaction confirmed the isolates to be Salmonella Typhimurium DT104. In conclusion, the findings of this survey call for the systematic and comprehensive domestic and international surveillance programs to determine the true rates of occurrence of AmpC-producing Salmonella both in the livestock and public health sectors.