Antimicrobial susceptibility phenotypes, resistance determinants and DNA fingerprints of Salmonella enterica serotype Typhimurium isolated from bovine in Southern Japan

Abstract

A longitudinal study was conducted in cattle to determine the antimicrobial resistance phenotypes, integron elements, resistance genes and pulsed-field gel electrophoresis fingerprints among Salmonella enterica serotype Typhimurium isolates. A total of 33 strains were isolated and categorised into Groups A, B and C during the period 1989–2004. Thirty-one strains (93.9%) showed resistance to ampicillin (A) encoded by bla OXA-1, bla TEM and bla PSE-1 genes; 84.8% showed resistance to chloramphenicol (C) encoded by floR and catA1; 97.0% were resistant both to streptomycin (S) and sulfamethoxazole (Su), the former encoded by aadA1 and aadA2; 100% were resistant to oxytetracycline (T) encoded by tetA, tetB and tetG; and 42.4% were resistant to kanamycin (Km) encoded by aphA1-Iab. Multidrug resistance types observed were ACSSuT-Km ( n = 13), ACSSuT ( n = 15), ASSuT ( n = 3) and SSuT ( n = 2). Class 1 integrons ranging from 1.0 kb to 1.9 kb were detected from 54.5% of isolates (18/33). Integrons were not detected initially (1989–1992), then during the 1993–1996 interval a high frequency of 1.0 kb and 1.2 kb amplicons were detected and during 2000–2004 the amplicon size increased to 1.7 kb and 1.9 kb. We report evidence of additional integration of resistance gene cassettes as shown by integrons with increased size. Finally, group B strains showed banding patterns indistinguishable from S. Typhimurium DT104 reference strain, indicating that the DT104 lineage existed on the island since 1993.