Epidemiological Characteristics of Salmonella enterica Serovar Typhimurium from Healthy Pigs in Japan

We characterized 53 Salmonella enterica ser. Typhimurium strains recovered from healthy pigs during 1998-1999 (n=12) and 2004-2005 (n=41) as to their carriage of DT104 spacer region, class 1 and 2 integrons, virulence genes (spvC, rck, and pefA), and XbaI- and BlnI-Pulsed-Field Gel Electrophoresis (PFGE) profiles. No DT104 strain was detected in 1998-1999, whereas 65.9% (27/41) of the strains in 2004-2005 were DT104 showing resistance to ampicillin, chloramphenicol, streptomycin, sulfonamides, tetracycline, and cephaloridine (R-type ACSSuT+). Class 1 intergron-associated genes, aadA2 (1.0-kbp amplicon) and pse1 (1.2-kbp amplicon), were found in all the DT104 strains (27/27). One strain showing resistance to streptomycin, sulfonamides, tetracyclin, and trimethoprim (R-type SsuTTm) harbored another class 1 integron-associated gene (dhfrXII-orfF- aadA2) on 1.9-kbp amplicon. Virulence gene spvC was found in 92.5% (49/53) and rck and pefA were found in 88.7% (47/53) of the strains, whereas spvC, rck, and pefA were found in all the DT104 strains. Ser. Typhimurium strains were categorized into four clusters (X1, X2, X3, and X4a/X4b) by XbaI-PFGE, or into nine clusters (B1, B2, B3a/B3b, B4, B5, B6, B7, B8, B9a/B9b) by BlnI-PFGE analyses. DT104 strains were restricted into X2, or into B2, B3a/B3b, and B6 clusters, indicating that our multidrug-resistant DT104 strains from healthy pigs might have derived from at least three independent clones, with the most widespread clone being the cluster B6 strains isolated in Kanto, Tokai, Chugoku, and Kyushu regions.