Antibiotic resistance and genetic properties of R plasmids in Salmonella isolates of swine origin in Japan.

A total of 84 Salmonella isolates of ten serovars from swine in Japan were tested for antibiotic susceptibility and detection of R plasmids. Of these, 58 (69.0%) were resistant to one or more antibiotic(s) such as tetracycline, streptomycin, sulfadimethoxine, and chloramphenicol; and 38 (65.5%) of the 58 resistance isolates (mainly Salmonella typhimurium isolates) had conjugative R plasmids. Among the 38 conjugative R plasmids, 36 thermosensitive R plasmids carrying tetracycline resistance were Fi- (no fertility inhibition) and belonged to incompatibility group Hl, whereas the R plasmids carrying streptomycin and sulfadimethoxine resistances coexisted in the same host, were Fi+, and were also compatible with the reference R plasmids tested. The epidemiologic significance of R plasmids derived from Salmonella isolates from swine is discussed.