Gentamicin-resistant Salmonella typhimurium phage type 204c: molecular studies and strain diversity in a putative bovine outbreak.

As part of the investigation of a putative bovine outbreak, 13 isolates of Salmonella typhimurium phage type 204c were subjected to plasmid analysis. Plasmid profiles suggested that several distinct strains were involved and these observations were supported by minor variations in antibiotic resistance pattern. Restriction enzyme fingerprinting and conjugational segregation of the plasmids confirmed these findings. Although 12 of the 13 isolates were resistant to gentamicin, resistance was conferred by 4 distinct plasmids; 3 of these belonged to Inc I and were distantly related on the basis of restriction fingerprints and the fourth was a resistant derivative of the 60 MDa S. typhimurium serotype-specific plasmid. The molecular evidence refuted the hypothesis that geographical and temporal clustering of these gentamicin-resistant isolates could be explained on the basis of a single epidmiological episode.