Variation in clonality and antibiotic-resistance genes among multiresistant Salmonella enterica serotype typhimurium phage-type U302 (MR U302) from humans, animals, and foods

Since 1990 multiresistant (MR) Salmonella enterica serotype Typhimurium def initive phage-type (DT) 104 (MR DT104) and closely related phage types have emerged as a worldwide health problem in humans and food animals. In this study the presence of the bla(CARB-2) (ampicillin), cmlA (chloramphenicol), aadA2 (streptomycin/spectinomycin), sul1 (sulphonamide), and tetG (tetracy cline) resistance genes in isolates of one such phage type, U302, have been determined. In addition bla(TEM) I primers have been used for the detectio n of TEM-type beta-lactamases. Isolates have also been characterized by pla smid profile and pulsed field gel electrophoresis (PFGE). Thirty-three of 3 9 isolates were positive for blaCARB-2, cmlA, aadA2, sul1 and tetG, four fo r bla(TEM), aadA2 and sul1, one for aadA2 and sul1, and one for blaTEM only . bla(TEM)-mediated ampicillin resistance was transferred to Escherichia co li K12 from three isolates along with other resistance markers, including r esistance to chloramphenicol, streptomycin, spectinomycin, sulphonamides, a nd tetracyclines. Strains carried up to 6 plasmids and 34 plasmid profiles were identified. Although the majority of strains (33/39) produced a PFGE p rofile identical to that predominant in MR DT104, six different patterns we re generated demonstrating the presence of various clones within MR U302. T he results show that the majority of the MR U302 strains studied possessed the same antibiotic resistance genes as MR DT104. However, isolates with di stinctive PFGE patterns can have different mechanisms of resistance to ampi cillin, chloramphenicol, streptomycin, sulphonamides, and tetracyclines. Su ch resistance genes may be borne on transmissible plasmids.