Parallel evolution of multidrug-resistance in Salmonella enterica isolated from swine.

The increase in frequency of Salmonella enterica resistant to antibiotics in food-producing animals is of great concern to public health. Determining the rate at which different resistance phenotypes are generated and maintained in the environment is thus of great importance. The distribution and evolution of antibiotic resistance and multidrug-resistance in 362 Salmonella stains as part of a cross-sectional study of the Canadian swine industry were investigated. The susceptibility of all isolates to 12 antimicrobial agents was tested and the statistical and phylogenetic distribution of resistance among strains characterized via multilocus sequence typing was studied to test the origin of multidrug-resistance in Salmonella. More than 25% of all isolates were multidrug-resistant, with predominance in serotype Typhimurium, a serotype of vital importance to public health. The strong associations between resistance phenotypes, which differ among serotypes and which is supported by the significant genetic distance between serotypes, was indicative of the independent acquisition of multidrug-resistance in at least two different serotypes, i.e. Typhimurium and Derby. The independent origin of multidrug-resistance in Salmonella indicates that strong selective pressures are present in the environment of the bacteria and that statistical and phylogenetic studies of antibiotic resistance are an essential part in the understanding and the control of the epidemic.