Characterization of resistance genes in multidrug-resistant Salmonella enterica serotype Typhimurium isolated from diseased cattle in France (2002 to 2007).

We report here the results of the survey of antimicrobial resistance in 148 serotype Typhimurium strains isolated from cattle in France from 2002 to 2007 and displaying more than two antimicrobial resistances. Salmonella enterica serotype Typhimurium of definitive phage type 104 strains that are commonly resistant to ampicillin-amoxicillin, chloramphenicol-florfenicol, streptomycin-spectinomycin, sulfonamides, and tetracycline (ACSSuT phenotype) harbored resistance genes clustered on a complex class 1 integron In104 of the Salmonella genomic island 1 (SGI1). In our isolates, the most common antimicrobial resistance pattern was ACSSuT (77.7%) or ACSSuT combined to additional resistances. SGI1 was detected in 143 strains and constituted thus the main structure involved in resistance to antimicrobials in these strains. In spite of the high recombination potential of In104, SGI1 variability was quite limited among these strains since only two SGI1 variants, SGI1-B and SGI1-C, were identified. One hundred and thirty-eight out of the 143 SGI1-positive isolates belonged to the DT104 complex. Pulsed-field gel electrophoresis profile A was the most prevalent in 135 SGI1-positive isolates, confirming the diffusion of the DT104 clone. However, changes in phages susceptibility have occurred in three serotype Typhimurium strains of phage type DT12, as they displayed the same pulsed-field gel electrophoresis profile as the SGI1-positive serotype Typhimurium DT104. No variant harboring an additional resistance gene was identified, but the risk of recombination between SGI1 and any other mobile structure carrying other antimicrobial resistance genes is still an issue in serotype Typhimurium.