Ten years of antimicrobial susceptibility testing of Salmonella from Danish pig farms

Objectives: This study analysed the trends in antimicrobial resistance in Salmonella serovars and phage types from pigs in Denmark from 1997 to 2006. Methods: Salmonella isolates collected through the Salmonella surveillance programme in pigs were serotyped and phage-typed, and susceptibilities to the following antimicrobials were determined: ampicillin, chloramphenicol, gentamicin, nalidixic acid, colistin, streptomycin, sulphonamide, tetracycline and trimethoprim. Results: No significant development of resistance occurred within the most important serovars, except Salmonella Typhimurium. A major decrease in Salmonella Typhimurium DT12 occurred from 46.5% in 1998 to 16.8% in 2006 while DT120, DT170 and DT104 increased. Throughout the study period, 80.9% of the DT12 isolates remained susceptible to the antimicrobials tested despite an increase in antimicrobial consumption in pigs during the period. In DT120, DT170 and DT104, only 20.1%, 33.1% and 23.0%, respectively, remained fully susceptible. Conclusions: The results support that the use of antimicrobial agents might select for multiple resistant clones and that this might be the driver of changes in antimicrobial resistance within a serovar, rather than an emergence of resistance within clones. The results of this study also support that susceptible serovars only slowly become resistant to the antimicrobials tested.