Changes in the antimicrobial resistance profiles of Salmonella isolated from the same Michigan dairy farms in 2000 and 2009

This study compared the antimicrobial resistance profiles of Salmonella collected from the same Michigan, USA dairy farms between the years 2000 and 2009. The specific objective was to understand the type and distribution of changes in antimicrobial resistance that occurred within farms over the past 10 years. Multinomial, multilevel models were constructed to estimate the differences in minimum inhibitory concentrations (MICs) between years. The MICs of most antimicrobials were significantly lower in 2009 than in 2000, but were higher for amikacin and gentamicin. Decreases in MICs were in part due to changes in the prevalence of multidrug resistant strains, but were also distributed across the susceptible population of isolates. The type and direction of within-farm changes in MICs were similar for the majority of farms. These results suggest a decrease in antimicrobial resistance and/or a change in the population structure of Salmonella that colonize dairy farms in Michigan.