Lower prevalence of antibiotic-resistant Salmonella on large-scale U.S. conventional poultry farms that transitioned to organic practices.

As a result of the widespread use of antibiotics in large-scale U.S. poultry production, a significant proportion of Salmonella strains recovered from conventional poultry farms and retail poultry products express antibiotic resistance. We evaluated whether large-scale poultry farms that transitioned from conventional to organic practices and discontinued antibiotic use were characterized by differences in the prevalence of antibiotic-resistant Salmonella compared to farms that maintained conventional practices. We collected poultry litter, water and feed samples from 10 newly organic and 10 conventional poultry houses. Samples were analyzed for Salmonella using standard enrichment methods. Isolates were confirmed using standard biochemical tests and the Vitek®2 Compact System. Antimicrobial susceptibility testing was performed by Sensititre® microbroth dilution. Data were analyzed using Fisher's exact test and generalized linear mixed models. We detected Salmonella in both conventional and newly organic poultry houses. Salmonella Kentucky was the predominant serovar identified, followed by S. Orion, S. Enteritidis, S. Gostrup and S. Infantis. Among S. Kentucky isolates (n=41), percent resistance was statistically significantly lower among isolates recovered from newly organic versus conventional poultry houses for: amoxicillin-clavulanate (p=0.049), ampicillin (p=0.042), cefoxitin (p=0.042), ceftiofur (p=0.043) and ceftriaxone (p=0.042). Percent multidrug resistance (resistance to ≥3 antimicrobial classes) was also statistically significantly lower among S. Kentucky isolates recovered from newly organic poultry houses (6%) compared to those recovered from conventional houses (44%) (p=0.015). To our knowledge, these are the first U.S. data to show immediate, on-farm changes in the prevalence of antibiotic-resistant Salmonella when antibiotics are voluntarily withdrawn from large-scale poultry facilities in the United States.