Antimicrobial Drug Resistance of Fecal Escherichia coli and Salmonella spp. Isolates from United States Dairy Cows

Authors: LUNDIN, JESSICA - USDA-APHIS; DARGATZ, DAVID - USDA-APHIS; WAGNER, BRUCE - USDA-APHIS; LOMBARD, JASON - USDA-APHIS; HILL, ASHLEY - USDA-APHIS; Ladely, Scott; Cray, Paula

Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2007
Publication Date: 2/1/2008
Citation: Lundin, J., Dargatz, D., Wagner, B., Lombard, J., Hill, A., Ladely, S.R., Cray, P.J. 2008. Antimicrobial Drug Resistance of Fecal Escherichia coli and Salmonella spp. Isolates from United States Dairy Cows. Foodborne Pathogens and Disease. 5(1):7-19.

Interpretive Summary: Salmonella is a zoonotic pathogen which can be transferred from animals to humans, most often through consumption of contaminated food. Infection with Salmonella can cause mild to severe gastroenteritis in humans while infection in food animals is often with clinical signs of disease. Antimicrobial resistance is a growing concern for public and animal health. Threats to public health could come from the transfer of pathogens from animals to people via indirect contact such as through food or by direct contact. In addition, concern has been raised for the potential transfer of resistance determinants from animals to humans through commensal bacterial flora such as Escherichia coli. Isolates of E. coli and Salmonella spp. from dairy cows on farms in 21 states were evaluated for resistance to a panel of 16 antimicrobial drugs. Resistance patterns for E. coli were compared to those of Salmonella spp. when they were isolated concurrently on the same farm or from the same fecal sample. Overall, most of the E. coli isolates (85.3%) and Salmonella spp. isolates (87.2%) were susceptible to all antimicrobials in the panel. The resistance profile for E. coli not isolated concurrently with Salmonella spp. was not different from those of E. coli that were isolated at the same time as Salmonella spp. with the exception of Tetracycline resistance which was more common among the E. coli isolated with Salmonella spp. The resistance patterns for E. coli and Salmonella spp. isolated concurrently were not different for any of the antimicrobials evaluated. The data from this study suggest that the majority of commensal E. coli and Salmonella spp. recovered from feces of dairy cows demonstrate no resistance to a broad range of antimicrobial drugs. Resistance patterns in commensal E. coli are similar to patterns observed for Salmonella spp. when isolates are recovered concurrently. However, resistance patterns for commensal E. coli were also similar whether they were found with or without Salmonella spp. These data are necessary to enable a more informed debate among scientists, commodity groups, government regulators, and animal industry personnel on the feasibility and likely efficacy of enforcing a zero tolerance policy for Salmonella and as a means to reduce the incidence of human salmonellosis.

Technical Abstract: Antimicrobial resistance is a growing concern for public and animal health. Threats to public health could come from the transfer of pathogens from animals to people via indirect contact such as through food or by direct contact. In addition, concern has been raised for the potential transfer of resistance determinants from animals to humans through commensal bacterial flora such as Escherichia coli. Isolates of Escherichia coli and Salmonella spp. from dairy cows on farms in 21 states were evaluated for resistance to a panel of 16 antimicrobial drugs. Resistance patterns for E. coli were compared to those of Salmonella spp. when they were isolated concurrently on the same farm or from the same fecal sample. Overall, most of the E. coli isolates (85.3%) and Salmonella spp. isolates (87.2%) were susceptible to all antimicrobials in the panel. The resistance profile for E. coli not isolated concurrently with Salmonella spp. was not different from those of E. coli that were isolated at the same time as Salmonella spp. with the exception of Tetracycline resistance which was more common among the E. coli isolated with Salmonella spp. The resistance patterns for E. coli and Salmonella spp. isolated concurrently were not different for any of the antimicrobials evaluated. The data from this study suggest that the majority of commensal E. coli and Salmonella spp. recovered from feces of dairy cows demonstrate no resistance to a broad range of antimicrobial drugs. Resistance patterns in commensal E. coli are similar to patterns observed for Salmonella spp. when isolates are recovered concurrently. However, resistance patterns for commensal E. coli were also similar whether they were found with or without Salmonella spp. Further studies are indicated to better understand the factors that influence the frequency of resistance in commensal E. coli and Salmonella spp. on dairy operations.