Antibiotic Resistance Patterns of Major Zoonotic Pathogens from All-Natural, Antibiotic-Free, Pasture-Raised Broiler Flocks in the Southeastern United States.

The use of antibiotics in agroecosystems has been implicated in the rise in antibiotic resistance (AR), which can affect environmental, animal, and human health. To determine the environmental impact of antibiotic use in agroecosystems, appropriate background levels of AR in agricultural environments in the absence of antibiotic application must be determined. Therefore, to determine background levels of AR in broiler production, four target microbes (Escherichia coli, Salmonella, Campylobacter, and Listeria) were isolated from 15 all-natural, antibiotic-free, pastureraised broiler flocks from six farms within the southeastern United States. The AR profiles of these isolates were characterized using the CDC National Antimicrobial Resistance Monitoring System for Enteric Bacteria (NARMS), and these resistance patterns were compared across target microbes and farms and throughout the life cycle of the flocks along the farm-to-fork continuum. Antibiotic resistances were most prevalent in Listeria and Salmonella and least prevalent in Campylobacter. Although E. coli and Salmonella were isolated from the same farms and characterized using the same NARMS plates, they exhibited distinct AR profiles, with Salmonella demonstrating clear farmspecific resistance patterns. Multidrug resistance rates (three or more antibiotics), in order of prevalence, were Listeria (63.9%), Salmonella (36.0%), E. coli (12.7%), and Campylobacter (1.4%). The results of this study demonstrate the variability in background AR among major food safety-related microbes, even when isolated from similar production and processing samples from the same farms, and indicate the need for the proper design of future broiler production studies to account for this highly dynamic background AR. [ABSTRACT FROM AUTHOR]