Prevalence and Antibiotic Resistance Pattern of Salmonella Serovars in Integrated Crop-Livestock Farms and Their Products Sold in Local Markets

Introduction: The mixed crop-livestock (MCL) and backyard farms are major contributors to organic food production, however foods from integrated MCL and backyard farms, including chicken, egg, and fresh produce, are potentially in greater risk with cross-contamination of enteric bacterial pathogens as they are grown in close proximity. Salmonella enterica, the most common foodborne pathogen which infects humans and a wide range of animal hosts, potentially contaminates these products.

Purpose: The purpose of this study was to investigate pre- and post-harvest levels Salmonella prevalence, serovar prevalence, and their antimicrobial resistance patterns in the MCL farming system environments and its products available in retail stores.

Methods: A total of 1,287 samples from conventional poultry farms/organic MCL farms and 1,377 samples from organic/conventional retail supermarkets or farmers markets were collected from Maryland and the DC metropolitan area. Salmonella was identified with biochemical tests and PCR assay, and the serovars was determined by molecular subtyping multiplex PCR. Antibiotic resistance of the isolates was determined with agar dilution method.

Results: Totally, 315 Salmonella isolates were recovered, with 17.44% and 5.88% (n=315), from MCL and conventional farms samples (P<0.001). At post-harvest level, the prevalence of Salmonella was 30.95%, 19.83%, and 8.38% in chicken meat (P<0.001) from farmers, organic, and conventional markets, respectively, and 16.81% and 6.06% in produce products (P<0.001) from farmers and organic markets, but none from conventional markets. From the isolated Salmonella, 34.50% was confirmed Salmonella Typhimurium, followed by Salmonella Heidelberg (10.86%) and Salmonella Enteritidis (9.90%). The overall multi-antibiotic resistance in recovered Salmonella was 23.81% versus 4.55% in conventional and MCL farms (P=0.004) and 66.67% versus 7.76% in conventional and farmers markets (P<0.001).

Significance: This study demonstrates the potential food safety risks associated with MCL systems in terms of Salmonella especially Salmonella Typhimurium contamination on produce products and poultry meat in farmers markets.