Prevalence and Antimicrobial Resistance Patterns of Major Foodborne Bacterial Pathogens in Mixed Crop-animal Farms and Its Products in Retail Stores and Farmers Markets

Introduction: Salmonella, Campylobacter jejuni and enterohemorrhagic Escherichia coli (EHEC) are the major causative bacterial agents of foodborne illness. Increasing numbers of outbreaks with these bacterial pathogens have been attributed to vegetables specifically organic products. Mixed crop-animal/back yard farms, possessing higher biosecurity and biosafety risks, are one of the major producers of organic products, supplying farmers market as well as organic stores. It is essential to understand possible ecological sources of contamination at the production level and the translation of contamination at the consumption level.

Purpose: To investigate the prevalence and antimicrobial susceptibility of Salmonella, C. jejuni and EHEC in pre-harvest and post-harvest products of mixed crop-animal farms.

Methods: Samples (>350) were collected from organic farms, farmers markets, organic and conventional supermarkets located either in Washington D.C. or Maryland. Portions of each sample were diluted 1:9 with buffered peptone water. After enrichment in broth containing 5% sheep blood, samples were streaked on agar; presumptive colonies were confirmed biochemically and PCR analysis. Antimicrobial susceptibility testing was performed using the agar dilution method. The data was analyzed using the χ² test.

Results: On organic farms surveyed, there was a significant (P = 0.01) difference in the occurrence of Salmonella and Campylobacter between environmental, animal feces, animal feed and vegetable samples, with prevalence of 35%, 70%, 22% and 20% for Salmonella and 15%, 36%, 12% and 16%, respectively. These numbers are significantly higher than conventional farms. There was no significant difference in the rates of antimicrobial resistance of Salmonella and C. jejuni recovered from those environments as well as organic and conventional products in markets and farms.

Significance: The study shows contamination at the farm level is significantly higher in mixed farms. These data also suggest that there is no difference in microbiological quality of organic products in markets compared to conventional products.