ABSTRACT
Produce is increasingly recognized as a reservoir of human pathogens and transferable antibiotic resistance genes. This study aimed to explore methods to characterize the transferable resistome of bacteria associated with produce. Mixed salad, arugula, and cilantro purchased from supermarkets were analyzed by means of cultivation- and DNA-based methods. Before and after a nonselective enrichment step, tetracycline (tet) resistant Escherichia coli were isolated and plasmids conferring tet resistance were captured by exogenous plasmid isolation. Tet resistant E. coli isolates, transconjugants and total community (TC)-DNA from the microbial fraction detached from leaves or after enrichment were analyzed for the presence of resistance genes, class 1 integrons and various plasmids by real-time PCR and PCR-Southern blot hybridization. Real-time PCR primers were developed for IncI and IncF plasmids. Tet resistant E. coli isolated from arugula and cilantro carried IncF, IncI1, IncN, IncH11, IncU and IncX1 plasmids. Three isolates from cilantro were positive for IncN plasmids and blaCTX-M-1. From mixed salad and cilantro, IncF, Inc11, and IncP-1β plasmids were captured exogenously. Importantly, whereas direct detection of IncI and IncF plasmids in TC-DNA failed, these plasmids became detectable in DNA extracted from enrichment cultures. This confirms that cultivation-independent DNA-based methods are not always sufficiently sensitive to detect the transferable resistome in the rare microbiome. In summary, this study showed that an impressive diversity of self-transmissible multiple resistance plasmids was detected in bacteria associated with produce that is consumed raw, and exogenous capturing into E. coli suggests that they could transfer to gut bacteria as well.
