Characterization of Multidrug-Resistant Escherichia coli Isolated from Two Commercial Lettuce and Spinach Supply Chains

Leafy green vegetables have increasingly been reported as a reservoir of multidrug-resistant pathogenic Enterobacteriaceae, with Shiga toxin-producing Escherichia coli frequently implicated in disease outbreaks worldwide. This study examined the presence and characteristics of antibiotic resistance, diarrheagenic virulence genes, and phylogenetic groupings of E. coli isolates (n = 51) from commercially produced lettuce and spinach from farms, through processing, and at the point of sale. Multidrug resistance was observed in 33 (64.7%) of the 51 E. coli isolates, with 35.7% (10 of 28) being generic and 100% (23 of 23) being extended-spectrum-beta-lactamase/AmpC producing. Resistance of E. coli isolates was observed against neomycin (51 of 51, 100%), ampicillin (36 of 51, 70.6%), amoxicillin (35 of 51, 68.6%), tetracycline (23 of 51, 45%), trimethoprimsulfamethoxazole (22 of 51, 43%), chloramphenicol (13 of 51, 25.5%), Augmentin (6 of 51, 11.8%), and gentamicin (4 of 51, 7.8%), with 100% (51 of 51) susceptibility to imipenem. Virulence gene eae was detected in two E. coil isolates from irrigation water sources only, whereas none of the other virulence genes for which we tested were detected. Most of the E. coli strains belonged to phylogenetic group B2 (25.5%; n = 13), B1 (19.6%; n = 10), and A (17.6%; n = 9), with D (5.9%; n = 3) less distributed. Although diarrheagenic E. coli was not detected, antibiotic resistance in E. coli prevalent in the supply chain was evident. In addition, a clear link between E. coli isolates from irrigation water sources and leafy green vegetables through DNA fingerprinting was established, indicating the potential transfer of E. coli from irrigation water to minimally processed leafy green vegetables.