Prevalence of beta-lactam drug-resistance genes in commensal Escherichia coli contaminating ready-to-eat lettuce

The objective of this study was to evaluate the prevalence of antibiotic resistance and beta-lactamdrug resistance genes in Escherichia coli isolated from ready-to-eat lettuce, obtained from local supermarkets in Northern California. Bags of lettuce were purchased from 4 chain supermarkets during three different periods--Oct 2018-Jan 2019, Feb 2019-Apr 2019 and May 2019-July 2019. From 91 packages of lettuce, we recovered 34 E. coli isolates from 22 (24%) lettuce samples. All E. coli isolates were genotyped by multilocus sequence typing(MLST), and we found 15 distinct sequence types (STs). Five of these genotypes (ST2819, ST4600, ST2432, ST1198 and ST5143) have been reported to cause infection in humans. Twenty (59%) E. coli isolates were found resistant to at least one of the antibacterial drugs. They included resistance to ampicillin(AMP, 85%) and ampicillin/ sulbactam(SAM, 50%), cefoxitin(FOX, 40%) and cefuroxime(CXM, 35%). We found 8 (40%) of 20 beta-lactamresistant E. coli isolates to carry blaCTX-M; 5 (25%) tested positive for blaSHV, while only 4 (20%) tested positive for blaTEM. Additionally, we identified a class A broad-spectrum beta-lactamaseSED-1 gene, blaSED, reported by others in Citrobacter sedlakiiisolated from bile of a patient. This study found that a large proportion of fresh lettuce carry beta-lactamdrug-resistant E. coli, which could serve as a reservoir for drug resistance genes that could potentially enter pathogens to cause human infections.