Changes in Gene Transcription Induced by Hydrogen Peroxide Treatment of Verotoxin-Producing Escherichia coli O157:H7 and Non-O157 Serotypes on Romaine Lettuce

Disease outbreaks of verotoxin-producing Escherichia coli (VTEC) O157:H7 and non-O157 serotypes associated with leafy green vegetables are becoming a growing concern. A better understanding of the behaviour of VTEC, particularly non-O157 serotypes, on lettuce under stress conditions is necessary for designing more effective control strategies. Hydrogen peroxide can be used as a sanitizer to reduce the microbial load in leafy green vegetables, particularly in fresh produce destined for organic market. In this study, we tested the hypothesis that hydrogen peroxide treatment of contaminated lettuce affects in the same manner transcription of stress-associated and virulence genes in VTEC strains representing O157 and non-O157 serotypes. Six VTEC isolates representing serotypes O26:H11, O103:H2, O104:H4, O111:NM, and O145:NM and O157:H7 were included in this study. The results indicate that 50 mM H2O2 caused a population reduction of 2.4 - 2.8 log10 (compared to non-treated control samples) in all six VTEC strains present on Romaine lettuce. Following the treatment, the transcription of genes related to oxidative stress (oxyR and sodA), general stress (uspA and rpoS), starvation (phoA), acid stress (gadA, gadB and gadW), and virulence (stx1A, stx2A and fliC) were dramatically downregulated in all six VTEC serotypes (P≤0.05) compared to not treated control samples. Therefore, VTEC O157:H7 and non-O157 serotypes on lettuce showed similar survival rates and gene transcription profiles in response to 50 mM H2O2 treatment. Thus, the results derived from this study provide a basic understanding of the influence of H2O2 treatment on the survival and virulence of VTEC O157:H7 and non-O157 serotypes on lettuce.