Prevalence of verotoxigenic Escherichia coli in irrigation waters and fresh produce in British Columbia, Canada

Irrigation water is a risk factor in the transmission of verotoxigenic Escherichia coli (VTEC) to growing food plants. To date, there have been no systematic attempts to examine VTEC occurrence in surface waters used to irrigate crops in British Columbia. The objectives of this study were to determine the seasonal prevalence and characteristics of VTEC in surface waters and fresh produce grown in the Lower Mainland of British Columbia. Water samples (n=330) were collected from irrigation ditches and streams in four distinct watersheds between November 2012 – November 2013. Leafy vegetables (n=79) and berry fruits (n=26) were obtained from seven local farm markets during the summer. Detection and isolation of VTEC in water and produce rinses were performed using a novel hydrophobic grid membrane filtration-verotoxin immunoblot method. Prospective VTEC isolates were characterized by multiplex PCR to determine virulence gene profiles (eaeA, hlyA, vt₁ and vt₂), serotyping, antibiotic resistance profiling and genotyping by BOX-PCR. Fecal coliform and E. coli counts were obtained on Petrifilm™. VTEC were not recovered from 105 samples of produce but were found in 19.1% of surface water samples. Seasonal prevalence rates were significantly different (P < 0.05), ranging from 34.2% to 15.7% and 13.3%, in winter, spring/summer and fall months, respectively. BOX-PCR revealed that 100 of 528 VTEC isolates were unique, including 3 of serotypes O157:H7/O157:NM, 16 of non-O157 “priority” serogroups O26, O103 and O111, and 81 from 32 additional serotypes with 10 distinct virulence gene profiles. Multiple antibiotic resistance was observed in 16.3% of isolates (n=104), and resistance to florfenicol (27.9%), streptomycin (13.5%), tetracycline (12.5%) and ampicillin (9.6%) was common. Fecal coliform and E. coli counts did not correlate with VTEC occurrence, and isolates were recovered from 21.7% of water samples with no E. coli detected. In conclusion, VTEC were not recovered from fresh produce despite their recurrence in surface waters. The diversity of VTEC recovered from surface waters suggests the need for further research to identify sources of contamination and factors leading to persistence. Water quality methods based on the presence of indicator species may not reliably predict the risk of contamination with VTEC.Land and Food Systems, Faculty ofGraduat