Purpose: To determine the seasonal prevalence rates and characteristics of STEC in irrigation waters and on fresh produce from three growing regions in British Columbia, Canada.
Methods: Water samples were collected monthly from irrigation ditches and streams in 20 sites located in three distinct watersheds (R, CA, LS) in British Columbia, Canada. Locally-grown leafy vegetables and berry fruits were obtained from seven farm markets during the summer months. A novel hydrophobic grid membrane filtration (HGMF) - Stx immunoblot method was used to detect and isolate STEC among the background microflora in samples of water and produce rinses tested without enrichment. Presumptive STEC isolates were tested for virulence genes (eaeA, hlyA, stx1 and stx2) by multiplex PCR and confirmed isolates were serotyped.
Results: STEC were not recovered from 105 samples of produce (79 vegetable, 26 berry). In contrast, STEC were isolated from 63 of 330 (19.1%) water samples. Prevalence rates varied with season, ranging from 34.2% to 15.7% and 13.3%, in winter, spring/summer and fall months, respectively. Isolates recovered from water included 9 (4.2%) of serotypes O157:H7 or O157:NM, 29 (13.6%) of non-O157 “priority” serogroups O26, O103 and O111, and 176 (82.2%) of 31 other serotypes. The 214 STEC isolates were randomly distributed among 11 distinct virulence gene patterns, providing evidence of considerable genotypic diversity in environmental STEC populations.
Significance: STEC were not recovered from fresh produce despite their frequent recovery in irrigation waters. Additional research is needed to clarify the role of biotic and abiotic factors on the transmission of STEC to field-grown produce.