Incidence and survival of non-O157 verocytotoxigenic Escherichia coli in soil.

This study estimated the incidence of non-O157 verocytotoxigenic Escherichia coli (VTEC) in farm pasture soils and investigated the survival of non-O157 VTEC in clay and sandy loam soils. Twenty farms were tested over a 12-month period by sample enrichment in tryptone soya broth plus vancomycin, followed by PCR screening for the presence of vt1 and vt2 genes. Of the 600 soil samples, 162 (27%), across all farms, were found to contain vt1 and/or vt2 genes. The enrichment cultures from the 162 PCR-positive samples were plated onto Chromocult tryptone bile X-glucuronide agar (TBX), presumptive VTEC colonies recovered, confirmed as VTEC by PCR and serotyped. Samples of the two predominant soil types in Ireland (clay and sandy) were homogenized, characterized in terms of pH, boron, cobalt, copper, potassium, magnesium, manganese, phosphorus, zinc and organic matter content, inoculated with washed suspensions of eight non-O157:H7 soil isolates and six bovine faecal isolates and stored at 10°C for up to 201 days. Inoculum survival rates were determined at regular intervals by recovering and plating soil samples on TBX. All inoculated non-O157 serotypes had highest D-values in the sandy loam soil with D-values ranging from 50·26 to 75·60 days. The corresponding range in clay loam soils was 31·60-48·25 days. This study shows that non-O157 VTEC occur widely and frequently in pasture soils and can persist in such environments for several months, with considerable opportunity for recycling through farm environments, and cattle, with clear potential for subsequent transmission into the human food chain. This is the first such study of non-O157 VTEC in farm soils and found that these VTEC are frequent and persistent contaminants in farm soils. In light of recent epidemiological data, non-O157 VTEC should be seen as an emerging risk to be controlled within the food chain.