Effect of diet on the concentration of complex Shiga toxin-producing Escherichia coli and EHEC virulence genes in bovine faeces, hide and carcass

An experiment was conducted to determine whether diets based on structural carbohydrate and/or simple sugars, as found in roughage and/or molasses-based diets, reduce the bovine faecal populations of Shiga toxin-producing Escherichia coli (STEC) isolates containing the eaeA and ehxA genes, referred to as complex STEC (cSTEC), compared with typical high starch, grain-based feedlot diets. In addition, whether commercial lairage management practices promote or diminish any diet-induced responses on the contamination of carcasses was also investigated. After 13 days on the dietary treatments total faecal E. coli numbers were approximately one log lower in the roughage (R) and roughage + 50% molasses (RM) diets compared with grain (G) fed animals, this difference varying between 0.5 and 1 log at lairage. Fermentation patterns were similar in the R and RM diets whereas decreased pH and enhanced butyrate fermentation pathways were associated with the G diet. A significant decrease in the faecal concentration of the eaeA gene occurred when animals were changed from high grain to R and RM diets for 6-13 days, compared with animals maintained on the G diet. Significantly lower concentrations of the ehxA gene were also associated with the R diet. Concentrations of the stx₂ gene however, were unaffected by diet. cSTEC were infrequently isolated, with the faecal concentrations of these organisms being low (< 3 log₁₀ MPN per g faeces). cSTEC were only isolated from animals fed G or RM diets, but were never isolated from cattle fed the roughage-based diet, with this diet-induced effect sustained following lairage. These organisms were not detected on the hide and carcass of animals found to shed cSTEC in their faeces and thus appeared uncontaminated with cSTEC. Crown