Effects of diet on rumen proliferation and fecal shedding of Escherichia coli O157 : H7 in calves

Calves inoculated with Escherichia coli O157:H7 and fed either a high-rough age or high-concentrate diet were evaluated for rumen proliferation and fec al shedding of E. coli O157:H7. Calves fed the high-roughage diet had lower mean rumen volatile fatty acid concentrations and higher rumen pH values t han did calves fed the high-concentrate diet. Despite these differences in rumen conditions, the calves fed the high-roughage diet did not have greate r rumen populations of E. coli O157: H7 and did not exhibit increased or lo nger fecal shedding compared with the calves fed the high-concentrate diet. Two calves shedding the highest mean concentrations off. coli O157:H7 were both fed the high-concentrate diet. There was a significant (P < 0.05) pos itive correlation between fecal shedding and rumen volatile fatty acid conc entration in calves fed a high-concentrate diet. The effects of diet on E. coli O157:H7 proliferation and acid resistance were investigated using an i n vitro rumen fermentation system. Rumen fluid collected from steers fed a high-roughage diet, but not from steers fed a high-concentrate diet, suppor ted the proliferation of E. coli O157:H7. Rumen fluid from steers fed a hig h-concentrate diet rapidly induced acid resistance in E. coli O157:H7. The impact of diet on fecal shedding of E. coli O157:H7 is still unclear and ma y depend on dietary effects on fermentation in the colon and on diet-induce d changes in the resident microflora. However, rapid development of acid to lerance by E. coli O157:H7 in the rumens of calves fed high-concentrate die ts, allowing larger populations to survive passage through the acidic aboma sum to proliferate in the colon, may be one factor that influences fecal sh edding in cattle on feed.