Fecal shedding and rumen growth of Escherichia coli O157 : H7 in fasted calves

Nine weaned calves aged from 8 to 12 weeks were fitted with rumen cannulas and were inoculated by cannula with 10(10) CFU of a five-strain mixture of nalidixic acid-resistant Escherichia coil O157:H7. Six calves were fasted f or 48 h on days 15 and 16 and days 22 and 23 after inoculation. Samples of rumen contents and feces were obtained daily to enumerate E. coli O157:H7 p opulations and to determine rumen volatile fatty acid (VFA) concentrations and rumen pH. Fasting resulted in a marked decrease in rumen VFA concentrat ions from a mean of 135 mmol/liter before the fast to a mean of 35 mmol/lit er during the second day of the fast. However, there was no correlation bet ween daily VFA concentration and daily rumen or fecal numbers of E. coil O1 57:H7 in any of the calves. Fasting generally had no significant effect on the rumen or fecal numbers of E. coli O157:H7. The exception was a single f asted calf that experienced a 3-log(10) CFU/g increase in fecal shedding du ring and after the first fast. Despite the consistent changes in VFA concen trations in fasted calves, the fluctuations in rumen numbers of E. coil O15 7:H7 in the rumen of fasted calves were minimal. At the end of the experime nt, E. coli O157:H7 was detected in either the rumen or omasum in two of th ree control calves at necropsy and in either the rumen or reticulum in five of six fasted calves. E. coil O157:H7 was detected in the colon in two of three control calves and in six of six fasted calves at necropsy. These res ults suggest that in cattle already shedding E. coli 0157:H7, feed withdraw al and the associated changes in rumen pH and VFA concentrations have littl e effect on fecal shedding and rumen proliferation of E. coil O157:H7.