Dynamics of Shiga toxin-producing Escherichia coli (STEC) in feedlot cattle

Feedlot cattle were monitored during fattening to determine changes in faec al shedding of Shiga toxin-producing Escherichia coli (STEC) and their rela tion to the coliform population. Faecal samples were enriched, screened for Shiga toxin genes (stx) by a polymerase chain reaction test and isolated u sing colony hybridization. During 117 d in the feedlot, there were differen ces in the numbers of coliforms shed and in the percentage of samples posit ive for stx. These fluctuations did not appear to be consistently related t o changes in feed or time in the feedlot. The mean log coliform count for s tx-positive samples (log 5.85 g(-1)) was similar to that for stx-negative s amples (log 6.00 g(-1)). The STEC isolates obtained from the first 5 d in t he feedlot belonged to eight serotypes. Later, one serotype (O136:H16) beca me the predominant STEC which appeared to be one clone as characterized by virulence determinants and pulsed-field gel electrophoresis.