Characterization of Shiga toxin-producing Escherichia coli isolated from feces of cattle in commercial feedlots
Shiga toxin-producing Escherichia coli (STEC) are potential food and waterborne zoonotic pathogens. The objective of this study was to characterize the general population of E. coli (EC) in feedlot cattle and determine the proportion that is STEC. Four commercial feedlots with approximately 1,000 pens were sampled over a three-year period. Thirty percent of the pens were randomly selected for study enrollment. Pens were sampled when they had been filled with cattle and again after cattle had been on feed for > 60 days. Sampling consisted of collecting 1-2 g of material from 20 fresh fecal pats on pen floors and combining them into a single mixed sample per pen. In total, 291 pens were sampled and processed for EC isolation at both time points. A total of 3,578 EC were isolated and stored after plating pooled fecal pats onto MacConkey agar. Isolates of EC were then screened by multiplex real-time PCR for the virulence genes stx1, stx2, and eae. Isolates positive for stx1 or stx2 were further characterized for i) variants of stx using PCR, ii) susceptibility to 15 antimicrobials using broth dilution, and iii) genetic relatedness after pulsed-field gel electrophoresis (PFGE) of Xba1 restricted DNA. In total, 60 (1.7%) isolates tested positive for stx1 (N = 27), stx2 (N = 30) or a combination of stx1 + stx2 (N = 3) genes. The most prevalent stx variant was stx1d, followed by stx2a, stx2g, stx1a, stx1a + stx2a, and stx2b (N = 18, 15, 8, 7, 3, and 2, respectively). Undefined stx1 and stx2 variants were present in 2 and 5 isolates, respectively. Seasonality and time of isolation did not affect prevalence of total stx1 or stx2 (P > 0.05). However the majority of isolates with stx1d (15/18) and stx2g (7/8) were detected at > 60 days. Only 10 STEC (0.3%) also tested positive for eae. Sampling time point did not affect the prevalence of antimicrobial-resistant STEC (P > 0.05). Overall, 69.8% of STEC were resistant to at least one antimicrobial. The most prevalent resistance was to tetracycline which was common to all resistant STEC. From PFGE, 46 subtypes were observed. These data indicate STEC from feedlot cattle are diverse and their prevalence is low among the general population of EC. In addition, the majority of STEC were eae-negative, thus unlikely to be associated with outbreaks of hemolytic uremic syndrome.
Keywords: STEC, Escherichia coli, cattle