Prevalence of Escherichia coli O157:H7 in the American bison (Bison bison).

Bison is becoming a popular meat source for consumers, but very little is known about the bison's status with respect to Escherichia coli O157:H7. We conducted a study to determine the prevalence and identify virulence genes and pulsed-field genetic types of E. coli O157:H7 in bison. Rectal contents and rectoanal mucosal swab (RAMS) samples were collected from a total of 342 bison at slaughter on seven different dates. Isolation of E. coli O157:H7 was by enrichment, immunomagnetic separation, and plating on selective medium, and identification was based on sorbitol fermentation reaction, indole production, and O157 agglutination test. An overall E. coli O157:H7 prevalence of 47.4% was observed. Fecal prevalence across sampling days ranged from 17 to 83%, with an average of 42.1%. The prevalence in the rectoanal mucosal region ranged from 2.2 to 50%, with an average of 19.9%. All E. coli O157:H7 isolates (n = 212) possessed eae, hlyA, fliC, and stx2 genes. The antiterminator Q gene, Q933, was present in 50.7% of fecal and 38% of RAMS isolates, and Q21 was present in 52.1% of fecal and 61.5% of RAMS isolates. The pulsed-field gel electrophoresis analysis of isolates revealed 11 types (> 95% Dice similarity) and 19 subtypes (100% Dice similarity). Two pulsed-field genetic types accounted for 76.4% of total isolates. Our study suggests that the prevalence of E. coli O157:H7 in rectal contents or on rectal mucosa of bison is variable, but relatively high overall and bison could serve as an important reservoir for human infection.