Origin of contamination and genetic diversity of Escherichia coli in beef cattle

Abstract

The possible origin of beef contamination and genetic diversity of Escherichia coli populations in beef cattle, on carcasses and ground beef, was examined by using random amplification of polymorphic DNA (RAPD) and PCR-restriction fragment length polymorphism (PCR-RFLP) analysis of the fliC gene. E. coli was recovered from the feces of 10 beef cattle during pasture grazing and feedlot finishing and from hides, carcasses, and ground beef after slaughter. The 1,403 E. coli isolates (855 fecal, 320 hide, 153 carcass, and 75 ground beef) were grouped into 121 genetic subtypes by using the RAPD method. Some of the genetic subtypes in cattle feces were also recovered from hides, prechilled carcasses, chilled carcasses, and ground beef. E. coli genetic subtypes were shared among cattle at all sample times, but a number of transient types were unique to individual animals. The genetic diversity of the E. coli population changed over time within individual animals grazing on pasture and in the feedlot. Isolates from one animal (59 fecal, 30 hide, 19 carcass, and 12 ground beef) were characterized by the PCR-RFLP analysis of the fliC gene and were grouped into eight genotypes. There was good agreement between the results obtained with the RAPD and PCR-RFLP techniques. In conclusion, the E. coli contaminating meat can originate from cattle feces, and the E. coli population in beef cattle was highly diverse. Also, genetic subtypes can be shared among animals or can be unique to an animal, and they are constantly changing.

FIG. 1.

Mean and standard error of the numbers of E. coli isolated at each stage of beef production. E. coli log₁₀ counts are per gram of feces in the pasture and feedlot, per 1,000 cm² on hides and carcasses, and per 200 g in ground beef. Data are for 10 animals. P, pasture; F, feedlot; H, hide; PC, prechilled carcasses; CC, chilled carcasses; GB, ground beef.

FIG. 2.

Ethidium bromide-stained agarose gel showing RAPD E. coli type 9 (≥80% similarity). Lanes: P, pasture; F, feedlot; H, hide; PC, prechilled carcasses; CC, chilled carcasses; GB, ground beef; M, DNA size marker in kilobases.