Genetic Diversity and Antimicrobial Resistance among Isolates of E.coli O157:H7 in Two Commercial Beef Feedlots

In this Canadian study, only one animal was a 'super-shedder' of E.coli O157:H7 at both sampling periods, six weeks apart. The numbers and types of E.coli O157:H7 isolated from the cattle varied between the sampling periods and depended on the sample (faeces or hide) but transmission of the bacteria from super-shedders to low-shedder pen mates appeared to be limited. Antimicrobial resistance among the isolates was generally low.
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Cattle shedding at least 104 colony-forming units (CFU) of Escherichia coli O157:H7 per gramme of faeces are described as 'super-shedders' and have been shown to increase transmission of E.coli O157:H7 to other cattle in feedlots, according to Kim Stanford of Alberta Agriculture and Rural Development in Lethbridge, Canada, and co-authors there and at Agriculture and Agri-Food Canada.

In a paper in BMC Veterinary Research, they explain that their study investigated relationships among faecal isolates from super-shedders (n=162), perineal hide swab isolates (PS) from super-shedders (n=137) and faecal isolates from low-shedder (<104 CFU per gramme of faeces) pen-mates (n=496) using pulsed-field gel electrophoresis (PFGE).

A subsample of these faecal isolates (n=474) was tested for antimicrobial resistance. Isolates of E.coli O157:H7 were obtained from cattle in pens (average = 181 head) at two commercial feedlots in southern Alberta with each steer sampled at entry to the feedlot and prior to slaughter.

The researchers found that only one steer maintained super-shedder status at both samplings, although approximately 30 per cent of super-shedders in sampling 1 had low-shedder status at sampling 2.

A total of 85 restriction endonuclease digestion clusters (REPC; 90 per cent or greater similarity) and 86 unique isolates (<90 per cent similarity) were detected, with the predominant REPC (30 per cent of isolates) being isolated from cattle in all feedlot pens, although it was not associated with shedding status (super- or low-shedder; P=0.94).

Only two of 21 super-shedders had faecal isolates in the same REPC at both samplings. Faecal and PS isolates from individual super-shedders generally belonged to different REPCs, although faecal isolates of E.coli O157:H7 from super- and low-shedders showed greater similarity (P<0.001) than those from PS. For 77 per cent of super-shedders, PFGE profiles of super-shedder faecal and PS isolates were distinct from all low-shedder faecal isolates collected in the same pen.

A low level of antimicrobial resistance (3.7 per cent) was detected and prevalence of antimicrobial resistance did not differ among super- and low-shedder isolates (P=0.69) although all super-shedder isolates with antimicrobial resistance (n=3) were resistant to multiple antimicrobials.

From their results, Stanford and co-authors concluded that super-shedders did not have increased antimicrobial resistance compared to low-shedder pen mates.

Their data demonstrated that PFGE profiles of individual super-shedders varied over time and that only one out of 162 steers was a super-shedder at both samplings. In these two commercial feedlots, PFGE subtypes of E.coli O157:H7 from faecal isolates of super- and low-shedders were frequently different as were subtypes of faecal and perineal hide isolates from super-shedders.


Stanford K., C.A. Agopsowicz and T.A. McAllister. 2012. Genetic diversity and antimicrobial resistance among isolates of Escherichia coli O157:H7 from feces and hides of super-shedders and low-shedding pen-mates in two commercial beef feedlots. BMC Veterinary Research, 8:178. doi:10.1186/1746-6148-8-178

Further Reading

You can view the full report (as a provisional PDF) by clicking here.

October 2012
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