title: 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.
abstract: BACKGROUND
  Cattle shedding at least 104 CFU Escherichia coli O157:H7/g feces are described as super-shedders and have been shown to increase transmission of E. coli O157:H7 to other cattle in feedlots. This study investigated relationships among fecal isolates from super-shedders (n = 162), perineal hide swab isolates (PS) from super-shedders (n = 137) and fecal isolates from low-shedder (< 104 CFU/g feces) pen-mates (n = 496) using pulsed-field gel electrophoresis (PFGE). A subsample of these fecal isolates (n = 474) was tested for antimicrobial resistance. Isolates of E. coli O157:H7 were obtained from cattle in pens (avg. 181 head) at 2 commercial feedlots in southern Alberta with each steer sampled at entry to the feedlot and prior to slaughter.
  
  
  RESULTS
  Only 1 steer maintained super-shedder status at both samplings, although approximately 30% of super-shedders in sampling 1 had low-shedder status at sampling 2. A total of 85 restriction endonuclease digestion clusters (REPC; 90% or greater similarity) and 86 unique isolates (< 90% similarity) were detected, with the predominant REPC (30% 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 2/21 super-shedders had fecal isolates in the same REPC at both samplings. Fecal and PS isolates from individual super-shedders generally belonged to different REPCs, although fecal isolates of E. coli O157:H7 from super- and low-shedders showed greater similarity (P < 0.001) than those from PS. For 77% of super-shedders, PFGE profiles of super-shedder fecal and PS isolates were distinct from all low-shedder fecal isolates collected in the same pen. A low level of antimicrobial resistance (3.7%) 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.
  
  
  CONCLUSIONS
  Super-shedders did not have increased antimicrobial resistance compared to low-shedder pen mates. Our data demonstrated that PFGE profiles of individual super-shedders varied over time and that only 1/162 steers remained a super-shedder at 2 samplings. In these two commercial feedlots, PFGE subtypes of E. coli O157:H7 from fecal isolates of super- and low-shedders were frequently different as were subtypes of fecal and perineal hide isolates from super-shedders.
authors.affiliation: Alberta Agriculture and Rural Development, Agriculture Centre, 100-5401 1st Ave, S, Lethbridge, AB, T1J 4 V6, Canada. kim.stanford@gov.ab.ca
authors.forename: Kim
authors.initials: K
authors.lastname: Stanford
authors.forename: Chelsey A
authors.initials: CA
authors.lastname: Agopsowicz
authors.forename: Tim A
authors.initials: TA
authors.lastname: McAllister
mesh_terms: D000416:Alberta; D000818:Animals; D057138:Bacterial Shedding; D002417:Cattle; D002418:Cattle Diseases / Q000382:microbiology* / Q000635:transmission; D015169:Colony Count, Microbial / Q000662:veterinary; D024881:Drug Resistance, Bacterial; D016521:Electrophoresis, Gel, Pulsed-Field / Q000662:veterinary; D004927:Escherichia coli Infections / Q000382:microbiology / Q000635:transmission / Q000662:veterinary*; D019453:Escherichia coli O157 / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification*; D005243:Feces / Q000382:microbiology; D014644:Genetic Variation / Q000502:physiology*; D008297:Male; D008826:Microbial Sensitivity Tests / Q000662:veterinary; D012867:Skin / Q000382:microbiology