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Comparison of sampling techniques for measuring the antimicrobial susceptibility of enteric Escherichia colirecovered from feedlot cattle

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  • 1 Centers for Epidemiology and Animal Health, USDA: Animal and Plant Health Inspection Service: Veterinary Services, 2150 Centre Ave, Building B, Mail Stop No. 2E7, Fort Collins, CO 80526-8117.
  • | 2 Centers for Epidemiology and Animal Health, USDA: Animal and Plant Health Inspection Service: Veterinary Services, 2150 Centre Ave, Building B, Mail Stop No. 2E7, Fort Collins, CO 80526-8117.
  • | 3 Animal Population Health Institute, College of Veterinary Medicine and Biological Sciences, Colorado State University, Fort Collins, CO 80523-1601.
  • | 4 Animal Population Health Institute, College of Veterinary Medicine and Biological Sciences, Colorado State University, Fort Collins, CO 80523-1601.
  • | 5 Department of Veterinary Preventive Medicine, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.
  • | 6 Department of Environmental and Radiological Health Sciences, College of Veterinary Medicine and Biological Sciences, Colorado State University, Fort Collins, CO 80523-1601.

Abstract

Objective—To evaluate the effectiveness of various sampling techniques for determining antimicrobial resistance patterns in Escherichia coli isolated from feces of feedlot cattle.

Sample Population—Fecal samples obtained from 328 beef steers and 6 feedlot pens in which the cattle resided.

Procedure—Single fecal samples were collected from the rectum of each steer and from floors of pens in which the cattle resided. Fecal material from each single sample was combined into pools containing 5 and 10 samples. Five isolates of Escherichia coli from each single sample and each pooled sample were tested for susceptibility to 17 antimicrobials.

Results—Patterns of antimicrobial resistance for fecal samples obtained from the rectum of cattle did not differ from fecal samples obtained from pen floors. Resistance patterns from pooled samples differed from patterns observed for single fecal samples. Little pen-to-pen variation in resistance prevalence was observed. Clustering of resistance phenotypes within samples was detected.

Conclusions and Clinical Relevance—Studies of antimicrobial resistance in feedlot cattle can rely on fecal samples obtained from pen floors, thus avoiding the cost and effort of obtaining fecal samples from the rectum of cattle. Pooled fecal samples yielded resistance patterns that were consistent with those of single fecal samples when the prevalence of resistance to an antimicrobial was > 2%. Pooling may be a practical alternative when investigating patterns of resistance that are not rare. Apparent clustering of resistance phenotypes within samples argues for examining fewer isolates per fecal sample and more fecal samples per pen. (Am J Vet Res 2002;63:1662–1670)