Within-sample and between-sample variation of antimicrobial resistance in fecal Escherichia coli isolates from pigs.

The present study was initiated to evaluate the effect of sampling time and within-sample variability on the diversity in antimicrobial resistance patterns in fecal Escherichia coli from healthy pigs. Isolates were tested against 11 antimicrobials. A total of 25 different profiles were observed, involving resistance to ampicillin, streptomycin, tetracycline, sulfonamides, trimethoprim, and/or a trimethoprim/sulfonamide combination. No isolates were resistant to enrofloxacin, gentamicin, or chloramfenicol, whereas resistance against neomycin and nalidixic acid was sporadically detected in isolates from grower pigs. A model that clusters pigs within-sampling time as a repeated factor and clusters isolates within individual pigs as a random factor was used. For sows, the variance component ratio of sampling time to residuals was 0.28-0.56 for the different antimicrobials (except ampicillin) and 0.85-1.79 for grower pigs. The variance components for within-sample variation were zero or close to zero, except in isolates from sows where resistance to ampicillin explained 14.8 times more of the variation compared to residuals. Thus, the effect of an animal's status at a given sampling time was more influential on the variability in antimicrobial resistance than within-animal diversity. We conclude that repeated sampling and analysis of one isolate per animal each time may be preferable for screening general tendencies, whereas several isolates have to be tested when individual animals are focused.