Antimicrobial susceptibility of generic Escherichia coli following administration of subtherapeutic antimicrobial drugs to feedlot cattle

Antibiotic resistance in bacteria has become a leading public health concern. While expected to play a role in development and spread of antibiotic resistant bacteria, feeding subtheraputic levels of antimicrobial drugs to feedlot cattle is still highly debated. The purpose of this study was to evaluate the impact of in-feed antimicrobial administration on the populations of generic Escherichia coli isolated from feedlot cattle and their susceptibility to common antimicrobial drugs. One thousand, six hundred and eleven animals were allocated to one of 16 pens; each pen was assigned one of four treatments. Treatments included: a) monensin (29.8 mg/kg dry matter) and tylosin (9.7 mg/kg dry matter); b) tylosin (9.7 mg/kg); c) monensin (29.8 mg/kg); and d) neither of the aforementioned antimicrobials. Fecal samples were collected from 40 animals per pen upon arrival, at re-implant, and harvest. Hide samples were collected from these same animals prior to shipment. A sub-sample of 10 animals were sampled in the plant post-stunning, post-hide removal and post-intervention. Samples were subjected to culture and enumeration. Multiple isolates collected from each sample were tested for susceptibility to a panel of antimicrobial drugs using a broth micro-dilution technique. A total of 7,097 isolates were collected and analyzed for antimicrobial susceptibility. Breakpoints were obtained from the Clinical Laboratory Standards Institute and the National Antimicrobial Resistance Monitoring System. Data were analyzed using various procedures of SAS.

Of the 7,097 isolates tested, 73.61% were resistant to at least one antimicrobial. The most commonly observed resistance was to sulfisoxazole, tetracycline, and streptomycin with 64.97%, 31.51% and 10.57% of the isolates showing resistance to the antimicrobials, respectively. The most common resistance pattern was to sulfisoxazole alone (33.61%) followed by a co-resistance pattern to both sulfisoxazole and tetracycline (10.17%) and single resistance to tetracycline (6.66%). Of the 7, 097 isolates, 32.61% showed resistance to at least 2 antimicrobials, and 3.21% of the isolates were resistant to at least 5 antimicrobials. The percentage of isolates from fecal samples that were resistant to at least one antimicrobial increased throughout sampling periods during the trial with 48.14%, 59.97%, and 92.14% exhibiting resistance at time of arrival, re-implant, and time of harvest, respectively. Antimicrobial treatment had no significant effect on estimated populations of resistant E.coli populations in fecal samples. Populations of E.coli resistant to nalidixic acid, streptomycin, ceftiofur and cefoxitin were lower at time of re-implant and harvest than at arrival (P < 0.05). Higher sulfisoxazole, tetracycline, and ampicillin-resistant populations were observed at harvest compared to arrival (P < 0.05). Results from this study indicate that while resistance to certain antimicrobials in E. coli is not uncommon, an increase in resistance is not due solely to the use of subtheraputic levels of antimicrobials. The impact of sampling period resulted in an increase in resistance parameters of some antimicrobial drugs and a decrease in others.