Purpose: To determine the prevalence of lactose-fermenting coliforms resistant to the 3GC cefotaxime (Lac+ CTX) in feces and on hides of feedlot cattle throughout a production cycle and molecularly characterize the resistant isolates.
Methods: Fecal samples (n = 1,446) and hide samples (n = 1,446) were taken on six occasions over the 10-month period which the study population of cattle (n = 763) resided at the feedlot. Samples were plated onto MacConkey agar supplemented with 4 mg/l cefotaxime to isolate Lac+ CTX. Prevalences were evaluated by chi-square with Bonferroni’s correction for multiple comparisons and comparisons with P values < 0.01 were considered significant. Molecular characterization included plasmid Inc group identification, plasmid size analysis, PCR for presence of blaCMY-2 gene, and pulsed field gel electrophoresis (PFGE) sub-typing.
Results: The fecal Lac+ CTX prevalence of 27.8% during the period of frequent ceftiofur use was higher than the 4.3% prevalence when cattle arrived at the feedlot and the 1.1% to 9.4% prevalences during periods of infrequent ceftiofur use at the feedlot. The 15.5% hide Lac+ CTX prevalence when cattle arrived at the feedlot was not different from the 13.9% prevalence during the period of frequent ceftiofur use and the 1.9 to 19.3% prevalences during periods of infrequent use. Molecular analysis of 383 resistant isolates revealed that the resistance plasmid conferring 3GC resistance, as well as resistance to other antibiotic classes, integrated into the chromosome of the most frequently isolated Lac+ CTX sub-type.
Significance: Lac+ CTX proportion of the total Lac+ coliform feedlot population increases during periods of frequent ceftiofur use but returns to baseline levels when ceftiofur is not used. Molecular analysis of resistant isolates revealed that the plasmid conferring resistance to 3GC integrates into the chromosome, which may contribute to maintenance of 3GC resistance in the absence of selective pressure (ceftiofur use).