Prevalence of Extended-Spectrum Beta-Lactamase Producing Coliform Bacteria in Fed Cattle Receiving Metaphylaxis

The use of antimicrobials – and antibiotics in particular – in livestock production for prevention, control and treatment of bacterial diseases has expanded significantly over the decades. Bacterial resistance to any particular antibiotic can be a natural property of the bacteria, or else acquired as a gene-based mechanism. A field trial was conducted to evaluate the long-term effects of two metaphylactic antibiotic regimens on extendedspectrum beta-lactamase (ESBL) producing fecal E. coli in feedlot cattle. A total of one hundred thirty-four crossbred beef steers (initial BW = 390 ± 18.4kg) were used in a randomized complete block design. Upon arrival (d -3), cattle from two sources received routine processing and their body weights were recorded. Steers were assigned randomly to treatment pens (11 to 12 head per pen; 12 pens total) with 1 to 2 steers randomly selected to be a sentinel control and receive no treatment to evaluate antimicrobial resistance among treatments within the pen. Each of the 3 treatments was replicated 4 times. The trial began on d 0 with the initial fecal sample collected and animals receiving the previously assigned 1 of 3 treatments consisting of: 1) control, no antimicrobial administered; 2) ceftiofur (Excede®, Zoetis, Kalamazoo, MI) administered 6.6 mg/kg BW in the caudal aspect of the left ear; or 3) tulathromycin (Draxxin®, Zoetis) administered 2.5 mg/kg BW subcutaneously in the neck. Subsequent fecal samples (n=804) were collected on d X, Y, Z… to determine presence of ESBL from E. coli iv bacteria and BW was obtained simultaneously to determine performance. There was no difference in health (data not shown) or performance (P ≥ 0.10) data observed in this study; however, there was a tendency for ADG to be reduced from d 14 to 28 in the ceftiofur-treated cattle (P = 0.10). No differences in carcass traits were observed (P ≥ 0.43). Extended spectrum ß-lactamase and AmpC bacteria were prevalent in the fecal samples across all days and treatment groups. There was no difference (P = XX) between treatment groups across days in terms of sample-level prevalence of ESBL or AmpC E. coli. A difference (P = XX) was observed, however, in the bacterial level prevalence and quantity of bacteria growing on MacConkey plus ceftriaxone (4µg/ml: MAC-CEF) plates only on d 7 for the ceftiofur treated group, when total coliform counts also were significantly reduced on plain MAC plates. On d 14 we reported that coliform counts had returned to baseline prevalencebefore antibiotic administration. At the time of harvest, no differences were detected among treatment groups. Among all fecal samples, 143 bacterial isolates exhibiting the ESBL/AmpC phenotype also exhibited resistance to as many as 9 classes and 13 individual antibiotics. Among these, 42 isolates were of the true ESBL phenotype while 101 were of the more common AmpC phenotype commonly found in North American cattle populations.