In view of the significant concerns regarding increasing antimicrobial resistance and the need for efficacious alternatives to the use of antimicrobials, the studies presented were based on two primary objectives: 1) To determine the effect of three feed-based subtherapeutic antimicrobials (FSAs; apramycin, chlortetracycline, and carbadox) and 2) the effect of rocine-derived mucosal competitive exclusion (PCE) culture on both the antimicrobial susceptibility profiles of commensal E. Coli populations and animal performance in growing pigs. For Objective 1, three replicate trails were conducted using growing piglets fed standard diets with and without subtherapeutic antimicrobials. For Objective 2, two replicate trails were coducted with a porcine-derived competitive exclusion culture replacig the subtherapeutic antimicrobials. Fecal samples were cultured for commensla E. coli at regular intervals from all piglets and antimicrobial susceptibility profiles of E. coli isolates were determined. Animal performance was also measured. Results from Objective 1 indicated significantly increased antimicrobial resistance in E. coli isolates from piglets fed FSAs when compared to controls (P<0.0001). However, resistance levels returned to baseline by the end of each trail. Piglets fet FSAs demonstrated higher average daily gains at Days 61 and 75 (Day 1: weight at end of carbodox-starter diet, Day 75: weight at end chlortetracycline grower diet) (P<0.001) and improved feed efficiency at Day 75 (P<0.0001). Results from Objective 2 indicated that streptomycin resistance in E. coli frm piglets subsequent to the first 2 doses (Days 0-1, P<0.0001) and then returning to baseline levels by Day 21 (weaning). Significant differences for strptomycin were observed again after the second 2 doses at weaning (Days 21-22), P<0.0001), the contribution of tetracycline resistance from sows and the environment was unclear. Piglets treated with the PCE culture demonstrated improved feed efficiencies when compared to contro piglets (P<0.005). These data suggest that some antimicrobials or classes of antimicrobnials may not lead to the development of persistently resistant E. Coli populations and may therefore be suitable for continued use. Additionally, data suggest that PCE cultures improve animal performance and increased resistance in E. coli appeared to be transient for some antimicrobials. Therefore, antimicrobial susceptibility testing of PCE cultures should be performed. Therefore, antimicrobial susceptibility testing of PCE cultures should be performed. Further research is warranted on the populations of resistant E. coli.