While use of antimicrobial drugs in livestock production has made a significant impact on animal health, welfare, and productivity, interest in suitable alternatives such as pre/probiotics, organic acids, and cultures of normal flora or "competitive exclusion" cultures from young animals has increased significantly in the wake of the antimicrobial resistance issue. The present study was undertaken to determine the effect of porcine-derived mucosal competitive exclusion (PCE) culture on both the antimicrobial susceptibility of commensal E. coli and on growth performance in piglets. Two replicate trials were conducted using growing piglets fed standard antimicrobial-free production diets. Piglets in the treatment group were orally dosed with PCE (10(10) cfu/mL) twice within 24 h of birth, at weaning, and 18-24 h post-weaning; control group piglets were dosed with sterile broth as a placebo. Fecal samples from all piglets were cultured for commensal E. coli at dosing times and when feed type was changed. A significantly higher proportion of E. coli from PCE-treated piglets demonstrated resistance to tetracycline (p < 0.0001), and streptomycin (p < 0.0001) when compared to controls. Resistance to streptomycin resistance in E. coli from piglets treated with PCE culture was variable, returning to baseline levels by day 21 (weaning). Piglets treated with the PCE culture demonstrated improved feed efficiencies when compared to control piglets (p < 0.005) during feeding of the starter and first growth diets. The PCE culture used in the present study had previously been shown to effectively exclude Salmonella in pigs. To the best of the authors' knowledge, this is the first report characterizing the effect of a competitive exclusion culture on antimicrobial resistance of commensal E. coli.