Based on its antimicrobial spectrum, it might have been expected that olaquindox would have had the greatest effect on E. coli numbers and these in fact were significantly lower than the control group when measured in the colon. However, it is surprising that the bacitracin zinc treatment reduced E. coli counts to an even greater extent than did olaquindox. This suggests that competitive exclusion may be as important a mechanism in controlling bacterial growth as are the direct effects of the antibiotics.