Antimicrobial resistance of fecal coliforms from pigs in a herd not exposed to antimicrobial agents for 126 months

Patterns of antimicrobial resistance were determined for lactose-fermenting fecal coliforms obtained during a 20-month period from pigs in a herd without exposure to antimicrobial drugs for 126 months. Mean percent resistance to cephalothin, chloramphenicol, kanamycin, naladixic acid, streptomycin and tetracycline was lower (P less than 0.01) for isolates obtained during the 20-month period (Group 2) than for isolates obtained during the first 13 months (Group 1) after the withdrawal of antibiotics. Mean tetracycline resistance was 40.5% for Group 2 isolates. Approximately 44% of the Group 2 isolates were resistant to the 10 antimicrobial agents compared with 74.9% for the Group 1 isolates (P less than 0.01). Multiple resistance was lower (P less than 0.01) in the Group 2 isolates than in the Group 1 isolates. Four resistance patterns (tetracycline, tetracycline-streptomycin, tetracycline-sulfisoxazole, tetracycline-streptomycin-sulfisoxazole) accounted for 74% of the resistant isolates.