Effect of age and housing location on antibiotic resistance of fecal coliforms from pigs in a non-antibiotic-exposed herd

The relationship of age and housing location to single antibiotic resistance, multiple antibiotic resistance, and resistance patterns of fecal coliforms obtained during a 20-month period from pigs in a herd that was not exposed to antibiotics for 126 months was determined. Bacteria resistant to single and multiple antibiotics were isolated more frequently (P < 0.01) from pigs under 7 months of age. A greater proportion of isolates from pigs over 6 months of age was sensitive to the 13 antimicrobial agents tested (P < 0.01), while a smaller proportion showed resistance to single (P < 0.05) and multiple (P < 0.01) antibiotics. More than 80% of the resistant isolates were resistant to tetracycline, streptomycin, or sulfisoxazole. Resistance was greater (P < 0.01) for pigs in the finishing unit than for those on pasture. Resistance to ampicillin, carbenicillin, and tetracycline was greater (P < 0.05) for pigs in the finishing unit than for those in the farrowing house. More isolates from pigs on pasture were sensitive to all antimicrobial agents tested (P < 0.01). A greater proportion of isolates from pigs in the finishing unit showed resistance to a single antibiotic (P < 0.01). The data from this study suggest that exposure to antibiotics is not the only factor that influences the prevalence of bacteria that are resistant to single and multiple antibiotics in the feces of domestic animals and that considerable research is needed to define the factors influencing antibiotic resistance in fecal bacteria.