Sub-therapeutic use of antibiotics and prevalence of antibiotic resistant bacteria on swine farms

This study was undertaken to compare the prevalence of antibiotic resistant bacteria (ARB) in manure, soil and dog fecal samples collected from swine farms using antibiotics (AU) and not using antibiotics (NAU) sub-therapeutically. A total of 12 farms (6 of each type) were surveyed for this study. All samples were screened for the presence of ARB by plating on Mueller Hinton II agar with or without antibiotic (tetracycline, tylosin, or monensin). Results showed a significantly higher (p<0.05) prevalence of tetracycline resistant bacteria (TC^r) and monensin resistant bacteria (MON^r) in manure samples from AU farms than from NAU farms. However, there was no significant difference in the prevalence of tylosin resistant bacteria (TYL^r) in manure samples between AU and NAU farms. Also, the difference in the prevalence of TC^r, TYL^r and MON^r in either soil or dog fecal samples between the two types of farm was non-significant. Antimicrobial susceptibility profile of common Gram negative bacterial isolates from these samples showed only small differences between AU and NAU farms. In general, isolates from AU farms showed higher resistance to ampicillin, spectinomycin, tetracycline and sulphonamides. E. coli isolates from dog fecal sample had significantly higher resistance from the AU than the NAU farms thus raising some concerns on the potential of ARB spread to humans on the farm. We also conclude that higher prevalence of ARB in swine manure samples from AU farms is not spreading to the terrestrial environment following land application of manure.