Submitted to: Proceedings of the International Conference on Emerging Infectious Diseases
Publication Type: Abstract Only
Publication Acceptance Date: 2/29/2004
Publication Date: 2/29/2004
Citation: Stanton, T.B., Stoffregen, W.C. 2004. Searching for baseline resistance - organic and feral pig tales. Proceedings of the International Conference on Emerging Infectious Diseases. Abstract p. 53. Interpretive Summary:
Technical Abstract: Background: Important for assessing the success of programs to reduce antibiotic resistance in farm animals is knowing what levels of reduction are possible. The goal of this research was to determine baseline levels of tetracycline-resistant intestinal bacteria in swine not fed antibiotics. Methods: Fecal samples were obtained from ten organically-raised swine from two Iowa farms that have not used antibiotics for at least four years and from nineteen feral swine remote from agricultural contact. Fecal bacteria were cultured aerobically on MacConkey agar (coliforms) and anaerobically on Me109M agar (Megasphaera elsdenii) or RTC agar (total anaerobes). The culture media contained 0, 16, or 64 ug chlortetracycline (CTC)/ ml. Genes encoding tetracycline resistance were detected and identified by using PCR assays differentiating thirteen tet classes. Results: Organic swine feces contained an estimated 6.7 x 10**6 coliforms/gm, with 27% insensitive to 64 ug CTC/ml. By contrast, coliforms in feral swine samples insensitive to CTC at 16 ug /ml were undetectable (Limit of detection = 5,000 cfu/gm). From organic swine, 46% (264/540) of M. elsdenii isolates grew at 64 ug CTC/ml, whereas none (0/91) from feral swine grew at 16 or 64 ug CTC/ml. At 64 ug CTC/ml, 16% (9 x 10**9 cfu/gm) of the anaerobes from organic swine were insensitive to the antibiotic and 0.3% (3.2 x 10**6), from feral swine were insensitive. The tetracycline resistance genes and the resistant bacterial species of organic swine were more diverse than those of feral swine. M. elsdenii CTC-resistant strains carried novel mosaic tetracycline resistance genes, products of recombination between tet(O) and tet(W) gene classes. Conclusions: Organically-raised swine shed diverse tetracycline-resistant bacteria in high numbers. Tetracycline-resistant bacteria in feral swine feces are at least 1000-fold fewer. Factors in addition to feeding antibiotics are implicated in the persistence of tetracycline resistance in organic swine. The finding of mosaic tet genes in M. elsdenii suggests that commensal intestinal microbes may serve not only as a reservoir of antibiotic resistance genes but as a testing ground for the evolution of those genes.