Submitted to: Safepork
Publication Type: Proceedings
Publication Acceptance Date: 3/22/2007
Publication Date: 5/2/2007
Citation: Ramlachan, N., Anderson, R.C., Andrews, K., Nisbet, D.J. 2007. A comparative study on the effect of subtherapeutic tylosin administration on select feral or domestic porcine gut microflora grown in continuous-flow culture. In: Proceedings of 7th International Safepork Symposium, May 9-11, 2007, Verona, Italy. p. 481-484. Interpretive Summary: Antibiotics are commonly used in human and veterinary medicine to treat infectious diseases and to improve production efficiency. Resistance to antibiotics can occur by a variety of mechanisms, with resistance to a certain class of antibiotics called macrolides being affected by enzymes that can degrade the antibiotic or pump it out of the bacterial cell. It is not known with certainty how easy it might be for bacteria to acquire resistance in natural settings such in the gut of a pig where many different types of bacteria may have to compete with one another to survive. Consequently, we conducted a laboratory experiment to model the effect of feeding low levels of the antibiotic tylosin on the acquisition of resistance by natural gut bacteria that were harvested from the intestinal tracts of a wild and a farm-raised pig. We found that this low level administration of tylosin promoted the enrichment of tylosin-insensitive bacterial populations from the farm-raised pig but not in the populations from the wild pig. These results will help antibiotic users to appropriately manage all antibiotics at their disposal. Ultimately, these results will help provide farmers, scientists and U.S. public health officials with important information to make sound, science-based decisions for good public health.
Technical Abstract: Continuous flow cultures of feral (culture FC) and domesticated (culture RPCF) pig gut microflora were established in steady state. Cultures, in duplicate, were continuously infused subtherapeutic (25 ug/ml) levels of tylosin and sampled at intervals to assess effects on total culturable anaerobes, Bacteroides spp. and Enterococcus spp. via plating on serial 10-fold dilutions to anaerobic Brucella blood agar, Bacteroides bile esculin agar, and M Enterococcus agar supplemented without or with 100 ug tylosin/ml (the latter to assess bacterial sensitivity to tylosin). Concentrations of total culturable anaerobes within culture FC decreased (P < 0.05) following 7 days tylosin administration. Concentrations of Bacteroides and Enterococcus decreased (P < 0.05) to near or below detectable levels (1.0 log10 CFU/ml) in culture FC following 7 days tylosin administration, and tylosin-insensitive colonies were recovered at low numbers (< 2 log10 CFU/ml) and did not persist. In contrast, concentrations of total culturable anaerobes, Bacteroides and Enterococcus in culture RPCF, while initially decreased upon initiation of tylosin administration, began to increase (P < 0.05) by as early as 4 days thereafter, with tylosin-insensitive colonies recovered as one of predominant populations. Under the conditions of this test, subtherapeutic administration of tylosin promoted the enrichment of tylosin-insensitive bacterial populations within RPCF cultures (originating from a traditionally reared domesticated pig) but not from FC cultures (originating from a feral pig). This suggest that gut microflora from conventionally reared swine may be more at risk to the development of antimicrobial resistance.