A comparative study on the effects of tylosin on select bacteria during continuous-flow culture of mixed populations of gut microflora derived from a feral and a domestic pig

Authors: Ramlachan, Nicole; Anderson, Robin; Andrews, Kathleen - Kate; Harvey, Roger; Nisbet, David

Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2007
Publication Date: 2/9/2008
Citation: Ramlachan, N., Anderson, R.C., Andrews, K., Harvey, R.B., Nisbet, D.J. 2008. A comparative study on the effects of tylosin on select bacteria during continuous-flow culture of mixed populations of gut microflora derived from a feral and a domestic pig. Foodborne Pathogens and Disease. 5:21-31.

Interpretive Summary: The gut of wild and domesticated animals contains billions of bacteria, each competing to survive within their very competitive environments. In order to learn more about how the different microbial populations from wild and domestic swine may adapt to antibiotics, we separately grew microbial populations from a wild and farm-raised pig in specially designed incubation vessels that mimic the gut environment. We found that while the microbial populations from the wild and farm-raised pig were generally similar in composition, there were some differences in the types of bacteria possessing natural resistance to a common antibiotic called tylosin that is sometimes fed to pigs. We also found that under the experimental conditions used here, these resistant microbial populations could be increased more easily by administering a low rather than a high level of tylosin, and that resistant populations from the wild pig did not persist as long as those from the farm-raised pig. Additionally, we found that the bacteria naturally-present in the microbial populations of the wild and farm-raised pig were able to out-compete and prevent colonization by an experimentally introduced non-native bacterium possessing high level resistance to tylosin. Thus, bacteria not native to the pig gut, even if possessing high levels of resistance to tylosin, are not necessarily well adapted to these environments. These results will help antibiotic users to appropriately manage all antibiotics at their disposal and ultimately will help provide farmers, scientists, and U.S. public health officials with important information to make sound, science-based decisions for the good of public health and animal production.

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 of 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 endogenous tylosin-insensitive anaerobes within the FC and RPCF cultures, while similar prior to tylosin administration (ranging from 8.8 to 9.2 log10 CFU/ml) responded differently during the experiments, with concentrations in RPCF cultures becoming enriched more than those in FC cultures during administration of either 25 or 100 ug tylosin/ml. Moreover, tylosin-insensitive anaerobes persisted at their increased concentrations after cessation of tylosin administration whereas concentrations from FC cultures decreased, albeit slightly. Concentrations of Bacteroides and endogenous Enterococcus spp. decreased to near or below detectable levels (1.0 log10 CFU/ml) in culture FC following administration of 25 or 100 ug tylosin/ml although tylosin-insensitive populations were present (> 1.0 log10 CFU/ml) before initiation of tylosin administration. Concentrations of endogenous tylosin-insensitive Bacteroides spp. were not enriched in FC cultures during 25 ug tylosin/ml administration but were enriched to > 5 log10 CFU/ml in RPCF cultures. Tylosin-insensitive Bacteroides spp. were not enriched when 100 ug tylosin/ml was administered to either FC or RPCF cultures. When administered at 25 or 100 ug tylosin/ml, populations of endogenous tylosin-insensitive Enterococcus sp. were enriched in RPCF cultures but not in FC cultures. An exogenous-sourced E. faecium containing high level resistance to tylosin was able to maintain itself at detectable levels when inoculated into either FC or RPCF cultures only when in the presence of tylosin. These results indicate that antibiotic pressure may enrich for antibiotic-insensitive bacteria of endogenous or exogenous origin but that the ability of non-native bacteria to persist is dramatically reduced in the absence of the antibiotic, likely due to its exclusion by native flora.