Farm animal manure-associated, antibiotic (AB)-resistant microorganisms and genes encoding this resistance are viewed as important sources for the observed increase of AB resistance in humans and animals. The data on relationships between the application of AB in swine farms, and presence and migration of AB resistance genes into the environment are contradictory. A systematic study of their profiles and abundances was conducted in and around three swine farms with different strategy of AB usage.
Three swine farms, with known histories of AB applications, were surveyed in this study. Tetracycline resistance (TCR) genes were used as models. Their presence and concentrations in swine farms and the adjacent environments were evaluated by tracking 16 different groups of TCR genes in corresponding microbial communities. Throughout the year, periodic samples were taken of feces, food, manure lagoons, lagoon-derived irrigation water, irrigated and non-irrigated soils, and adjacent streams and ponds.

TCR genes were observed in all three farms including the AB-free “finisher” branch of one of them. Some of these genes likely originated from animal feed, but most of the TCR genes were selected by farm environments. The profiles of these genes were composed of “transient” and “persistent” TCR genes, and with exception of one gene were quite similar in all three farms. This exceptional gene was exclusively associated with AB usage by farrowing animals and piglets. No TCR genes were observed around the AB-free farm branch. The highest environmental occurrence of TCR genes was observed around the farm with the highest AB usage. Furthermore, at this farm, the concentrations of most persistent and environmentally mobile genes increased during the passage of manure through the system of lagoons. Observed environmental occurrence of these genes was related to spills from the lagoons and irrigation with manure.

Project findings demonstrated that not all AB resistance genes should be monitored in swine farms. The “transient” genes remain in animal feces or the directly receiving them lagoons, and do not pose a threat to the environment. Evidences of the AB usage-dependent proliferation of the “persistent” TCR genes and of their environmental occurrence in proximal soils and water bodies call for the improvement of current protocols for AB administration. To estimate the threat that these genes pose to humans, their longevity and mobility in soils and water bodies should be examined. On the other hand, the observation of a gradual increase in TCR gene concentrations in consequent lagoons calls for a comparative study of different manure-handling practices, such as separate lagoons for manure from AB-treated and untreated animals, and may call for new engineering solutions for manure treatment lagoon systems. Finally, more attention should be paid to the feed that is the source of some persistent AB resistance genes in farms. Potential action here may include the industry-wide quality control and on-site pre-treatment of animal feed.
Contact: Prof. Andrei L. Barkovskii, GCSU,