Swine antibiotic induces phage-mediated gene transfer in swine gut

What is it about?

FDA regulations on agricultural antibiotic use have focused on antibiotics that are important for human medicine. Carbadox is an antibiotic not used in humans but frequently used on U.S. pig farms. It is important to study possible side-effects of carbadox use because it has been shown to promote bacterial evolution, which could indirectly impact antibiotic resistance in bacteria of clinical importance. Previous research from our research group has shown that carbadox induces phages in swine gut bacteria and in Salmonella Typhimurium, a foodborne pathogen. Phages are viruses that infect bacteria and are important because they kill bacteria and they transfer genetic material between cells. In the current study, we analyzed whether carbadox caused changes in gene expression of the swine gut bacterial community and in the fecal phage diversity. The results showed that by two days after the initiation of carbadox in the feed, the bacteria in the carbadox-fed pigs were expressing different genes than the bacteria in the non-medicated pigs. These differences showed that the bacteria in the carbadox-fed pigs were not multiplying or metabolizing carbohydrates as they normally would. Interestingly, these differences also showed that phages were being induced in the carbadox-fed pigs two days after carbadox initiation. Importantly, the phage genetic material isolated in this study encoded antibiotic resistance genes that could provide resistance to antibiotics that are important in human medicine, indicating that human-relevant antibiotic resistance genes are mobile between bacteria via phages. This study highlights the collateral effects of antibiotics and demonstrates the need for considering diverse antibiotic effects whenever antibiotics are being used or new regulations are considered. The results of this study are important for scientists, policy-makers, regulators, and farmers who are interested in devising strategies for improving antibiotic stewardship in agriculture.

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