There is growing concern about the fate of antibiotics and antibiotic residues in the environment. These compounds can potentially select for antibiotic resistant bacteria, and this selection can occur in the environment if the antibiotic is still in an active form. The swine industry uses considerable amounts of certain antibiotics, such as tylosin and tetracycline. Although measured levels of these antibiotics in waterways associated with swine operations have been low, they may still be high enough to influence the persistence and dissemination of antibiotic resistance. In a previous study funded by the National Pork Board, we evaluated the effect of low levels of chlortetracycline in river water on the levels of tetracycline resistance in the bacteria in the water. We used a laboratory-based system to evaluate these effects. The results of that previous study suggested that the low levels of tetracycline observed in the waterways of the U.S. may not have a strong biological effect. In other words, these low tetracycline levels may not be high enough to select for tetracycline resistant bacteria and thus may not pose a threat to human or animal health through an increase in antibiotic resistant bacteria. However, when high doses of chlortetracycline were added to the river water, we observed an increase in the levels of tetracycline resistant bacteria.
The objective of this current study was to evaluate the effects of tylosin in river water on antibiotic resistance levels in the bacterial populations in the water. We conducted this current study in a similar fashion to the chlortetracycline study. All experiments were conducted in the laboratory. Tylosin in varying concentrations was added to river water. The water was then observed over time. In this current study, increasing concentrations of tylosin were associated with decreasing total levels of Gram-positive cocci bacteria. However, tylosin had no effect on the levels of tylosin or erythromycin resistance in these bacteria. Based on the results of this study, it would appear that tylosin at varying concentrations, including levels detected in rivers in the U.S., does not select for tylosin-resistant bacteria under controlled laboratory conditions. The results of this study are based on laboratory experiments which may not mimic reality. Consequently, additional studies should be conducted in the field to validate the findings of this study. Ultimately, the results of these studies will help the swine industry manage the potential risks associated with the environmental release of antimicrobial compounds from farms.
