Characterization of Antibiotic Resistant Fecal Indicators in Runoff from Fields Amended with Swine Manure.

The application of animal manures to agricultural soil is a widespread practice.  One concern associated with food animal manures is the introduction of antibiotic drugs, AR bacteria, and AR genes that may be spread through and across agroecosystems, with the potential to adversely impact the treatment of human disease.  Despite this widespread concern, there is very little information available regarding the numbers and types of resistant bacteria in specific agricultural ecosystems, or data regarding relationships between different types of resistance. In this study, two types of AR bacteria were quantified in runoff from fields amended with swine manure, and characterized for resistance to twelve common antibiotic drugs.  Swine manure was broadcast onto 0.75 m x 4 m plots containing Aksarben silty clay loam soil. Tetracycline and bacitracin-resistant fecal indicator bacteria were enumerated from the source manure and soil before manure application, and from runoff that was collected on three consecutive days post-manure application, for a total of 52 samples.  Isolates were characterized using standard disk diffusion assays.  Tetracycline resistance was common in the source manure for both Gram negative and Gram positive fecal indicators, and tetracycline resistant bacteria were commonly isolated from the runoff. Most pre-application soils were negative for tetracycline resistant fecal indicators.   No bacitracin resistant organisms were isolated.  Multiply drug resistant (MDR) fecal indicators were present in both the source manure and in runoff.  In general, the runoff isolates had similar MDR patterns to the source manure, although some heterogeneity was observed.  In conclusion, AR bacteria from swine manure can persist on agricultural fields, and be transported from agricultural fields in runoff.  The types of bacteria and drugs being measured impacts the results, and data from this study support the idea that the amount of AR bacteria detected depends on the specific type of resistance being measured.