Antibiotic resistance (AR) is a growing public health problem worldwide. Concentrated animal feeding operations (CAFOs) where antibiotics are used extensively are likely sites for the introduction of antibiotics, antibiotic resistant bacteria and antibiotic resistance genes (ARGs) into the environment. Indeed, ARGs have been detected in lagoon water, irrigation ditch water, agriculturally impacted river sediments and groundwater.

While soil is a natural reservoir of ARGs, CAFOs may impact this reservoir by adding ARGs to soil microbial community, and horizontal gene transfer (HGT) plays an important role in the dissemination of ARGs. Furthermore, residual antibiotics can exert a selective pressure on HGT. Few studies have attempted to investigate these processes because of the technical difficulties associated with working with soil.

Our study focuses on tetracycline resistance (Tc^R). For decades tetracyclines have been used as growth promoters in food animals including chicken. We investigated the occurrence of Tc^R in a waste-affected subsurface at a broiler chicken farm on the Maryland Eastern Shore. Several Tc^R genes were identified from subsurface samples. A mobilizable Tc^R plasmid was identified. This plasmid was shown to transfer to, and confer Tc^R on, a phylogenetically distant recipient bacterial strain at high frequency. Other Tc^R genes were also shown to transfer. Higher transfer frequencies were observed when tetracycline selection was present.

650 million pounds of chicken manure is produced in Maryland annually

Our results reflect that tetracycline is indeed a selective pressure in the proliferation and dissemination of Tc^R in microbes from a chicken-waste impacted subsurface. The high groundwater table at our field site could significantly raise the risk of AR migrating from the subsurface into the groundwater, and eventually into hydraulically connected surface waters such as the Chesapeake Bay. Our results can help assess the real effects of antibiotic selection on the spread of AR in a manure-affected subsurface. This ability is crucial for improving groundwater and downstream water quality.