In the United States nearly 80% of all antibiotics are used in the livestock industry. This use has become a major health concern given the potential for increased antibiotic resistance in human pathogens. While such concerns have obvious human health implications, the impact of increased antibiotic resistance on microbial communities and the ecosystem processes they regulate remains unknown. Given the ~13 thousand metric tons of antibiotics entering natural systems annually, we expect marked impacts on microbial community composition, ecosystem processes, and even other components of ecological communities (i.e. plants or animals). We expect that this cascade of effects will be driven by the response of soil bacterial communities whereby the increased level of antibiotics entering a system acts as a selective mechanism leading to communities exhibiting increased antibiotic resistance. In turn, we predict that greater antibiotic resistance may result in a response in the microbial community akin to a stress response, indicated by greater mass specific respiration and decreased microbial immobilization of nutrients. To investigate the effect of antibiotic inputs on microbial communities and ecosystem processes, we present the results of multiple studies, from national to local scale, and terrestrial to aquatic systems.
Results/Conclusions
We find, nationally, that the presence of antibiotics can lead to, as much as, a 5-fold increase in antibiotic resistance genes in soil microbial communities. Soil bacterial communities also tend to shift towards taxa with both known resistance to antibiotics and human health concern. These changes, particularly the increase in antibiotic resistance genes, are linked to a 2-fold increase in microbial mass specific respiration, suggesting less efficient carbon cycling. At the local scale, we find that additions of antibiotic laced manure induce shifts in microbial activity that is specific to the type of antibiotic. The changes in turn seem to feed-back and influence plant communities. In aquatic systems, we find increasing evidence that antibiotics may influence leaf litter decomposition and may have knock-on effects on aquatic invertebrates. Together these compiled results suggest that antibiotic inputs to ecosystems not only have human health implications but may also have substantial environmental impacts.