The authors of the study (including researchers from Michigan State University, where AGua’s author is currently enrolled) found 149 unique antibiotic resistance genes among soil samples of material from manure processing and land disposal compared with control soil samples. The abundance of antibiotic resistance genes was directly correlated with the concentration of antibiotics and trace metals (1). These antibiotic resistant microbes pose a serious threat to global human health because they are not removed by treatment or processing, and manure from livestock factory farms is typically disposed of by spreading on surrounding farm land as fertilizer. Once applied to the soil the microbes are able to share their genes with other microbes and travel through the environment, entering streams and water supplies. Read more about this in the link to reference 1 below or this NY Times article.
(1) Tiedje et al 2013 PNAS antibiotic resistant genes Chinese swine farms