Figure 2.

The role of the environment in the recruitment of antibiotic resistance genes (ARGs) to human pathogens includes four major steps: (1) emergence of novel resistance factors in the environment, (2) mobilization onto mobile genetic elements, (3) transfer of ARGs to human pathogens, and (4) dissemination of ARGs into the human microbiome. The width of each arrow roughly corresponds to the assumed frequency of each event, although this is of course largely speculative. Many events are likely more frequent when antibiotic selection pressure is stronger or recurrent. Still, due to the much larger numbers of environmental bacteria than human- and animal-associated, environmental emergence and mobilization of ARGs is probably common on a global scale. Note that the focus of this figure is on the processes involving the external environment.