ABSTRACT
Most genes encoding resistance are complex—they are usually not just newly developed mutations in bacteria, occurring when people or animals receive antibiotic therapy. Resistance is mostly encoded by resistant genes that are already carried by bacteria in the environment, humans or animals, with the genes then acquired by other bacteria (Davies and Davies, 2010; D’Costa et al., 2011; Aminov, 2009; Finley et al., 2013). Most resistance genes have been in the environment for very long periods, well before antibiotics were developed for medical purposes, some potentially for billions of years (Davies and Davies, 2010; D’Costa et al., 2011; Aminov, 2009; Finley et al., 2013). Wild animals that have never been exposed to antibiotics carry resistant bacteria (but at low levels), as do people who died in the preantibiotic era and those currently living in very remote regions, such as the Amazon (Shirley et al., 2000; Clemente et al., 2015).
