Antibiotics have been introduced to or spread to almost every ecosystem on earth
The environment is a huge source of antibiotic resistance. In fact, long before humans discovered and began to use antibiotics they were already present in the environment. Microbes produce antibiotics to gain a growth advantage and to defend themselves against competing organisms.The level of resistance genes detected in agricultural soil and in surface water is increasing. Recent work has even detected resistance genes in the air.
The life cycle of an antibiotic does not end when livestock are treated or a patient swallows a pill. In most cases, the antibiotic is excreted into urine or feces, ranging from 10-90% of still active compounds. These then go through the sewage system to waste-water treatment plants, but the treatment does not remove all the antibiotics. Antibiotics either leave the plant in treated water, or they become part of the sewar sludge and are introduced into the environment when the sludge is used as fertilizer or a filling material. Similarly, animal feces end up in manure storage tanks/lagoons, and the manure is used as fertilizer. When humans eat crops grown in manured soils or produce irrigated with contaminated water, they are exposed. Antibiotics also enter the water directly from pharmaceutical production facilities.