Even unrelated bacteria can gain resistance from their neighbors in a phenomenon called horizontal gene transfer. Resistance to antibiotics is encoded in DNA, the genetic blueprint for life. Bacteria can fuse and exchange plasmids and sometimes chromosome fragments. Increase in mobilizing plasmids has been associated with use of antibiotics in swine husbandry. These plasmids have a broad host range and are able to cross genus lines during transfer.
Viruses can infect bacteria and fungi, passing along genes from one infected organism to the next.
These genes sometimes encode resistance factors. The use of antibiotic growth promoters in animal husbandry may increase the amount of free phage in intestines and may contribute to the spread of antibiotic resistance. Some diseases are no longer treatable with antibiotics. Microbial populations have been shown to change due to exposure to antibiotics and antibacterial agents. We are seeing a rise in antibiotic resistance all around the world.