Antibiotics are also used at therapeutic levels to treat and cure animals that are ill, or at high risk of becoming ill. For example, respiratory illness afflicts large numbers of young cattle, particularly as weather conditions change. Chicks are vulnerable to a disease called colibacillosis, and to potentially fatal intestinal infections. Thus they are often treated as a group via their feed or water or by direct injection where possible. For example, penicillin doesn t work at all on Salmonella, and never has.
Bacteria can also develop resistance by random changes in their genetic makeup that is, by mutation.
The discovery, in the late 1950s, that bacteria can swap genes raised the possibility that genes coding for resistance to a certain antibiotic could be transferred and perhaps find their way to a dangerous pathogen best treated by that same antibiotic. Bacteria have proven themselves very resourceful at developing resistance sooner or later. Resistance to antibiotics is a consequence of their use. Over-use and misuse of antibiotics speeds up the development of resistance. The resistance issue has stirred more than four decades of analysis and research, beginning in 1956 when the U.S. National Academy of Sciences first studied it.