Broad-spectrum antibiotic their greater potential to disrupt a larger variety of normal human flora. After continued exposure to an antibiotic, bacteria may develop changes in their structure or function that make them resistant to the antibiotic. These resistant organisms will live, while the susceptible organisms will die, leaving the population of bacteria entirely resistant to a given antibiotics. For example, after the discovery of penicillin and its subsequent use to treat bacterial infections, bacteria were found to have begun producing an enzyme, penicillinase, which rendered the penicillin molecule inactive. In response to this newly-acquired resistance, newer penicillins were produced that could not
Antibiotic use in livestock in its inaugural 2013 report on antibiotic resistance threats in the United States, the CDC identified resistant non-typhoidal "Salmonella" and "Campylobacter" as "serious threats" and called for improved surveillance and intervention in food production moving forward. There are other bacteria as well, where research is evolving and revealing that bacterial resistance acquired through use in livestock may be contributing to disease in humans. Examples of these include Enterococcus (including "E. coli" 0157) and "Staphylococcus aureus". For foodborne illness from "E. coli", which is still not typically treated with antibiotics because of associated risk of renal failure, increasing rates of antibiotic-resistant