Bacterial resistance is driven largely by antibiotic use, says White. The highest resistance is often to drugs that are the easiest to obtain those sold over the counter. Many bacteria that cause food-borne illness can infect many animal species. Chickens carry most Campylobacter jejuni infections, and eggs can carry Salmonella enteritidis. If these bacteria are resistant to antibiotics in other animals, they'll be resistant to antibiotics when they infect us.
Antibiotics usually kill bacteria by interfering with an essential aspect of bacterial life.
Penicillin inhibits the synthesis of the bacteria cell wall a structure lacking in animals. The fluoroquinolones interfere with bacterial reproduction. Sulfa drugs interfere with essential bacterial enzymes, and erythromycin inhibits bacterial protein synthesis. Bacteria resistant to these antibiotics use one of several mechanisms. One is to simply inactivate the antibiotic.