Why does antibiotic resistance occur?

When bacteria are exposed to an antibiotic, they either die or adapt. Those that survive carry genes that protect them against the antibiotic and pass those genes on to other bacteria. The picture below illustrates an antibiotic-resistant bacterial cell. Since bacteria multiply very quickly and can be easily spread among people, resistant bacteria can easily occur in places like hospitals and nursing homes, where a lot of people are gathered and antibiotic use is high. The risk of spreading resistant bacteria is even greater if the facilities are overcrowded and hygiene and sanitation is poor.

Sometimes, doctors feel pressured by their patients’ expectations to prescribe an antibiotic. More than 50% of antibiotic prescriptions are given without any evidence that the illness being treated is due to a bacterial infection. Physicians often want to avoid costly and lengthly laboratory tests that would determine the cause of the illness. Antibiotics can not be used to treat illnesses caused by viruses, such as colds, influenza, and viral pneumonia. Of the 235 million doses of antibiotics consumed by humans per year, 20-50% of them are unnecessary.

Another factor that contributes to resistance is that when patients are prescribed antibiotics for a just cause, many do not finish their medication. This allows resistant bacteria to survive more easily. The practice of saving unused medication to treat themselves or others at a later date can also lead to resistant strains.

Also contributing to antibiotic resistance is the widespread use of antibiotics to promote weight gain and to control disease in cattle, pigs, and chickens. Forty to fifty percent of antibiotics produced are used in livestock feed. This leads to an increase of resistant bacteria in these animals, which is then spread to humans. ^{7, 9}