Development of resistance is a natural phenomenon that has existed since time immemorial. Resistance can result from changes in the bacterium’s own genome or if resistant genes are received from other bacteria.
Compared with non-resistant bacteria, resistant bacteria are more likely to have difficulty establishing themselves under normal circumstances because they are less robust. Wherever antibiotics are used, however, resistant bacteria gain a survival advantage over non-resistant microbes. They are able to survive and replicate themselves. The increased use of antibiotics in human and veterinary medicine has accelerated the spread of antibiotic resistance.