Like any other living population bacteria are dynamic and respond to environmental pressures.  The change in the frequency of genetic charcteristics in  populations of bacteria is a naturally occurring process (evolution)  that has  proved troublesome to humans as we attempt to control infections of pathogenic bacteria.  Antibiotics , which first came into clinical use in the 1940's, are chemical agents taken internally to control bacterial infection.  During the course of evolution characteristics may arise in a few bacteria that alter the targets of antibiotic action;  a number of  may influence whether the occurence of this trait increases in the population.