How can drug resistance develop in microorganisms?

The use of antibiotics over the last 60 years has led to the development of drug- resistant strains of bacteria.These bacterial strains always existed in the microbial population, but they never needed to use their resistance mechanisms because they were never confronted with the antibiotic. With widespread antibiotic use, the susceptible bacteria died off rapidly, and the surviving bacteria were those with resistance. They
quickly multiplied to form populations of drug- resistant microorganisms. While methicillin- resistant Staphylococcus aureus (MRSA) is a well- known example of a drug- resistant strain, many other organisms are showing alarming rates of drug resistance due to the selective pressures exerted by the increased use of antibiotics.