Explain how bacteria can become resistant to antibiotics.

Sometimes, a chance mutation in bacterial DNA allows certain bacteria to become resistant to certain types of antibiotic. When an antibiotic is given, only the non-resistant strains of bacteria will be killed, and the resistant strains will survive. The resistant strains then have no competition, so are able to reproduce and increase the size of the resistant population. For this reason, antibiotics are no longer used to treat non-serious infections in an attempt to slow down the rate of development of resistant strains.