How can bacteria become resistant to antibiotics and how can doctors reduce the number of bacteria that become resistant?

-A mutation in a pathogen may produce a new strain. This new strain may become resistant to an antibiotic through natural selection. The new strain may spread rapidly as the individual is not immune to it and the antibiotic does not kill the bacteria. An example of a resistant strain is MRSA. 

-Doctors can reduce the number of bacteria that become resistant by encouraging patients to complete the whole course of antibiotics, not prescribing antibiotics for viral infections and only prescribing antibiotics for more serious bacterial infections.