How does bacteria become resistant to antibiotics?

Antibiotics work by poisoning molecules in bacteria. Most cases of medically significant antibiotic resistance are not due to mutations but to complex enzymes that inactivate the poison. Bacteria can either have inherited or acquire these enzymes form other bacteria.
( Bacteria can pick up genes from dead bacteria and by exchanging genes with other bacteria in their environment)

Some cases of resistance result from a mutation where the bacteria loses the molecules which the poison is suppose to attack. These mutated bacteria survive better than normal bacteria in the presence of the antibiotic poisons. However these mutated bacteria do not survive as well as normal bacteria outside of the presence of the antibiotics.

An example would be the blind fish of Death Valley. These fish live in total darkness in underground rivers. The sharp rocks in the underground caverns caused great damage to the fishes eyes. As a result the fish have mutated losing their eyes. Tough tissue has replaced the sensitive eye tissue. These fish do better than normal unmutated fish in the darkness but would be less successful in normal conditions.