How has MRSA become resistant to antibiotics? How did antibiotics originally kill the bacteria?
Investigators found that the bacteria had dropped from its chromosomes a gene called katG which codes for the production of enzymes. These enzyme proteins allowed the antibiotics to attack the bacteria. The mutation that caused the loss of genetic information for making these enzymes protected the bacteria from the antibiotics.
"Apparently the cells had paid a price to defend themselves from the drug. They had made an evolutionary trade-off, giving up part of their own adaptive equipment for the sake of survival." (The beak of the Finch page 260 Jonathan Weiner)
By losing genetic information the bacteria became more simple and less susceptible to the antibiotic. The mutation makes the bacteria less adaptable in a normal environment but makes the bacteria resistant to the antibiotics.