Sometimes this happens naturally because of a genetic mutation that makes them resistant to a particular antibiotic. While some bacteria may be eradicated with the antibiotic, the ones with the genetic mutation survive and pass along this genetic resistance to their offspring. Subsequent generations of bacteria are then also resistant to the same antibiotic.
Other times the adaptation is unintentionally forced upon the bacteria through evolutionary stress. Repeated exposure to an antibiotic
(by over-prescription, misuse, or through consumption of antibiotic-fed livestock) forces bacteria to adapt in order to survive. Evolution and natural selection lead to the continued survival of only those bacteria able to withstand exposure to the antibiotic.
Why the worry?
Already, there are a number of superbugs causing concern in hospitals, including Methicillin resistant Staphylococcus aureus (MRSA), Vancomycin resistant Enterococcus (VRE), Clostridium difficile (C.dif), and Carbapenem resistant Enterobacteriaceae (CRE), to name a few.
It is estimated that by 2050, around 40% of infections could be resistant to first-line antibiotics. Combined with the lack of new antibiotic development, it may eventually become harder, or even near impossible, to treat even simple infections.
So 90 years after the first patients were treated with antibiotics, bacterial infections may once again become a threat!