“What causes resistance to antibiotics where the drug previously was effective in treating the bacterial infection?”
Exactly as MEno said .
Basically there are some mutations that allow Bacteria to withstand Abx. for example E coli can generally be killed by Beta LActam abx ( such as Cephlasporins ) However some have a a gene codign for an enzyme Beta-lactamase that destroys the antibiotic. These were likely always around but not in sufficent number to cause harm. However with the overuse of antibiotics these ESBL producing organisms Ie resitent ones have been selected for and now are more prevelant to the point where they can and do cause significant harm.
Basically Survival of the fittest
“What can be done to avoid bacteria mutating to resist antibiotics?
Exactly as Meno said “USe less antibiotics” This is the responsibilty of both providers and patients. Patients oftne feel deprived when told cold was likely viral and didnt require antibiotics, and they waited (and paid!!!) for “nothing” IF you trust your doctor, and they say you dont need antibiotics dont insist on some.
If you dont trust your doctor then get a new one.
The other thing that can help is completing the ocurse of antibiotics. While this seems contradictory to the first piece of advice, it is believed that not completing a course might leave behind some microbes that arent fully eradicated and allow for these bacteria to proliferate