Hi
Mutations are responsible for increasing the antibiotic resistance of some bacterial strains.
As we know that antibiotics are the drugs that target specific strain or type of bacteria, and kill them. These are designed against specific disease causing gene of that bacteria strain. However, as the genome of bacteria is very small in size, and bacteria undergo hundreds of multiplication events in very small span of time. Therefore, upon replicating, there occur chances of mutations in genetic sequence of bacteria.
When the genetic sequence of bacteria is changed even by one base, the drug wont be able to target and kill that strain of bacteria. And researchers need to design new antibiotic against new strains of bacteria.
Antibiotics resistance has become huge issue across the world as it takes alot of time (years) to design and approve an antibiotic.
Hope it help!