Such bacteria become resistant to the antibiotics by virtue of such inactivating enzymes. The natural antibiotics, i.e. those produced by different organisms, sometimes do not possess all the desirable properties. They have to be chemically modified to give these properties. Such modified antibiotics are called semi-synthetic ones.
Antibiotics usually kill bacteria, but in this case the mutation means the bacteria cannot be destroyed by the antibiotic. This evolution of antibiotic resistance in bacteria is an example of.