The first step in the emergence of resistance is a genetic change in a bacterium. There are two ways that can happen. Many antibiotics work by inactivating an essential bacterial protein.
Genetic change can remove that protein.
Also, mutations in the target protein can prevent the antibiotic from binding or it if does bind, prevent it from inactivating the target protein. Genetic change can also lead to increased production of the antibiotic s target enzyme so that there are too many of them and the antibiotics cannot inactivate them all. Alternatively, the bacterium may produce an antibiotic-inactivating enzyme . As well, the bacterium may alter the permeability of its cell membrane, or wall to the antibiotic. The second way for a bacterium to gain resistance is for an existing antibiotic-resistant gene to transfer from one bacterium to another bacterium.