Many bacteria acquire resistance to one or more of the antibiotics to which they were formerly susceptible. Bacteria develop resistance by acquiring genes encoding proteins that protect them from the effects of the antibiotic. The genes are often found on plasmids which spread easily from one bacterium to another - even from one species of bacterium to another. These enzyme break the beta-lactam ring at the position shown with the green arrow in the diagram of penicillin G . Likewise synthesis of cephalosporinases defeats the cephalosporins .
Some bacteria modify their DNA gyrase thus defeating quinolones .
Bacteria may modify the structure of their peptidoglycan wall and thus avoid the inhibitory effects of antibiotics like cycloserine . These multidrug-resistant strains are particularly common in hospitals where antibiotic use is heavy, and the patients often have weakened immune systems. How transposons contribute to multidrug resistance. The figure illustrates the simplest method of the several available for measuring antibiotic resistance. Disks of several different antibiotics are pressed on the surface of the agar.