Mechanisms of resistance One of the most worrisome mechanisms of resistance involves the ability of bacteria to exclude antimicrobial agents from the cell. Some gram-negative pathogens have an efflux pump that can remove fluoroquinolones fast enough to protect the bacteria from the drug's effects, thus inactivating the antibiotic. Resistance to an antibiotic may also result from a change or alteration in the target site at which the drug acts. Some strains of bacteria alter the protein composition of their outer cell membrane to prevent a drug from reaching its target site. This is a common method of resistance to the penicillins.
Metabolic bypass is another common way in which bacteria can become resistant to antibiotics.
In this case, the antibiotic interferes with the metabolic pathway by blocking or interfering with a critical step. Some resistant bacteria produce enzymes that cleave or alter the molecular structure of an antibiotic, thereby rendering it ineffective. The most common enzymes that mediate bacterial resistance are the beta-lactamases. These enzymes open the beta-lactam ring, a molecular structure that is characteristic of the penicillins and cephalosporins and linked to the activity of both classes of drugs. Thus, the greater the stability of the beta-lactam ring, the less chance there is of resistance developing.