Epidemiology and mechanisms of antimicrobial resistance.

Bacterial evolution and the emergence of antimicrobial drug resistance continue to interfere with the successful treatment of infections by both community- and hospital-based physicians. Resistance has emerged to even the newer, most potent antimicrobial agents. Although generalizations can be made about the appropriateness of antimicrobial agents on the basis of published susceptibility patterns, significant regional, demographic, and interinstitutional variables exist that require each hospital to establish its own antibiotic data base and antibiogram. In particular, multiresistant pathogens occur infrequently in acute care community hospitals compared with tertiary care centers. Important clinical factors that promote the emergence of drug-resistant flora include prolonged therapy, the persistence of foreign bodies, sequestra, or prostheses, and the inadequate surgical debridement of necrotic tissue or abscesses. Antibiotic resistance may occur through changes in the permeability of the cell wall or outer membrane, by alteration of the antimicrobial binding or target site, and by inactivation or modification of the drug by bacterial enzymes. These mechanisms are reviewed. In particular, gram-negative beta-lactamases, methicillin-resistant staphylococci, multiresistant enterococci, and the emergence of fluoroquinolone resistance are discussed in detail.