[Mechanisms underlying resistance of the bacteria responsible for nosocomial infections].

Resistance of nosocomial bacteria to antimicrobials is due to both natural and acquired factors. Production of inactivating enzymes (beta-lactamases, aminoglycoside detoxifying enzymes) is the most common mechanism in Gram-negative bacteria. The concomitant presence of other mechanisms such as abnormalities in the targets (for fluoroquinolones), permeability, or efflux of antimicrobials, explains why many nosocomial strains are resistant to multiple drugs. In Gram-positive bacteria, the main mechanism involves modifications in the bacterial targets of antimicrobials; for instance, changes in penicillin-binding proteins lead to penicillin resistance in enterococci and to methicillin resistance in staphylococci, whereas changes in peptidoglycans result in glycopeptide resistance in enterococci. As is the case for Gram-negative bacteria, the concomitant presence of other mechanisms is common, and as a result many nosocomial strains are multiresistant.