[Extended spectrum beta-lactamases in clinical isolates of Klebsiella pneumoniae].

Resistance of Klebsiella pneumoniae isolates to broad spectrum cephalosporins is mainly attributable to the production of extended-spectrum beta-lactamases. They are plasmid-mediated beta-lactamases derived from TEM or SHV enzymes by mutations that alter the configuration of the active site to expand their spectrum of activity. These enzymes are ineffective against cephamycins with an alpha-methoxy substituent in the C7 position such as cefoxitin, cefotetan and moxalactam. They can be distinguished by determination of isoelectric point, substrate profile, sensitivity to inhibitors, molecular weight and amino acid sequence. Extended spectrum beta-lactamases are divided into four groups: TEM, SHV, ampC derivates and unclassified beta-lactamases. Multiple resistance to beta-lactams is often accompanied by resistance to aminoglycosides, sulphonamides, tetracyclines, chloramphenicol and trimethoprim. Besides plasmid-mediated beta-lactamases hyperproduction of chromosomal beta-lactamases can be responsible for the resistance to the new cephalosporins in some strains of Klebsiella pneumoniae. Chromosomal resistance is intrinsic and is not transferable.