Beta-lactamase-mediated resistance and opportunities for its control.
Clinical use of beta-lactams has selected for beta-lactamase-producing organisms. Numerous beta-lactamases are known, and sequencing allows them to be divided into four Classes, A to D, with Classes A and C being the most important. Pharmaceutical chemists have responded to the spread of beta-lactamase-producing organisms by developing stable agents and inhibitors. Stability in penicillins and cephalosporins is achieved by attaching a bulky substituent to the amino group of 6-aminopenicillanic acid or 7-aminocephalosporanic acid, or by replacing the hydrogen on carbon 6 (penicillins) or 7 (cephalosporins) with an alpha-methoxy group. In carbapenems, stability is achieved by incorporation of a simple trans-6-hydroxyethyl group. Beta-lactamase-inhibitory activity occurs in many beta-lactam classes but only clavams and penicillanic acid sulphones have been developed specifically as beta-lactamase inhibitors. These inhibit most Class A and some Class D enzymes but act poorly against Class B and C enzymes. Their success is affected by the amount of enzyme, the permeability of the bacterial cell wall, the partner beta-lactam and the pH. Piperacillin/tazobactam, which combines a good inhibitor of Class A enzymes with a broad-spectrum, easily-protected penicillin, has wide activity against common pathogens, the major exceptions being strains of Enterobacter, Serratia and Citrobacter freundii that produce large amounts of Class C enzymes, and Gram-positive cocci with modified penicillin-binding proteins. Beta-lactamase-stable beta-lactams and inhibitor combinations overcome many existing resistance mechanisms but are themselves selecting new resistances. Few new beta-lactams able to overcome these resistances are advanced in development and consequently the opportunities for control lie mostly in the more prudent use of compounds already available.
Central Public Health Laboratory, London, UK. d.m.livermore@mds.amw.ac.uk
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D000900:Anti-Bacterial Agents / Q000737:chemistry* / Q000494:pharmacology; D001419:Bacteria / Q000187:drug effects / Q000201:enzymology*; D018440:beta-Lactam Resistance; D065093:beta-Lactamase Inhibitors; D001618:beta-Lactamases / Q000378:metabolism*; D047090:beta-Lactams
D000900:Anti-Bacterial Agents; D065093:beta-Lactamase Inhibitors; D047090:beta-Lactams; D001618:beta-Lactamases