The epidemiology of beta-lactamases.

Chromosomally mediated species-specific beta-lactamases, as well as plasmid-mediated beta-lactamases, contribute to bacterial resistance to beta-lactam antibiotics. Chromosomally mediated enzymes confer primary resistance to some drugs and secondary resistance, by mutation to over-production of the enzyme. By far the most prevalent and most important of the, more than thirty, described plasmid-mediated beta-lactamases are those of the TEM group. They can be found in nearly all Gram-negative bacterial species of clinical importance. Furthermore these enzymes have changed their specificity by mutation so that recently described TEM enzymes hydrolyse even third generation cephalosporins. Although there is no change in the quantity of these enzymes, as far as the percentage of producer strains in any species is concerned, there has been a change in quality. The enzymes are further distributed to new species and hydrolyse more so-called 'stable' beta-lactam compounds.