New beta-lactamases in gram-negative bacteria: Diversity and impact on theselection of antimicrobial therapy

Of the 340 discrete beta -lactamases that have been identified, the most im portant groups of enzymes that are continuing to proliferate include the pl asmid-encoded cephalosporinases, the metallo-beta -lactamases, and the exte nded-spectrum beta -lactamases. Resistance to specific beta -lactam-contain ing antimicrobial agents frequently can be traced to a single beta -lactama se, but this task is becoming more difficult for the clinical microbiology laboratory. Other factors, such as multiple beta -lactamase production, tra nsferable multidrug-resistance genes, alterations in outer-membrane porins, and possible antibiotic efflux, all may contribute to a resistance phenoty pe. Appreciation of these factors may help the physician make a more inform ed decision when choosing therapy to try to avoid selection of even more pa thogenic strains.