Extended-spectrum beta-lactamases.

The first beta-lactamase was identified in an isolate of Escherichia coli in 1940. To date, there are >130 TEM-type and >50 sulfhydryl variable (SHV)-type beta-lactamases, mainly in E. coli, Klebsiella pneumoniae, and Proteus mirabilis but also in other members of the Enterobacteriaceae family and in some nonenteric organisms, such as Acinetobacter species. The incidence of expanded-spectrum beta-lactamases (ESBLs) varies, depending on which area of the globe the isolates originate from. ESBLs render the oxyimino-cephalosporins ineffective, and ESBL-producing organisms frequently also possess resistance factors to other classes of antibiotics, such as aminoglycosides and fluoroquinolones, and possibly also piperacillin-tazobactam and cefepime. These results suggest that microbiology laboratories should routinely test for the presence of these strains among their isolates and that the antibiotics of choice for infections believed to be caused by these types of organisms are the carbapenems.