The production of beta-lactamases is the most common cause of antimicrobial resistance in gram-negative bacilli. Two beta-lactamases, extended-spectrum beta-lactamases (ESBLs) and inducible beta-lactamases (IDBLs), are presently seen with increasing frequency. Treatment of severe infections due to gram-negative bacilli producing these beta-lactamases is problematic. In vitro susceptibility testing of ESBL-producing organisms remains difficult to interpret; clinical isolates sensitive to a particular antimicrobial agent at standard inoculum may become resistant when a higher inoculum is employed. Laboratory detection of IDBL-producing organisms is not yet commercially available. Due to the lack of sufficient outcome data, clinicians often have no choice but to use carbapenems empirically for the treatment of severe infections caused by organisms suspected of producing these enzymes. The widespread empiric use of carbapenems is of concern and may potentially precipitate increased bacterial resistance to this class of antimicrobial agents.
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