Beyond Susceptible and Resistant, Part II: Treatment of Infections Due to Gram-Negative Organisms Producing Extended-Spectrum β-Lactamases

The production of β-lactamase is the most common mechanism of resistance to β-lactam antibiotics among gram-negative bacteria. Extended-spectrum β-lactamases (ESBLs) are capable of hydrolyzing most penicillins, extended-spectrum cephalosporins, and aztreonam, but their activity is suppressed in the presence of a β-lactamase inhibitor. Serious infections with ESBL-producing isolates are associated with high rates of mortality, making early detection and adequate medical management essential to ensure optimal patient outcomes. Much controversy has centered on the recommendations for testing and reporting of antibiotic susceptibility of potential ESBL-producing organisms. The latest version of the Clinical Laboratory Standards Institute (CLSI) susceptibility reporting guidelines, published in 2010, no longer advocates for phenotypic testing of ESBL-producing isolates. From newer studies demonstrating a correlation between organism minimum inhibitory concentration (MIC) and clinical outcome, along with pharmacokinetic/pharmacodynamic (PK/PD) modeling demonstrating the importance of the MIC to achieving therapeutic targets, the CLSI has assigned lower susceptibility breakpoints for aztreonam and most cephalosporins. The new guidelines recommend using the lower MIC breakpoints to direct antibiotic selection. This article reviews the microbiology and epidemiology of ESBLs, the recent change in CLSI susceptibility reporting guidelines for ESBLs, and the clinical and PK/PD data supporting the relationship between in vitro susceptibility and clinical outcome. Finally, considerations for antimicrobial selection when treating patients with infections caused by ESBL-producing organisms from various sources are discussed.