Cephalosporin MIC Distribution of Extended-Spectrum-β-Lactamase- and pAmpC-Producing Escherichia coli and Klebsiella Species

Abstract

The acquisition of ␤-lactamases in members of the Enterobacteriaceae family poses a challenge to antimicrobial susceptibility testing in the clinical laboratory. We correlated the distribution of the MICs for Klebsiella spp. and Escherichia coli with the presence of extended-spectrum ␤-lactamase (ESBL) and plasmid-mediated AmpC ␤-lactamase (pAmpC) genes. A total of 264 isolates were subjected to cefazolin, ceftriaxone, cefotaxime, ceftazidime, cefepime, and aztreonam agar dilution MIC determination; ESBL screening and confirmatory testing by the methods of the Clinical and Laboratory Standards Institute (CLSI); and for isolates for which the MICs of extended-spectrum cephalosporins were >1 g/ml or the MICs of cefpodoxime were >4 g/ml, PCR amplification and sequencing of the ESBL and pAmpC genes. PCR was positive for 73/81 isolates (45 isolates with an ESBL gene alone, 24 isolates with a pAmpC gene alone, with 4 isolates with both genes). Compared to PCR, confirmatory testing by the CLSI method yielded a sensitivity and a specificity of 98.0 and 96.3%, respectively; there were six false-positive results and one false-negative result. No distinction in the MIC distribution was apparent between isolates with the ESBL gene and isolates with the pAmpC gene. A substantial percentage of the isolates with PCR-confirmed ESBL and/or pAmpC genes fell within the current CLSI susceptible category. For a ceftazidime, ceftriaxone, or cefotaxime MIC of >2 g/ml, a dichotomy existed between isolates with and without ESBL and pAmpC genes in most cases. This suggests that the presence of the ESBL and the pAmpC enzymes may yield similar MICs of extended-spectrum cephalosporins, many of which fall within the current nonresistant categories. Lowering of the current CLSI breakpoints for cephalosporins appears to be warranted.

Impact of Revised Broad-Spectrum Cephalosporin Clinical and Laboratory Standards Institute Breakpoints on Susceptibility inEnterobacteriaceaeproducing AmpC β-Lactamase

“…A recent study showed that when the revised CLSI breakpoints were applied to 264 isolates of E. coli and Klebsiella species, the rates of susceptibility to cefotaxime and ceftazidime were reduced by 8% and 3%, respectively [11]. Similarly, we found that the revised susceptibility breakpoint criteria led to little change in the rates of susceptibility of cefotaxime (0.9%) and ceftazidime (3.0%) to Enterobacteriaceae producing AmpC β-lactamase.…”

Detection of extended spectrum beta-lactamases-producing isolates and effect of AmpC overlapping

“…According to the Clinical Laboratory Standards Institute (CLSI) recommendations, routine antimicrobial susceptibility testing should include screening for ESBL production, employing cefpodoxime, ceftazidime, aztreonam, cefotaxime, or ceftriaxone followed by phenotypic confirmation for the positive cases, based on demonstrating the effectiveness of the screening antibiotic in the presence of a beta lactamase inhibitor [11,12,13]. Results are issued with the aim of preventing inappropriate use of cephalosporins or monobactams in the setting of ESBL production [12].…”

The Use of Noncarbapenem β-Lactams for the Treatment of Extended-Spectrum β-Lactamase Infections

“…The current European Committee on Antimicrobial Susceptibility Testing (EUCAST) and Clinical and Laboratory Standards Institute (CLSI) susceptibility breakpoints for cefepime are 1 µg/mL and 8 µg/mL (accounting for drug dosing), respectively [22,23]. There is concern that the current CLSI cefepime breakpoint leaves some ESBL enzymes in the susceptible range (ie, "hidden resistance)" [24].…”