|Extended-Spectrum Beta-Lactamase (ESBL) Activity|
Illustrated in the image is the surface of a disk diffusion test including a 30 mg ceftazidime disk (left) and a combintation 30/10 mg ceftazidime/clavulanic acid disk (right). Observe in the photograph that the zone of inhibition around the the combination ceftazidime/clavulanic acid disk (right) is at least 5 mm larger than around the clavulanic acid disk (left). This observation that the presence of clavulanic acid, a beta-lactamase inhibitor, has resulted in such a large increase in the zone of inhibition indicates that an extended-spectrum beta-lactamase (ESBL)is being produced. Additionally, the Clinical and Laboratory Standards Institute (CLSI) Performance Standards for Antimicrobial Testing Standards, published January 2011, proposes in the M100-S21document, table 2A-S1, that cefotaxime (30 mg) and cefotazime-clavulanic acid (30/10 mg) testing per performed alone AND in combination with the ceftazidime and ceftazidime/clavulanic acid testing previously described. When an organism is producing an ESBL, the susceptibility to individual cephalosporins cannot be predicted, thus requiring that each drug must be tested individually.
|Extended-Spectrum Beta-Lactamases (ESBLs) Review|
Clinical and Laboratory Standards Institute (CLSI). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard-Eighth Edition. CLSI document M07-A8. Wayne, PA: 2009. Clinical and Laboratory Standards Institute (CLSI). Performance Standards for Antimicrobial Susceptibility Testing; Twenty-First Informational Supplement. CLSI document M100-S21. Wayne, PA: 2011. It is important to detect ESBL-producing stains of Klebsiella pneumoniae, K. oxytoca, Escherichia. coli, and Proteus mirabilis as treatment failure may occur if the wrong cephalosporin is selected. These enzymes have the ability to hydrolyze third-generation cephalosporins and aztreonam, but are inhibited by clavulanic acid. ESBL-producing organisms can also exhibit co-resistance to many other classes of antibiotics. In January 2010, the Clinical and Laboratory Standards Institute (CLSI) published revised cephalosporin (cefazolin, cefotaxime, ceftazidime, ceftizoxime, and ceftriaxone) and aztreonam interpretive criteria for Enterobacteriaceae. The criteria for cefepime and cefuroxime (parenteral) did not change. Laboratories using these new criteria detailed in the M100-S21 document, table 2A, published in January 2011, no longer need to routinely test for extended-spectrum beta-lactamases (ESBLs) prior to reporting results. IF using the new criteria, there is no longer a need to change the interpretive criteria for cephalosporin's, aztreonam or penicillin's from susceptible to resistant before reporting. IF reporting moxalactam, cefonicid, cefamandole, or cefoperazone for E. coli, Klebsiella, or Proteus species, ESBL testing should still be performed as outlined in CLSI document M100-S21, supplemental table 2A-S1. If these isolates test ESBL positive, they should be reported as resistant. These drugs have limited availability in many countries, so the interpretive criteria were not evaluated by CLSI. CLSI notes that ESBL testing could still be useful for epidemiology and infection control purposes. ESBL testing should also continue to be performed until the new CLSI interpretive criteria is implemented.