Use of Microdilution Panels with and without β-Lactamase Inhibitors as a Phenotypic Test for β-Lactamase Production among Escherichia coli , Klebsiella spp., Enterobacter spp., Citrobacter freundii , and Serratia marcescens

Abstract

Over the past decade, a number of new β-lactamases have appeared in clinical isolates of Enterobacteriaceaethat, unlike their predecessors, do not confer β-lactam resistance that is readily detected in routine antibiotic susceptibility tests. Because optimal methodologies are needed to detect these important new β-lactamases, a study was designed to evaluate the ability of a panel of various β-lactam antibiotics tested alone and in combination with β-lactamase inhibitors to discriminate between the production of extended-spectrum β-lactamases, AmpC β-lactamases, high levels of K1 β-lactamase, and other β-lactamases in 141 isolates of Escherichia coli,Klebsiella pneumoniae, Klebsiella oxytoca,Enterobacter cloacae, Enterobacter aerogenes,Citrobacter freundii, and Serratia marcescens possessing well-characterized β-lactamases. The microdilution panels studied contained aztreonam, cefpodoxime, ceftazidime, cefotaxime, and ceftriaxone, with and without 1, 2, and 4 μg of clavulanate per ml or 8 μg of sulbactam per ml and cefoxitin and cefotetan with and without 8 μg of sulbactam per ml. The results indicated that a minimum panel of five tests would provide maximum separation of extended-spectrum β-lactamase high AmpC, high K1, and other β-lactamase production in Enterobacteriaceae. These included cefpodoxime, cefpodoxime plus 4 μg of clavulanate per ml, ceftazidime, ceftriaxone, and ceftriaxone plus 8 μg of sulbactam per ml. Ceftriaxone plus 2 μg of clavulanate per ml could be substituted for cefpodoxime plus 4 μg of clavulanate per ml without altering the accuracy of the tests. This study indicated that tests with key β-lactam drugs, alone and in combination with β-lactamase inhibitors, could provide a convenient approach to the detection of a variety of β-lactamases in members of the family Enterobacteriaceae.

Cefotaxime-resistant Citrobacter freundii in isolates from blood in a tertiary teaching hospital in Northern Taiwan

“…15 Clavulanate's failure to lower MICs in ESBL producers has been hypothesized to relate in large part to its ability to induce the AmpC type b-lactamase. 16 With over 200 known ESBLs, DNA sequencing remains the gold standard for detection and identification of specific types. Both the SHV-12 and CTX-M-27 genes in our isolates were carried in plasmids.…”

New -Lactamases in Gram-Negative Bacteria: Diversity and Impact on the Selection of Antimicrobial Therapy

“…It is quite likely that an ESBL will not be identified when cefotaxime is tested against an organism with a strong clavulanate-insensitive, cefotaxime-hydrolyzing group 1 cephalosporinase that is produced together with a weak clavulanate-sensitive, cefotaximehydrolyzing ESBL. In addition, if cefpodoxime is used in the ESBL screening test, false-positive results based upon an initial MIC of у2 mg/mL frequently are seen [30] because of high rates of cefpodoxime hydrolysis by clavulanate-insensitive group 1 cephalosporinases, as well as by ESBLs.…”

DETECTION OF EXTENDED-SPECTRUM -LACTAMASE ENZYMES (ESBLs) PRODUCED BY ESCHERICHIA COLI URINARY PATHOGENS AT ASSIUT UNIVERSITY HOSPITAL

“…Thus, the presence of an ESBL can be masked by the expression of an AmpC-type enzyme in the same strain (by masking any synergy with ESBLs) 40 . As a result, when testing for ESBL production in these organisms, we should consider using sulfones such as tazobactam and sulbactam as β-lactamase inhibitors 41 . Although the exact reasons are not completely understood, the pressure to use non-β-lactam antibiotics may be responsible for the coexistence of ESBL and AmpC.…”