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Utility of the VITEK 2 Advanced Expert System for Identification of Extended-Spectrum Lactamase Production in Enterobacter spp

Utility of the VITEK 2 Advanced Expert System for Identification of Extended-Spectrum Lactamase Production in Enterobacter spp,Mitchell J. Schwaber,Sh

Utility of the VITEK 2 Advanced Expert System for Identification of Extended-Spectrum Lactamase Production in Enterobacter spp   (Citations: 6)
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Accurate identification of extended-spectrum -lactamase (ESBL) production is essential for the appropriate reporting of antimicrobial susceptibility results, since ESBLs render peni- cillins, cephalosporins, and aztreonam inadequate for treat- ment of serious infections (2). While guidelines exist for ESBL detection in Escherichia coli, Klebsiella spp., and Proteus mira- bilis isolates only (2), these enzymes are produced by a wide variety of other gram-negative organisms as well, including Enterobacter spp. (3, 8, 12, 13). The VITEK 2 Advanced Expert System (AES; bioMerieux, Durham, NC) is an automated system that uses the antimicro- bial susceptibility data generated to suggest the phenotype of the tested isolate and thereby determine susceptibility or re- sistance to antibiotics not tested (10). It has been used success- fully to determine ESBL presence in E. coli and Klebsiella spp. (14, 16). Although ESBL detection in Enterobacter spp. by earlier automated systems, including VITEK, was hampered by the production of AmpC -lactamase (17), the more re- cently developed AES has not been evaluated for Enterobacter specifically. Given the high proportion of ESBL production among Enterobacter isolates at our institution and elsewhere (8, 12) and the fact that the AES includes ESBL production among the resistance mechanisms it suggests for Enterobacter isolates, we sought to determine the accuracy of the AES in ESBL detection among clinical isolates of Enterobacter. Forty unique-patient isolates of Enterobacter were included (30 E. cloacae isolates and 10 E. aerogenes isolates). These epidemiologically unrelated isolates were identified phenotyp- ically as ESBL producers via the disk diffusion method estab- lished for E. coli, Klebsiella spp., and P. mirabilis (2, 3), using 30-g cefotaxime-, ceftazidime-, and cefepime-impregnated disks with and without 10 g clavulanic acid (cefotaxime- and ceftazidime-containing disks made by Oxoid (Basingstoke, Hampshire, England); cefepime-containing disks were pre- pared in-house). Although CLSI does not recommend the disk diffusion method for Enterobacter, we and others have previ- ously noted a strong correlation between demonstration of a clavulanic acid effect and the presence of an ESBL gene (3, 12, 15). E. coli ATCC 25922 and Klebsiella pneumoniae ATCC 700603 were used as negative and positive controls for ESBL production, respectively. Susceptibility profiles and suggested resistance mechanisms were recorded by the AES for each isolate. All isolates not identified as ESBL producers by VITEK were subjected to further study in order to demonstrate ESBL presence. Screen- ing for the common ESBL gene families (TEM, SHV, OXA, and CTX-M) was performed by PCR (primers recorded in Table 1). Isolates for which PCR failed to amplify a known ESBL gene were subjected to further phenotypic testing to confirm the presence of a -lactamase able to hydrolyze ceftri- axone and inhibited by clavulanate, i.e., an ESBL. -Lactamase activity of sonicated cells was measured spectrophotometri- cally using 25 M ceftriaxone in 50 mM phosphate buffer (pH 7.0) as the substrate. Inhibition by 2.5 M clavulanate was measured after 5-min preincubation. In addition, these isolates
Published in 2006.
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