Evaluation of Etest MBL for Detection of bla _IMP-1 and bla _VIM-2 Allele-Positive Clinical Isolates of Pseudomonas spp. and Acinetobacter spp

Abstract

The Etest MBL (AB BIODISK, Solna, Sweden) correctly differentiated all 57 isolates of Acinetobacter spp. and Pseudomonas aeruginosa with the bla IMP-1 allele and 135 of 137 (98.5%) Acinetobacter spp. and Pseudomonas spp. isolates with the bla VIM-2 allele. The Etest MBL was reliable for detecting the IMP-1-and VIM-2-producing Pseudomonas and Acinetobacter isolates.The IMP-and VIM-type and other acquired class B metallo-␤-lactamase (MBL)-producing gram-negative bacilli have been increasingly isolated from clinical specimens (10, 15). Among the imipenem-nonsusceptible isolates, 62.1% of the 58 Pseudomonas aeruginosa isolates had a VIM-type MBL in a Greek study (3), while 11.4% of 387 Pseudomonas spp. isolates and 14.2% of 267 Acinetobacter spp. isolates had either an IMP-or VIM-type MBL in a Korean study (7).MBL can hydrolyze ␤-lactams from all classes except the monobactams (1). Higher mortality has been reported in patients infected with the IMP-1-producing strains (5). Therefore, the reliable detection of the MBL-producing strains is essential for the optimal treatment of infected patients and to control the nosocomial spread of resistance. However, the current NCCLS document (12) does not contain any method for detecting MBL.MBL-producing strains can be detected with double-disk synergy tests (8, 13) and a microdilution method (11). While the former method is only qualitative, the latter is not simple for routine testing. Walsh et al. (16) reported that the Etest MBL strip (AB BIODISK, Solna, Sweden) had 100% sensitivity and specificity, but their findings were based on testing only 12 IMP-1-type strains and did not include the prevalent VIM-2-type strains. The performance of Etest MBL was influenced by the choice of the Mueller-Hinton agar brand (16) but may have also been influenced by the level of imipenem resistance of the MBL-producing strains (17, 18) and possibly by other testing conditions.In this study, the performance of Etest MBL was evaluated with 199 bla VIM-2 or bla IMP-1 allele-positive isolates, and the factors that may influence the routine use of the test were also investigated.( Strains. The strains were isolated from the clinical specimens at Korean hospitals between 1995 and 2003, and imipenem susceptibility was tested with the disk diffusion method (12) using 10-g disks and Mueller-Hinton agar (Becton Dickinson, Cockeysville, Md.). MBL screening was tested with the Hodge test and a double-disk synergy test (6,8). The bla IMP-1 and bla VIM-2 alleles were detected by PCR (9). The isolates, which were imipenem resistant but MBL negative by the screening tests and PCR, were used as the negative controls. The test strains were stored at Ϫ76°C until used for the study.Evaluation of Etest MBL. The evaluations were performed at two laboratories, one in Korea and the other in Sweden. The Swedish laboratory used the strains that were transported in semisolid cystine trypticase agar (Becton Dickinson) vials at ambient temperature. Several colonies from a 24-h culture plate were used to prepare the ...

Evaluation of Phenotypic Tests for Detection of Metallo-β-Lactamase-Producing Pseudomonas aeruginosa Strains in China

“…Our results were in accordance with those obtained by Berges et al but not with those obtained by Picão et al (2,15). Most previous studies evaluating MBL phenotypic detection were performed under distinct experimental conditions, jeopardizing comparison of their results (6,11,14). The sizes of inhi- bition zones produced by ␤-lactam-IMBL combinations may differ according to the way that IMBL is incorporated into the ␤-lactam disks (1).…”

“…In the current study, we added the IMBL solutions directly on ␤-lactam disks already placed on the agar plate, as described by Picão et al, whereas some authors first prepare and freeze IMBL-␤-lactam disks; thus, the results of our CD assay may be comparable to those of studies using the same methodology (1,15,25). It has been suggested that the selection of the optimal MBL screening method be based not only on bacterial species but also on the strains collected and the local prevalence of MBL producers (11,15).…”

“…However, due to the high cost and unavailability of Etest strips, many clinical microbiology laboratories use alternative screening methods, such as the double-disk synergy test (DDST) and the combined disk (CD) assay. Although the DDST and the CD assay are simple to perform and cheaper than the MBL Etest, they have shown discordant results, depending on the employed methodology, ␤-lactam substrates, MBL inhibitors (IMBL), and bacterial genus tested (7,11,14,15). Standardization of a phenotypic method for screening for MBL-producing isolates is of crucial importance.…”

Metallo-β-Lactamase Detection: Comparative Evaluation of Double-Disk Synergy versus Combined Disk Tests for IMP-, GIM-, SIM-, SPM-, or VIM-Producing Isolates

“…isolates (11,14,17,21,25); (v) a lack of results stratified according to pathogen/species, in which the SN and SP values reflect the overall performance of all isolates (2,14,17); and (vi) accurate statistical analysis usually is not carried out to interpret and validate their results (2,11,21,37,38). This is the first study to assess the accuracy of phenotypic methods to detect all of the major types of mobile MBLs described (GIM, IMP, SIM, SPM, and VIM) that are produced by diverse bacterial genera with distinct imipenem susceptibility patterns.…”

“…Although they are simple to perform and cheaper than genotypic methods, they have shown discordant results depending on the employed methodology, ␤-lactam substrates, MBL inhibitors (IMBL), and bacterial genus tested (11,14,17,21,26). In addition, SPM-, GIM-, and SIM-producing pathogens rarely have been evaluated by these studies.…”