Detection of vancomycin resistance in enterococci by the Alamar MIC system

Zabransky, R.J.; DiNuzzo, Anthony R.; Woods, Gail L.

doi:10.1128/jcm.33.4.791-793.1995

Cited by 20 publications

(17 citation statements)

References 9 publications

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“…Several studies have evaluated the abilities of commercial and reference antimicrobial susceptibility testing methods to detect vancomycin resistance in Enterococcus isolates (3,(9)(10)(11)(12)(13)(14)(15)(16). Those studies which have evaluated current versions of automated or semiautomated commercial systems, such as the Vitek system (bioMerieux, Inc.), the MicroScan Rapid system (Baxter Health Care Corp., West Sacramento, Calif.), or the Alamar MIC system (Alamar Biosciences, Sacramento, Calif.), have shown that these methods do not reliably detect vancomycin resistance in enterococci (11,13,15,16). Reasons for the poor performance of these systems likely include the composition of growth media and the relatively short incubation times that are used.…”

Section: Discussionmentioning

confidence: 99%

“…One limitation of all of the above studies may be the inconsistent definition for the "gold standard" or reference susceptibility result. Some investigators have compared test results of the study method to a broth dilution result (11,13), while others have compared the results of the study method to results obtained by using either agar dilution (9,14), disk diffusion (15), the Micro Scan Rapid method (15,16), or E-test method (12) or by comparing results to the vancomycin resistance genotype (3,10,11). Furthermore, for two of these studies, evaluations were limited to Enterococcus isolates with high-level vancomycin resistance (3,12).…”

Section: Discussionmentioning

confidence: 99%

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Comparison of agar dilution, broth microdilution, E-test, disk diffusion, and automated Vitek methods for testing susceptibilities of Enterococcus spp. to vancomycin

et al. 1997

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An evaluation was undertaken to determine the optimal method for testing the susceptibilities of 100 clinical isolates and two reference strains of Enterococcus spp. to vancomycin in vitro. Six testing methods were studied by using the following media and incubation times: agar screen with the Synergy Quad Plate (Remel, Lenexa, Kans.), an in-house-prepared brain heart infusion (BHI) agar plate, and an in-house-prepared Mueller-Hinton (MH) agar plate, all incubated for 24 or 48 h; broth microdilution (Sensititre Just One Strip; AccuMed International, Inc. , West Lake, Ohio) with BHI or cation-adjusted MH broth incubated for 24 or 48 h; agar dilution with BHI or MH agar incubated for 24 or 48 h; epsilometer test (E test; AB BioDisk, Solna, Sweden) with BHI or MH agar incubated for 24 or 48 h; disk diffusion with BHI or MH agar incubated for 24 or 48 h; and the automated Vitek method with the gram-positive susceptibilityStaphylococcus aureus card and R02.03 software (bioMerieux, Inc., Hazelwood, Mo.). Growth failures occurred with MH media (n ‫؍‬ 6) but not with BHI media. One growth failure occurred with the Vitek method. Results for each testing method for each Enterococcus strain were interpreted as susceptible, intermediate, or resistant according to current National Committee for Clinical Laboratory Standards (NCCLS) criteria and compared to the vancomycin resistance genotype (i .e., vanA, vanB, vanC-1, or vanC-2/3). For all methods, extension of the incubation time from 24 h to 48 h either produced no difference in the results or gave poorer results. The following methods produced no very major or major interpretive errors: broth microdilution with BHI media incubated for 24 h, agar dilution with BHI media incubated for 24 or 48 h, and E test with BHI media incubated for 24 or 48 h. Unacceptable frequencies of very major errors (>1%) occurred with all methods for which MH media were used. Minor interpretive errors were frequent with all methods. These minor interpretive errors also occurred most frequently with Enterococcus strains with vanC genes, which encoded low-level vancomycin resistance (MIC < 8 g/ml), as opposed to Enterococcus strains which possessed vanA or vanB genes, which encoded higher-level vancomycin resistance (MIC > 64 g/ml). Modification of NCCLS breakpoints, especially for motile Enterococcus spp. (E. casseliflavus, E. flavescens, and E. gallinarum), may resolve this problem; however, in the current study, one E. faecalis strain and one E. faecium strain carried only the vanC gene. The agar screen method may also require reformulation. The current agar screen plate contains 6 g of vancomycin per ml, which may not detect all low-level resistance associated with vanC genotypes. Nevertheless, the clinical significance of this low-level vancomycin resistance remains unknown.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Comparison of agar dilution, broth microdilution, E-test, disk diffusion, and automated Vitek methods for testing susceptibilities of Enterococcus spp. to vancomycin

et al. 1997

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“…Vancomycin resistance in enterococci has recently become a clinical problem due to a lack of alternative therapies (4,5,14,19). Three major phenotypes of vancomycin-resistant enterococci have been described (12,28). These are the vanA strains, which show high-level vancomycin resistance (MICs of Ͼ32 g/ml) and resistance to teicoplanin; vanB strains, which have variable resistance to vancomycin (MICs of 4 to Ն128 g/ml) and susceptibility to teicoplanin; and vanC motile strains, which show intrinsic resistance to low levels of vancomycin (MICs of 2 to 32 g/ml) and susceptibility to teicoplanin.…”

Section: Discussionmentioning

confidence: 99%

“…Automated systems, including the Vitek and MicroScan systems, have been developed to identify and to determine the antimicrobial susceptibility of enterococci (1,2,18,22,23,(25)(26)(27)(28). Previous studies have shown conventional MicroScan panels to be reliable in the identification of Enterococcus spe-cies, even though the data bank includes only E. faecalis, E. faecium, E. durans, and E. avium (24).…”

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confidence: 99%

Revised approach for identification and detection of ampicillin and vancomycin resistance in Enterococcus species by using MicroScan panels

et al. 1996

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The frequency of antimicrobial agent-resistant enterococci is increasing, making accurate identification and screening for susceptibility essential. We evaluated the ability of MicroScan Positive Breakpoint Combo Type 6 panels (Dade MicroScan Inc., West Sacramento, Calif.) to identify Enterococcus species and to detect ampicillin and vancomycin resistance. A total of 398 well-characterized Enterococcus isolates from two institutions were inoculated into MicroScan panels, into conventional biochemical assays, and into ampicillin and vancomycin agar dilution media. Resistance was verified by the broth macrodilution method. MicroScan panels accurately detected resistance to ampicillin in 132 of 132 enterococcal isolates, while three isolates for which the MICs were <16 g/ml were classified incorrectly by MicroScan panels as resistant. No beta-lactamaseproducing enterococci were detected. All 64 isolates showing resistance to vancomycin (MICs >32 g/ml) were correctly classified by MicroScan panels. Seven isolates for which the vancomycin MICs were 8 and 16 g/ml were incorrectly classified as susceptible by MicroScan panels, while eight isolates for which the MICs were 4 g/ml were incorrectly labeled as intermediate. Fourteen of these 15 isolates were subsequently identified as motile enterococci. Overall, there were three major errors in susceptibility testing for ampicillin and 15 minor errors for vancomycin. Conventional testing confirmed the identity of 181 Enterococcus faecalis isolates, 157 E. faecium isolates, and 60 isolates of other species; however, 56 of these 60 isolates were misidentified by the MicroScan panels. After recognition of this problem, a reviased approach which included tests for pigment, motility, and sucrose fermentation was devised. In combination with these additional assays, the conventional MicroScan panels accurately identified the 56 originally misidentified isolates. In summary, the ability of MicroScan panels to detect vancomycin and ampicillin resistance in enterococci was confirmed. Our study found that the inability of MicroScan panels to identify enterococci other than E. faecalis and E. faecium can be compensated for by the addition of standard assays.

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“…The system can be used for susceptibility testing of gram-positive and gram-negative bacteria and for yeasts. It has been evaluated for gram-negative bacteria by Baker et al (3), for yeasts by Pfaller and colleagues (14,15), and for enterococci (vancomycin only) by Tenover et al (18) and Zabransky et al (22). The evaluations for gram-negative bacteria and yeasts indicated that the Alamar method is a satisfactory alternative to conventional MIC methods (3,14,15).…”

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confidence: 99%

Evaluation of Alamar colorimetric broth microdilution susceptibility testing method for staphylococci and enterococci

Baker

Tenover

1996

J Clin Microbiol

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We compared the results of the Alamar broth microdilution susceptibility testing method with the results of the National Committee for Clinical Laboratory Standards reference broth microdilution method for 119 gram-positive organisms. The strains were tested for their susceptibilities to 20 antimicrobial agents. Only appropriate antimicrobial agents were evaluated for each species of bacteria. Absolute categorical agreement between the reference method and the test method was 91.5% for enterococci, 99.8% for oxacillin-susceptible staphylococci, and 97.4% for oxacillin-resistant staphylococci. Essential agreement (percent complete agreement plus percent minor errors) was >99% for all organisms tested. The results for enterococci showed no very major errors, one major error with ofloxacin, and numerous minor errors with the quinolones. However, all except one of the minor errors were within ؎1 log 2 dilution of the reference result. For staphylococci, only 2 very major errors (one each with chloramphenicol and oxacillin), 1 major error (chloramphenicol), and 15 minor errors (multiple drugs) were observed. The Alamar colorimetric system was easy to use and the results were easy to read. It appears to be an acceptable method for antimicrobial susceptibility testing of staphylococci and enterococci.

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Detection of vancomycin resistance in enterococci by the Alamar MIC system

Cited by 20 publications

References 9 publications

Comparison of agar dilution, broth microdilution, E-test, disk diffusion, and automated Vitek methods for testing susceptibilities of Enterococcus spp. to vancomycin

Comparison of agar dilution, broth microdilution, E-test, disk diffusion, and automated Vitek methods for testing susceptibilities of Enterococcus spp. to vancomycin

Revised approach for identification and detection of ampicillin and vancomycin resistance in Enterococcus species by using MicroScan panels

Evaluation of Alamar colorimetric broth microdilution susceptibility testing method for staphylococci and enterococci

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