Detection of vancomycin resistance in enterococci by the Alamar MIC system

The Resazurin Microplate Method for Rapid Detection of Vancomycin Resistance in Enterococci

Vancomycin-resistant enterococci (VRE) are a serious challenge for physicians because of the limited treatment options for infections caused by this organism. Prevention of VRE transmission in hospitals requires early detection of infected or colonized patients. Therefore rapid and correct detection of vancomycin resistance is essential. In this study, we use the resazurin microplate method (RMM), which is a modification of the NCCLS and BSAC broth microdilution methods to rapidly determine the susceptibilities of clinical enterococci isolates to vancomycin. The alteration in the RMM was relevant to the final bacterial count. In this method, inoculum that was 10-fold higher than standard methods was used. A total of 80 enterococci, including 11 VRE isolates and 6 vancomycin intermediate isolates, were screened with this modified colorimetric broth microdilution method. After 4 h of incubation 30 microl of 0.01% resazurin solution were added to each well and the plates were reincubated for color change for 5-10 min. The MICs were obtained at the 4th h. The results were in exact agreement with the NCCLS and the BSAC microdilution methods. Absolute and essential agreements were 100% and there were no minor, major or very major errors. In conclusion, this modified colorimetric broth microdilution method can be used as a reliable, easy, cheap and rapid method for early detection of VRE. Moreover, this method has the potential of being used to test the susceptibilities of different bacteria to other antibiotics.

Multiple applications of Alamar Blue as an indicator of metabolic function and cellular health in cell viability bioassays

Accurate prediction of the adverse effects of test compounds on living systems, detection of toxic thresholds, and expansion of experimental data sets to include multiple toxicity end-point analysis are required for any robust screening regime. Alamar Blue is an important redox indicator that is used to evaluate metabolic function and cellular health. The Alamar Blue bioassay has been utilized over the past 50 years to assess cell viability and cytotoxicity in a range of biological and environmental systems and in a number of cell types including bacteria, yeast, fungi, protozoa and cultured mammalian and piscine cells. It offers several advantages over other metabolic indicators and other cytotoxicity assays. However, as with any bioassay, suitability must be determined for each application and cell model. This review seeks to highlight many of the important considerations involved in assay use and design in addition to the potential pitfalls.

A resazurin-based biosensor for organic pollutants

A new rapid biosensor method employing the dye resazurin as an indicator of bacterial respiration has been developed to provide a rapid, facile and specific biosensor for environmental contaminants that does not rely on genetic modification techniques, is suitable for a high-throughput multiwell format, and is ideally suited to resource-constrained environmental monitoring situations. This whole-cell biosensor has been applied to the test analyte toluene using natural toluene-degrading bacteria as the biological component and is competitive with more complex recombinant approaches. The redox-driven biosensor is dependent on the catabolism of a specific compound, concomitantly reducing the redox indicator resazurin to provide the analytical signal in a whole-cell biosensor assay.

Resazurin microtiter assay plate: simple and inexpensive method for detection of drug resistance in Mycobacterium tuberculosis

A method for detecting multidrug-resistant Mycobacterium tuberculosis by using a reduction of resazurin is described. Eighty clinical isolates were evaluated against isoniazid and rifampin; results at 7 days were compared with those of the proportion method. Specificity and sensitivity were excellent. The method is simple, inexpensive, and rapid and might be used with other antituberculosis drugs.

Rapid antibiotic susceptibility testing via electrochemical measurement of ferricyanide reduction by Escherichia coli and Clostridium sporogenes

Electrochemical measurement of respiratory chain activity allows rapid and reliable screening for antibiotic susceptibility in microorganisms. Chronoamperometry and chronocoulometry of suspensions of aerobically cultivated E. coli combined with the non-native oxidant potassium hexacyanoferrate(III) (ferricyanide) yield signals for reoxidation of the reduction product ferrocyanide that are much smaller if the E. coli has been incubated briefly with an effective antibiotic compound. Chronocoulometric results, obtained following 20-min incubation with antibiotic and 2-min measurement in assay buffer containing 50 mM ferricyanide and 10 mM succinate, at +0.50 V vs Ag/AgCl at a Pt working electrode, were compared with traditional disk diffusion susceptibility testing, which requires overnight incubation on agar plates; the results show significantly lower accumulation of ferrocyanide in all cases in which growth inhibition was observed in the disk diffusion assay. A range of antibiotic compounds (13) were examined that possess different mechanisms of action. Quantitative determination of IC50 values for penicillin G and chloramphenicol yielded values that were 100-fold higher than those obtained by standard turbidity methods after 10-h incubation; this is likely a result of the very brief (10 min) exposure time to the antibiotics. Addition of 5 microM 2,6-dichlorophenolindophenol, a hydrophobic electron-transfer mediator, to the assay mixture allowed susceptibility testing of a Gram-positive obligate anaerobe, Clostridium sporogenes. This rapid new assay will facilitate clinical susceptibility testing, allowing appropriate treatment virtually as soon as a clinical isolate can be obtained.

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

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 susceptibility Staphylococcus 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 < or = 8 microg/ml), as opposed to Enterococcus strains which possessed vanA or vanB genes, which encoded higher-level vancomycin resistance (MIC > or = 64 microg/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 microg 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.

Microplate alamar blue assay versus BACTEC 460 system for high-throughput screening of compounds against Mycobacterium tuberculosis and Mycobacterium avium

In response to the need for rapid, inexpensive, high-throughput assays for antimycobacterial drug screening, a microplate-based assay which uses Alamar blue reagent for determination of growth was evaluated. MICs of 30 antimicrobial agents against Mycobacterium tuberculosis H37Rv, M. tuberculosis H37Ra, and Mycobacterium avium were determined in the microplate Alamar blue assay (MABA) with both visual and fluorometric readings and compared to MICs determined in the BACTEC 460 system. For all three mycobacterial strains, there was < or = 1 dilution difference between MABA and BACTEC median MICs in four replicate experiments for 25 to 27 of the 30 antimicrobics. Significant differences between MABA and BACTEC MICs were observed with 0, 2, and 5 of 30 antimicrobial agents against H37Rv, H37Ra, and M. avium, respectively. Overall, MICs determined either visually or fluorometrically in MABA were highly correlated with those determined in the BACTEC 460 system, and visual MABA and fluorometric MABA MICs were highly correlated. MICs of rifampin, rifabutin, minocycline, and clarithromycin were consistently lower for H37Ra compared to H37Rv in all assays but were similar for most other drugs. M. tuberculosis H37Ra may be a suitable surrogate for the more virulent H37Rv strain in primary screening of compounds for antituberculosis activity. MABA is sensitive, rapid, inexpensive, and nonradiometric and offers the potential for screening, with or without analytical instrumentation, large numbers of antimicrobial compounds against slow-growing mycobacteria.

A new fluorometric assay for determination of osteoblastic proliferation: effects of glucocorticoids and insulin-like growth factor-I

A novel fluorometric proliferation assay, AlamarBlue (AB), was used to study the proliferative capacity of isolated human osteoblasts (hOBs). AB is an oxidation-reduction indicator that yields a fluorescent signal in response to metabolic activity. The assay was performed by replacing the experiment media in a microtiter plate with a 10% AB solution and measuring fluorescence after a 3-8-hour incubation. The assay was optimized with respect to incubation time, cell density, and AB concentration. When the results of the AB assay were compared with cell counting in a Bürker chamber there were consistently good correlations (r > 0.9), regardless of the agonist with which the cells were treated. The mean intraassay coefficient of variance (CV) values were 9.9-11.8% in experiments where osteoblasts were treated for 12 days with insulin-like growth factor-I (IGF-I; 100 nM), or dexamethasone (1 micro;M). IGF-I dose dependently, at and above 1 nM, stimulated proliferation of hOBs. This effect was detectable after 3 days and reached 130-140% of untreated controls after 12 days in culture. The effects of dexamethasone (DEX) on the proliferation rate of hOBs were more complex. In short-term cultures, 3 days, DEX dose dependently stimulated proliferation. However, at and above 6 days, DEX exerted a biphasic effect, with stimulation seen at 1-10 nM and a marked inhibition of cell proliferation at and above 100 nM. dexamethasone, hydrocortisone, prednisolone, and deflazacort had almost identical biphasic effects on osteoblastic proliferation in 12 day cultures with a stimulation seen at 1-10 nM, and a marked inhibition down to 50-60% of untreated controls at and above 100 nM. When IGF-I (0. 1-100 nM; 12 day culture) was combined with different doses of DEX, IGF-I still dose dependently stimulated the proliferation rate in hOBs regardless of the amount of DEX added. The stimulatory effect of DEX (10 nM, 12 days culture) was additive to the effect of 100 nM IGF-I. We conclude that AB is an easy and reliable assay for osteoblastic cell proliferation, well suited for large scale studies of cell growth using small amounts of cells, and that IGF-I partly reverses the glucocorticoid-induced inhibition of osteoblastic proliferation.

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

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 log2 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.

Evaluation of commercial vancomycin agar screen plates for detection of vancomycin-resistant enterococci

Brain heart infusion-6-micrograms/ml vancomycin agar plates obtained from five commercial sources (B-D Microbiology Systems, Carr-Scarborough Microbiologicals, MicroBio Products, PML Microbiologicals, and REMEL) were evaluated with 714 enterococci for detection of vancomycin resistance. All 465 (100%) vancomycin-resistant enterococci (MIC > or = 32 micrograms/ml) were detected by each manufacturer's agar screen plate, and each manufacturer's agar screen plate detected at least 99% of the 177 vancomycin-susceptible enterococci (MIC < or = 4 micrograms/ml). Detection of the 72 vancomycin-intermediate enterococci (MIC = 6 to 16 micrograms/ml) ranged from 94% for B-D Microbiology Systems to 99% for PML Microbiologicals.

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

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 micrograms/ml were classified incorrectly by MicroScan panels as resistant. No beta-lactamase-producing enterococci were detected. All 64 isolates showing resistance to vancomycin (MICs > or = 32 micrograms/ml) were correctly classified by MicroScan panels. Seven isolates for which the vancomycin MICs were 8 and 16 micrograms/ml were incorrectly classified as susceptible by MicroScan panels, while eight isolates for which the MICs were 4 micrograms/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 revised 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.

Reevaluation of contemporary laboratory methods for detection of antimicrobial resistance among enterococci

OBJECTIVE: To evaluate broth microdilution, disk diffusion, Etest and Vitek Systems for susceptibility testing of enterococci. METHODS: Susceptibility testing of a panel of 149 enterococci (99 vancomycin-resistant) strains, using the study methods, was performed and the results compared. RESULTS: For vancomycin susceptibility testing, categorical agreement of disk diffusion, Etest and Vitek with the reference broth microdilution test was > 95%. For aminoglycoside and ampicillin testing, categorical agreement between Etest and Vitek was 98 to 100%. CONCLUSIONS: Disk diffusion, Etest and Vitek have acceptable performance for detection of vancomycin resistance of Van A and Van B phenotypes among enterococci.

Factors influencing the vitek gram-positive susceptibility system's detection of vanB-encoded vancomycin resistance among enterococci

Studies were conducted to identify factors contributing to the inability of the Vitek Gram-Positive Susceptibility system (GPS; bioMerieux, Vitek, Inc., Hazelwood, Mo.) to reliably detect vanB-mediated vancomycin resistance among enterococci. To some extent the accuracy of the GPS depended on a particular strain's level of resistance, as all isolates for which vancomycin MICs were > or = 128 mu g/ml were readily detected but detection of resistance expressed by several strains for which MICs were < or = 64 mu g/ml was sporadic. Factors besides the level of resistance were studied in two vanB strains. For one strain (Enterococcus faecium U8304), the ability of GPS to detect resistance was accurate and consistent, while for the other (Enterococcus faecalis V583), GPS results were inconsistent and unreliable. Using these isolates, we established that growth medium had the most notable effect on the detection of resistance. In the absence of vancomycin, Vitek GPS broth supported growth comparable to that obtained with brain heart infusion broth for both E. faecium U8304 and E. faecalis V583. However, in the presence of vancomycin the growth patterns changed dramatically so that neither VanB strain grew well in Vitek broth, and growth of V583 was barely detectable after 8 h of incubation. In contrast, good growth of both strains was observed in brain heart infusion broth supplemented with vancomycin. Additionally, the same medium effect was observed with other inducibly resistant VanB strains. In conclusion, although Vitek broth can support good enterococcal growth, this medium does not sufficiently support expression of vancomycin resistance by certain strains to allow them to be detected by the Vitek automated system. Furthermore, this observation establishes that the type of growth medium used can substantially influence the expression of vancomycin resistance and indicates that medium-based strategies should be explored for the enhancement of resistance detection among commercial systems.