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Journal of Clinical Microbiology, February 2006, p. 553-560, Vol. 44, No. 2
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.2.553-560.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Rapid Susceptibility Testing of Medically Important Zygomycetes by XTT Assay

Charalampos Antachopoulos,¹ Joseph Meletiadis,¹ Emmanuel Roilides,^1,2 Tin Sein,¹ and Thomas J. Walsh^1*

Immunocompromised Host Section, Pediatric Oncology Branch, National Cancer Institute, Bethesda, Maryland,¹ Third Department of Pediatrics, Aristotle University, Hippokration Hospital, Thessaloniki, Greece²

Received 9 September 2005/ Returned for modification 22 September 2005/ Accepted 10 November 2005

Top Abstract Introduction Materials and Methods Results Discussion References

 
The XTT colorimetric assay quantifies fungal growth by measuring fungal metabolism and has been used successfully for susceptibility testing of Aspergillus species after 24 and 48 h of incubation. In the present study using 14 clinical isolates of Zygomycetes (Rhizopus oryzae [5 isolates], Cunninghamella spp. [3 isolates], Mucor spp. [3 isolates], and Absidia corymbifera [3 isolates]), significant metabolic activity was demonstrated before visual or spectrophotometric detection of fungal growth by performing the XTT assay as early as 6 h after inoculation. Testing of susceptibility to amphotericin B, posaconazole, and voriconazole was subsequently performed using the XTT method (100 µg/ml XTT, 25 µM menadione) at 6, 8, or 12 h after inoculation and the CLSI (formerly NCCLS) M38-A method with visual and spectrophotometric MIC determinations at 24 h after inoculation. Concentration-effect curves obtained with the use of the E_max model (a sigmoid curve with variable slope) were comparable between the early XTT and spectrophotometric readings at 24 h. Complete inhibition of early metabolic activity with the azoles was delayed in comparison to that with amphotericin B. Using appropriate cutoff levels, agreement was demonstrated between the early XTT and 24-h spectrophotometric or visual readings. In particular, for MIC-0 (the lowest drug concentration showing absence of visual growth) of amphotericin B, overall agreement levels were 90 to 93% for the 6-h XTT assay and 100% for the 8- and 12-h time points. For MIC-0 of posaconazole, agreement levels were 86% for the 6-h XTT and 93 to 100% for the 8- and 12-h time points. The overall agreement levels for MIC-0 and MIC-2 (the lowest drug concentration showing prominent reduction of growth compared with the control well) of voriconazole (compared with 24-h spectrophotometric readings) were 93 to 98% for the 8- and 12-h XTT assays. These results support the use of the XTT method for rapid MIC determination for Zygomycetes.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Infections caused by Zygomycetes have emerged as an increasingly important cause of morbidity and mortality among severely immunocompromised patients, including those with hematological malignancies and hematopoietic stem cell transplant recipients. The prognosis of zygomycosis in these patients remains extremely poor, despite antifungal treatment (10, 16, 18, 19). For years, amphotericin B has been the drug of choice for these devastating infections. Of the recently introduced second-generation triazoles, voriconazole is not active against the Zygomycetes, while posaconazole has demonstrated activity in vitro, in animal models, and in case reports (4-6, 8, 20-22). As zygomycosis may be rapidly fatal, timely determination of the susceptibilities of clinical isolates to these antifungal agents is important.

A colorimetric method, based on reduction of the tetrazolium salt 2,3-bis{2-methoxy-4-nitro-5-[(sulfenylamino) carbonyl]-2H-tetrazolium-hydroxide} (XTT) by mitochondrial dehydrogenases, has been recently introduced for in vitro susceptibility testing of filamentous fungi. This method quantifies fungal growth by measuring fungal metabolism, using appropriate concentrations of XTT and an electron transfer agent, such as menadione (1, 11, 12). For Aspergillus species, MICs determined by the XTT method at 24 and 48 h were comparable with those obtained by the National Committee for Clinical Laboratory Standards (now known as the Clinical and Laboratory Standards Institute [CLSI]) method (12). The sensitivity of the XTT assay may be increased depending on the menadione concentration (11). High menadione concentrations could increase the rate of XTT conversion and theoretically allow the detection of metabolic activity associated with small, early fungal growth that is not yet detected visually or spectrophotometrically. In this case, it is likely that the XTT method might even be useful for early determination of MICs of antifungal agents. This hypothesis has been previously discussed (12) but has not been investigated.

In preliminary studies using the XTT assay for zygomycete isolates, high metabolic activity was observed even in the presence of small fungal growth (assessed spectrophotometrically) at 12 or 24 h postinoculation and preceded increases in biomass (C. Antachopoulos, J. Meletiadis, E. Roilides, T. Sein, and T. J. Walsh, Abstr. 45th Intersci. Conf. Antimicrob. Agents Chemother., abstr. M-1003, 2005). The present study investigated whether significant metabolic activity with the XTT assay could be demonstrated for the Zygomycetes before hyphal growth is macroscopically or spectrophotometrically detected (at 6 or 8 h postinoculation) and whether changes in this early metabolic activity in the presence of antifungal agents could be used for rapid susceptibility testing of these filamentous fungi.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Isolates. A collection of 14 clinical zygomycete isolates was used, including the following species: Rhizopus oryzae (5 isolates), Cunninghamella bertholletiae (2 isolates), Cunninghamella echinulata (1 isolate), Mucor circinelloides (2 isolates), Mucor indicus (1 isolate), and Absidia corymbifera (3 isolates). The reference strains Candida krusei 6258 and Candida parapsilosis 22019 were used as quality controls.

Medium. RPMI 1640 medium with L-glutamine but without bicarbonate (Cambrex Bio Science, Inc., Walkersville, MD) buffered to pH 7.0 with 0.165 M 3-N-morpholinopropanesulfonic acid (MOPS) (Sigma-Aldrich, St. Louis, MO) was used as the assay medium.

Inoculum. Sporangiospores were harvested after isolates were subcultured in potato dextrose agar at 35°C for 3 days and were suspended in normal saline containing 0.025% Tween 20. The sporangiospore suspensions were counted with a hemacytometer and then diluted in RPMI in order to achieve the desired concentrations. Inoculum sizes were verified by quantitative colony counts on Sabouraud dextrose agar plates.

XTT and menadione. XTT (Sigma-Aldrich, St. Louis, MO) was dissolved in normal saline at concentrations of 1, 0.5, and 0.25 mg/ml. Menadione (Sigma-Aldrich, St. Louis, MO) was initially dissolved in absolute ethanol at a concentration of 10 mg/ml and subsequently added to the above-mentioned XTT solutions at concentrations of 125, 31.25, and 7.81 µM for each solution. In this way, a total of nine solutions with different XTT-menadione concentrations were prepared.

XTT assay for measurement of early metabolic activity. In the XTT assay for measurement of early metabolic activity, the abilities of the above-mentioned XTT-menadione combinations to detect early metabolic activity of Zygomycetes were investigated. Briefly, 10⁴ sporangiospores/ml of four clinical zygomycete isolates (one each of R. oryzae, C. bertholletiae, M. circinelloides, and A. corymbifera) were incubated in RPMI 1640 at 37°C in 96-well microtitration plates at a volume of 200 µl/well. After 6 or 8 h of incubation, 50 µl of one of the above-mentioned XTT-menadione solutions was added to each well, as previously described (11, 12). All nine solutions were used, resulting in final concentrations of 200, 100, and 50 µg/ml XTT, each combined with 25, 6.25, or 1.56 µM menadione, respectively. The microtitration plates were further incubated at 37°C for 2 h in order to allow conversion of XTT to its formazan derivative. These plates were then shaken for 1 to 2 min (Wallac Plate Shake 1296-004; Wallac OY, Turku, Finland) until complete dissolution of formazan derivatives was achieved. XTT conversion was measured as optical density (OD) with a microtitration plate spectrophotometric reader (Elx808; Bio-Tek Instruments, Winooski, VT) at 450 nm. For each well, XTT conversion was calculated after subtraction of the background OD, which was the OD of a simultaneously incubated well with 200 µl of RPMI and 50 µl of XTT-menadione solution but no inoculum. Four replicates were used for each species, time point, and XTT-menadione concentration.

Relationship between early XTT conversion and fungal biomass. An assay investigated whether the rate of early XTT conversion, obtained with the different XTT-menadione combinations, in fact correlated with the magnitude of fungal biomass, using a previously described approach (11). Briefly, the XTT assay was performed at 6 or 8 h of incubation, as described above, using 10-fold serially diluted inocula (10² to 10⁶ sporangiospores/ml) for each of the above-mentioned four clinical zygomycete isolates. The relationship between XTT conversion (OD at 450 nm [OD₄₅₀]) and fungal inoculum (log₁₀) was assessed by linear regression analysis, and the slopes and goodness of fit (r²) were reported for each XTT-menadione combination.

Antifungal-susceptibility testing. MICs were determined for all 14 isolates of Zygomycetes. The broth microdilution method of the CLSI (M38-A) (7, 15) was followed using 96-well microtitration plates. Posaconazole (Schering-Plough Research Institute, Kenilworth, NJ) was dissolved in dimethyl sulfoxide. Amphotericin B (Bristol-Myers Squibb, Princeton, NJ) and voriconazole (Pfizer Inc., New York, NY) were dissolved in sterile distilled water. Serial dilutions of the antifungal agents were prepared to yield concentrations of 0.03 to 32 µg/ml for posaconazole, 0.015 to 16 µg/ml for amphotericin B, and 1 to 1,024 µg/ml for voriconazole at a final volume of 200 µl after inoculation. The trays were maintained at –70°C until the day of testing.

Incubation and MIC determination. After the microtitration trays were defrosted, they were inoculated with a final concentration of 2.5 x 10⁴ sporangiospores/ml from each of the 14 isolates and incubated at 37°C. For each isolate, four different 96-well trays were used, corresponding to four different incubation periods: 6, 8, 12, and 24 h. For each time point, an additional tray was prepared, whose wells contained the above-mentioned drug concentrations at a final volume of 200 µl of RPMI, but no fungal inoculum. These trays served to provide background OD measurements for the XTT and spectrophotometric readings.

(i) XTT assay for early MIC determination. At 6, 8, and 12 h after inoculation, the corresponding trays were removed from the incubator, and 50 µl of saline containing 0.5 mg/ml XTT with 125 µM menadione was added to each well to obtain final concentrations of 100 µg/ml XTT and 25 µM menadione. The trays were further incubated for 2 h at 37°C. After shaking of the trays, XTT conversion was measured at 450 nm. After subtraction of the corresponding background OD, the relative OD₄₅₀ for each well and drug concentration, in relation to the control well, was calculated as the following fraction: OD₄₅₀ of the drug-containing well/OD₄₅₀ of the control well. Due to the lack of previous data regarding the effects of different antifungal agents on the early metabolic activity of Zygomycetes, appropriate cutoff levels for determination of MIC-0 and MIC-2 (see below) and comparison with the CLSI method were employed after the results were obtained and the concentration-effect curves of metabolic activity were analyzed.

(ii) CLSI method. After 24 h of incubation, the corresponding trays were removed from the incubator and fungal growth was assessed visually and spectrophotometrically. For visual assessment, a concave mirror was used. The MIC-0 was determined as the lowest drug concentration showing absence of visual growth and was recorded for all three agents. The MIC-2 was determined as the lowest drug concentration showing prominent reduction of growth compared with the control well and was recorded only for posaconazole and voriconazole. For spectrophotometric assessment, hyphal growth was measured at 405 nm, as previously described (5, 12). After subtraction of the corresponding background OD, the relative OD₄₀₅ for each well and drug concentration, in relation to the control well, was calculated as follows: OD₄₀₅ of the drug-containing well/OD₄₀₅ of the control well. The lowest drug concentrations showing relative ODs of 10% and 50% were determined to be the MIC-0 and MIC-2, respectively. Again, only MIC-0 was recorded for amphotericin B.

Experiments of antifungal susceptibility testing using the early XTT and CLSI methods were repeated in triplicate.

Modeling and data analysis. For four of the zygomycete isolates (one each of R. oryzae, C. bertholletiae, M. circinelloides, and A. corymbifera), for the purpose of modeling, the relative OD values obtained with the XTT assay (6, 8, and 12 h) or spectrophotometric reading at 405 nm (24 h) were analyzed by nonlinear regression analysis, using a four-parameter logistic model (a sigmoid curve with variable slope) known as the E_max model. Deviation from the model was tested by the runs test, and goodness of fit was checked by the r² value. The best-fit values of the EC₅₀, which is the drug concentration producing 50% of the E_max (maximum relative OD); of the slope m, which describes the steepness of the curve; and of the bottom of the curve were recorded and compared for the early (6-, 8-, and 12-h) and 24-h time points for each isolate. Comparison of best-fit values for these parameters was performed with one-way analysis of variance, followed by Dunn's test for multiple comparisons. The results of comparisons of the EC₅₀ and bottom-of-the-curve values were used to employ appropriate cutoff levels for determination of MIC-2 and MIC-0, respectively, with the early XTT method (6, 8, and 12 h). Statistical analysis was performed using the GraphPad (San Diego, Calif.) Prism software.

For purposes of comparison with CLSI methods, MICs for all 14 isolates obtained by using the above cutoff levels for the early XTT assays were compared with corresponding values from visual and spectrophotometric (405-nm) readings at 24 h. The percentage of relative (within one dilution) agreement between the MICs obtained with different methods and time points for each species and for all isolates was calculated and reported.

Reproducibility. In order to determine the reproducibility of the XTT, visual, and spectrophotometric methods for each time point and antifungal agent, the median MIC of those obtained with the three experiments was calculated for each isolate. The relative interexperimental agreement was subsequently defined as the percentage of MICs (obtained for each method, time point, and antifungal agent) that were within one twofold dilution from the median MIC (12).

Top Abstract Introduction Materials and Methods Results Discussion References

 
Early metabolic activity. Significant metabolic activity was demonstrated with the XTT assay at the 6- and 8-h time points for all four isolates tested, with OD₄₅₀s ranging from 0.17 to 1.06 for those wells treated with 200, 100, or 50 µg/ml XTT and 25 µM menadione at 6 h (Fig. 1). At this early time point, the use of different XTT concentrations resulted in comparable conversion rates. In contrast, the use of lower menadione concentrations (6.25 and 1.56 µM) resulted in markedly reduced XTT conversion, independently of the concentration of XTT (Fig. 1). Consequently, XTT-menadione combinations containing 25 µM of menadione were used exclusively in further assays of early metabolic activity.

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FIG. 1. Metabolic activity of Zygomycetes assessed by the XTT method at 6 h postinoculation, using different concentrations of XTT (200, 100, and 50 µg/ml) and menadione (MEN) (25, 6.25, and 1.56 µM). The error bars indicate standard errors of the means.

Relationship between early XTT conversion and fungal biomass. A linear relationship was demonstrated between early XTT conversion and fungal inoculum (P < 0.001). Of the three combinations of 200, 100, and 50 µg/ml XTT with 25 µM menadione, a higher median (range) r² was obtained with the concentrations of 100 µg/ml XTT and 25 µM menadione [0.91 (0.86 to 0.98)]. For the combinations of 200 or 50 µg/ml XTT with 25 µM menadione, the median (range) r² values were 0.85 (0.76 to 0.95) and 0.88 (0.86 to 0.92), respectively. The slope [median (range)] of the regression line obtained with 100 µg/ml XTT was 0.42 (0.41 to 0.46). Use of 200 or 50 µg/ml XTT resulted in very high (median, 0.81; range, 0.66 to 0.84) or low (median, 0.22; range, 0.20 to 0.23) slopes, respectively. Since we aimed for a linear relationship with intermediate slope (high or low slopes would result in fast or slow XTT conversion, respectively), we employed the combination of 100 µg/ml XTT and 25 µM menadione for early antifungal susceptibility testing of zygomycete isolates.

Antifungal susceptibility testing. In agreement with the previous findings, sufficiently high XTT conversion rates (mean OD₄₅₀, 0.69; range, 0.19 to 1.67) were obtained when the combination of 100 µg/ml XTT and 25 µM menadione was used for MIC determination at the 6-h time point for all 14 isolates. Examination of microtitration plates under the inverted microscope at the end of the 2-h incubation period following addition of the XTT-menadione solution (i.e., 8 h, in total, after inoculation) demonstrated that XTT conversion increased with progression from germination to the formation of hyphae (Fig. 2).

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FIG. 2. Photomicrographs (x82) of adjacent wells with serially decreasing concentrations of voriconazole at completion of the 6-h XTT assay (total, 8 h after inoculation). Gradual increases in XTT conversion rates are manifested by color changes from yellow to orange and coincide with germination of M. circinelloides conidia and progression to the formation of hyphae. MIC-0s were 32, 16, and 16 µg/ml according to the 24-h visual and spectrophotometric and 6-h XTT readings, respectively. GC, growth control. Voriconazole was used for this example because it provided more gradual concentration-effect curves than those of other agents (see Results), thus allowing a better demonstration of the different stages of transition from nongerminated conidia to elongated hyphae.

Modeling of XTT and spectrophotometric measurements. For the four isolates of Zygomycetes (one each of R. oryzae, C. bertholletiae, M. circinelloides, and A. corymbifera) for which data were analyzed by nonlinear regression analysis, the E_max model fit the spectrophotometric (24-h) and XTT (6-, 8-, and 12-h) readings very well; r² ranged between 0.97 and 0.99, and no statistical deviations from the model were found (P = 0.17 to 0.99; runs test). No statistical differences were found between the best-fit values of EC₅₀ obtained with the spectrophotometric and colorimetric methods for posaconazole and voriconazole (P > 0.05 by one-way analysis of variance) (Table 1). For amphotericin B, P was <0.05 for comparison of the 24-h spectrophotometric and 6- or 8-h XTT readings; however, the differences in EC₅₀ values did not exceed 1 dilution (Table 1). The slopes of the concentration-effect curves were also similar between the different methods (P > 0.05) but varied according to the antifungal agent used: higher absolute m values (steeper curves) were obtained with amphotericin B and lower values were obtained with voriconazole (data not shown). A significant difference, however, between the spectrophotometric (405-nm) and early XTT methods was found in the best-fit values of the bottom of the curve for posaconazole and voriconazole (P < 0.05) (Table 1). For these agents, the bottom values of the curve obtained with the early XTT assays were significantly higher than those obtained with the 24-h spectrophotometric readings. Bottom values were highest at the 6-h time point and decreased gradually at 8 and 12 h (Table 1). This difference in the bottom values of the sigmoid curve between the early XTT and spectrophotometric methods for the azoles, in comparison to amphotericin B, is also illustrated in Fig. 3 and suggests that for the azole agents, significant XTT conversion could occur at the early time points (up to almost 40% of control OD₄₅₀ for the 6-h time point), even in the presence of drug concentrations equal to or greater than the MIC-0. Thus, suppression of the metabolic activity of Zygomycetes by inhibitory concentrations of the azoles was gradually achieved and was not complete at the early time points.

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TABLE 1. Best-fit values of EC50 and bottom of the sigmoid curve obtained with the Emax model for early XTT (6-, 8-, and 12-h) and spectrophotometric (24-h) readings

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FIG. 3. Concentration-effect sigmoid curves based on 6-h XTT and 24-h spectrophotometric (SPEC) readings for R. oryzae. For posaconazole (A) and voriconazole (B), the bottom values of the curve for the 6-h XTT measurements were significantly higher (relative OD450, 0.4 and 0.2, respectively) than those of 24-h spectrophotometric readings (relative OD405, 0). For amphotericin B (C), the bottom values of both curves were similar (relative ODs, 0). The error bars indicate standard errors of the means.

The comparability of EC₅₀ values obtained with the E_max model suggested that for determination of MIC-2 by the early XTT assays, the cutoff level of 50% (also employed for the spectrophotometric readings at 24 h) would probably provide good agreement between the different methods. However, the differences in the bottom values of the curve suggested that in order to determine MIC-0 for the azoles by the early XTT method, cutoff levels greater than 10% (which was used in the spectrophotometric method) should be employed. Based on the E_max model data (Table 1), these cutoffs could be on the order of 20 to 40% for the 6-h time point and 10 to 20% for the 8- and 12-h time points. Indeed, the MIC-0 values obtained with the above range of cutoff levels were compared with those obtained with spectrophotometric (405-nm) or visual readings, and the cutoffs giving the best agreement are presented in Table 2. For amphotericin B, MIC-0 values obtained using the 10% cutoff for the 6- and 8-h XTT method were in very good agreement with the spectrophotometric method (90 and 95%, respectively). However, the threshold of 5% increased the agreement for the 8-h time point to 100%.

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TABLE 2. Cutoff levels of the relative OD for end point determination of MIC-0 of zygomycete isolates by the XTT method at different time points, based on best agreement with the spectrophotometric and visual (CLSI) methods

Comparison of MICs. Comparable MICs of zygomycete isolates were obtained with the visual (CLSI), spectrophotometric (405-nm), and early XTT methods using the cutoffs shown in Table 2 for different time points. Differences in median MICs obtained with different methods for zygomycete species did not exceed 1 dilution, with the exception of median MICs of voriconazole for Mucor spp. and A. corymbifera, which were within 2 dilutions (Table 3).

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TABLE 3. MICs of zygomycete isolates determined by the early XTT (6-, 8-, and 12-h) and CLSI methodsa

The relative levels of agreement between the different methods are presented in Table 4. The overall agreement levels between the spectrophotometric (405-nm) and visual (CLSI) methods of MIC determination at 24 h ranged between 83 and 100% for different agents, with 83% being the agreement for MIC-0 of voriconazole and 100% being the agreement for MIC-0 of amphotericin B. Lower agreement levels were observed between the two methods in the determination of MICs of posaconazole and voriconazole against Cunninghamella spp. (Table 4). Whenever differences were observed between these two methods, visual MICs tended to be higher than spectrophotometric ones (Table 3).

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TABLE 4. Percent relative agreement within ±1 dilution between MICs determined with the early XTT and visual or spectrophotometric methods for Zygomycetes

For amphotericin B, the overall agreement levels of MICs obtained with the early XTT assays compared to the spectrophotometric and visual readings ranged from 90 to 93% for the 6-h time point to 100% for those of 8 and 12 h (Table 4). Lower agreement levels were observed for Cunninghamella spp. with the 6-h XTT assay.

For posaconazole, the overall agreement was 86% for MIC-0 obtained with the 6-h XTT versus spectrophotometric readings and even higher for the 8- and 12-h time points (93 to 100% for MIC-0) (Table 4). Lower agreement levels, however, were observed between the 6-h XTT and visual MIC-0 (76%) and between the 6-h XTT and spectrophotometric MIC-2 (81%). Again, among different species, lower agreement levels were observed in the determination of MICs for Cunninghamella spp., in particular, MIC-2.

For voriconazole, the MIC-0 and MIC-2 obtained by the 8- and 12-h XTT methods showed an overall agreement of 93 to 98% with the spectrophotometric method but showed less with the visual readings (71 to 86%) (Table 4). The overall agreement levels of MIC-2 determined by the 6-h XTT method for voriconazole were 88% compared with the spectrophotometric method and slightly lower (83%) with the visual method. Lower agreement levels, however (67 to 74%), were observed for the MIC-0 values obtained with the 6-h XTT compared to visual or spectrophotometric readings.

Reproducibility. The relative interexperimental agreement levels for MIC-0 and MIC-2 ranged between 98 and 100% for all methods, time points, and antifungal agents, with the exception of MIC-0 determined by the 6-h XTT method for voriconazole and MIC-2 determined by the 24-h spectrophotometric method for posaconazole, for which the agreement was 95% (standard error, 3%).

Top Abstract Introduction Materials and Methods Results Discussion References

 
In the present study, we demonstrated that MICs of Zygomycetes can be obtained in a reproducible way with the XTT assay up to 16 h earlier than with current standard methods. This early determination of MICs was based on the demonstration of significant metabolic activity of these fungi before detection of significant growth macroscopically or spectrophotometrically (mean OD₄₀₅ for zygomycete isolates at 8 h postinoculation, 0.008; range, –0.004 to 0.03).

Several methods for rapid susceptibility testing of filamentous fungi have been developed in the past, including a radiometric assay measuring the inhibition of ¹⁴CO₂ production (14), a flow cytometry assay of conidial viability (3), a fluorescence-based microplate assay (2), and a turbidometric method assessing the lag phases of fungal growth curves in the presence of antifungal drugs (13). Some of these methods may use expensive equipment or potentially hazardous substances, and they have been evaluated only for amphotericin B. The fluorescence-based microplate assay generated MICs in 16 h for Aspergillus spp., but it remains to be evaluated for other filamentous fungi, including Zygomycetes (2). The use of turbidometric growth curves required up to 12 h of incubation for determination of the amphotericin B resistance of R. oryzae isolates (13). The XTT method presented here is a simple way of measuring fungal metabolism that can be performed in most laboratories.

When menadione is used as an electron transfer agent, the rate of conversion of XTT to its formazan derivative could be affected by the concentrations of both the XTT tetrazolium salt and menadione (11). As shown in Fig. 1, at the early time point of 6 h, the effect of menadione on XTT conversion seemed to be more significant than that of the XTT concentration. A high (25 µM) menadione concentration was required to obtain significant conversion rates. It should be emphasized, however, that while the use of the XTT assay for rapid determination of MICs could probably be evaluated for other fungi as well, the concentrations of XTT and menadione that should be used may need to be modified, due to potential differences in growth properties and metabolic activities among different classes or genera.

Another factor affecting XTT conversion at these early time points is the size of the inoculum, which correlated linearly with the OD₄₅₀ values obtained. The results from our studies of early metabolic activity (Fig. 1), as well as from the subsequent antifungal susceptibility studies (where the range of viable inoculum, based on quantitative colony counts, was 0.7 x 10⁴ to 3.9 x 10⁴ sporangiospores/ml), suggested that, within the range of inocula suggested by the CLSI guidelines (0.4 x 10⁴ to 5 x 10⁴ sporangiospores/ml) (15), sufficient XTT conversion occurred at the early time points of 6 and 8 h to allow MIC determination. However, deviations beyond the lower limit of the suggested range should probably be avoided, since they may result in significantly reduced XTT conversion (with 10³ sporangiospores/ml, an OD₄₅₀ as low as 0.04 was recorded for some isolates [data not shown]).

Although CLSI recommendations for filamentous fungi suggest reporting only MIC-0 for the azoles (15), others have also used MIC-2 (5, 8). Hence, we determined both MIC-0 and MIC-2 for these agents. For determination of MICs at 24 h, we used both visual and spectrophotometric (405-nm) readings of the trays. Spectrophotometric readings may be more precise and reproducible than visual assessment of fungal growth, which is often subjective (17). For spectrophotometric determination of MIC-0, thresholds of both 10% and 5% have been employed for filamentous fungi (5, 12). We used the 10% threshold because it provided greater agreement with the visual readings than 5% (a 2 to 3% increase in overall agreement for each agent). The levels of agreement obtained between visual and spectrophotometric readings were comparable to those of previous reports (5, 12).

An important finding was that the azole agents at concentrations equal to or greater than MIC-0 do not completely suppress the metabolic activity of Zygomycetes during the first 12 h after inoculation, as manifested by the trend of relative OD₄₅₀s for the 6-, 8-, and 12-h time points (Table 1 and Fig. 3). The gradual decrease of relative OD₄₅₀ values observed with inhibitory drug concentrations over these time points was due in part to the increase of the OD₄₅₀ values of the control wells and to a decrease in the absolute OD₄₅₀ values of the corresponding wells. In contrast, amphotericin B at inhibitory concentrations almost completely suppressed metabolic activity at the 6- or 8-h time points for all isolates. This differential effect on the early metabolic activity of Zygomycetes was probably the result of different mechanisms of action of the azoles and polyenes against these molds (9) and necessitated the use of cutoff levels for the azole agents different than those for amphotericin B in the early XTT assays (Table 2).

Using appropriate cutoff levels, the MIC-0s determined at the 6-h time point for amphotericin B and posaconazole were in very good agreement with the 24-h spectrophotometric (and visual, for amphotericin B) readings. MICs for all agents obtained at the 8- or 12-h time point were also in very good agreement with the 24-h readings (Table 4). This agreement was usually greater with the spectrophotometric readings than the visual ones, especially for voriconazole. This discrepancy was not surprising, as voriconazole demonstrated lower levels of overall agreement between spectrophotometric and visual readings at 24 h than posaconazole and amphotericin B, in both our study (Table 4) and previous reports (5).

A notable finding in this study was the lower level of agreement between the visual and spectrophotometric readings and between the early-XTT and 24-h MICs for Cunninghamella spp. than for other species. While it is difficult to draw conclusions due to the small number (3) of Cunninghamella isolates, it should be noted that the major "contributor" to disagreement, for most comparisons, was the C. echinulata isolate. When the two C. bertholletiae isolates were evaluated separately, their levels of agreement between visual and spectrophotometric readings at 24 h were 100% for MIC-0 of posaconazole and voriconazole and 50% for MIC-2 of posaconazole. In addition, the levels of agreement between the 6-h XTT and 24-h spectrophotometric readings were 100% for MIC-0 of amphotericin B and posaconazole, 50% for MIC-2 of posaconazole, and 83% for MIC-0 of voriconazole.

In conclusion, medically important Zygomycetes demonstrate significant metabolic activity with the XTT assay before visual or spectrophotometric detection of fungal growth. This early metabolic activity can be inhibited by antifungal agents, although complete inhibition with the azoles is delayed in comparison to amphotericin B. Using appropriate cutoff levels for each agent and time point, the XTT assay performed at 6, 8, or 12 h after inoculation can determine MICs that are in very good agreement with those obtained with visual or spectrophotometric readings at 24 h. These data now establish a foundation for conducting a multicenter trial for validation of the determination of MICs for Zygomycetes by the early XTT assay. The use of the XTT method for rapid susceptibility testing should also be investigated for other medically important filamentous fungi.

 
This work was supported in part by the Intramural Research Program of the National Institutes of Health and the National Cancer Institute.

 
* Corresponding author. Mailing address: Pediatric Oncology Branch, National Cancer Institute, CRC, Rm. 1-5750, MSC 1100, 10 Center Drive, Bethesda, MD 20892. Phone: (301) 402-0023. Fax: (301) 480-2308. E-mail: walsht{at}mail.nih.gov.

Top Abstract Introduction Materials and Methods Results Discussion References

 

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Journal of Clinical Microbiology, February 2006, p. 553-560, Vol. 44, No. 2
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.2.553-560.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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