Collaborative investigation of broth microdilution and semisolid agar dilution for in vitro susceptibility testing of Candida albicans

Lack of correlation of 24- vs. 48-h itraconazole minimum inhibitory concentrations with microbiological and survival outcomes in a guinea pig model of disseminated candidiasis

A 'trailing' effect has been commonly observed when azole antifungals are tested against Candida spp. Previous experience with fluconazole indicates that 24-h minimum inhibitory concentration (MIC) values are more compatible endpoints when compared with clinical outcomes. We evaluated the trailing effect of Candida isolates tested with itraconazole in a guinea pig model of systemic candidiasis. Survival and organ burden were only significantly affected by using a higher dose of itraconazole, irrespective of the MIC differences at 24 and 48 h. A fluconazole-resistant strain with susceptible dose-dependent MICs to itraconazole was successfully treated with high-dose itraconazole. Our data suggests that survival and microbiological response depend more on drug dosing than on the trailing phenotype of the isolates.

Influence of sub-inhibitory concentrations of conventional antifungals on metabolism of Candida albicans and on its adherence to polystyrene and extracellular matrix proteins

Five antifungal agents with different mechanisms of action were compared for their ability to affect mitochondrial dehydrogenase activity and adherence capacity of Candida albicans to polystyrene and extracellular matrix proteins. Only amphotericin B inhibited mitochondrial dehydrogenase activity when the culture medium was supplemented with galactose. 5-Fluorocytosine and terbinafine did not affect this activity, whereas itraconazole and fluconazole improved it. Furthermore, in these experimental conditions, the effect of sub-inhibitory concentrations of antifungals on adherence was dependent on the tested antifungal and the adherence surface: amphotericin B inhibited adherence to polystyrene and fibrinogen, but improved adherence to extracellular matrix. For all surfaces tested, when culture medium was supplemented with galactose, fluorocytosine did not affect adherence, and itraconazole, fluconazole and terbinafine inhibited adherence. Our results also confirmed the influence of the carbohydrates: sub-minimum inhibitory concentrations (MIC) of itraconazole increased or did not modify the mitochondrial metabolism of yeasts when the culture medium was supplemented with galactose, but this antifungal always decreased mitochondrial metabolism when the culture medium was supplemented with glucose. These data indicate that antifungals used below their MIC values can have various effects. It is important to distinguish the effects of antifungals on the metabolism of C. albicans from effects on its adherence capacity. The former effects are linked to the viability of the yeast and the latter depends on the colonization of cellular as opposed to inert surfaces.

Candida: a causative agent of an emerging infection

Incidences of infections due to Candida have increased over the last 15-20 y. This increase in the incidence and the high associated mortality rate despite therapy has focused the attention on this disease and prompted investigators to undertake research aimed at understanding the pathogenesis of this disease as well as methods to treat it. This paper discusses recent developments in the Candida field and the impact they have on patient management.

Antimicrobial susceptibility testing of dematiaceous filamentous fungi: effect of medium composition at different temperatures and times of reading

We examined the effects of the composition of five media on the MICs of three antifungal agents (amphotericin B, flucytosine and ketoconazole) against 24 dematiaceous fungi at different temperatures and times of reading. Media studied were RPMI 1640, tryptic soy broth (TSB), yeast nitrogen broth (YNB), casein-yeast-glucose broth (CYG) and high-resolution medium (HR). The degree of reproducibility was determined with ketoconazole and 5-fluorocytosine and the media TSB and RPMI showed the best results with ketoconazole, while the best results for flucytosine were in CYG. Reproducibility was higher at 25 degrees than at 30 degrees C. The media and incubation time were shown to have a significant influence on the MICs of the three antifungals, but there were no interactions between these variables and temperature, except in the case of ketoconazole, where media-temperature and media-incubation time interactions were significant. There were noteworthy differences between the MICs obtained with the different media.

Antifungal susceptibility testing of yeasts: uses and limitations

With recent developments in the field of mycology, such as increased incidence of fungal infections, the introduction of newer, safer antifungals, and the emergence of resistance, the need for clinically relevant antifungal susceptibility testing methods is obvious. Studies performed over the past decade have allowed the National Committee for Clinical Laboratory Standards Subcommittee on Antifungal Testing to achieve consensus on a new standardized broth dilution method for in vitro susceptibility testing of yeasts (NCCLS M27-A). Once the reproducibility of the M27-A document was established, tentative breakpoints for fluconazole and itraconazole were derived. The availability of a standardized procedure for determining the minimum inhibitory concentrations (MICs) of antifungal agents is an important tool in drug discovery and development. In addition, it provides means for detection of resistant strains and, in the case of oropharyngeal candidiasis, means for patient management. Copyright 2000 Harcourt Publishers Ltd.

[Fluconazole-resistant Candida species from HIV infected patients with recurrent Candida stomatitis: cross resistance to itraconazole and ketoconazole]

In vitro susceptibility to fluconazole of Candida species isolated from 83 HIV-infected patients treated with fluconazole because of recurrent Candida stomatitis was monitored over a period of two years. A microdilution assay with high-resolution antifungal assay (HR) medium and RPMI 1640-medium were compared. In vitro less susceptible and fluconazole resistant C. species were observed in 23 patient at the end of the study. The Candida isolates recovered from oral rinsing fluids at the beginning and the end of study were tested for crossresistance to itraconazole and ketoconazole. Susceptibility to ketoconazole and to itraconazole was reduced using RPMI 1640-medium. Susceptibility of the isolates to fluconazole was not influenced by the assay medium. In 21 patients in vitro fluconazole resistant or less susceptible C. albicans were observed. 9 of 21 isolates showed crossresistance to itroconazole and ketoconazole, in 10 isolates only crossresistance to itraconazole was observed. During fluconazole treatment double infections due to combination of C. albicans and C. glabrata or C. krusei increased from 20% to 78% C. krusei was resistant to the three azoles. C. glabrata was less susceptible (4-8 mg/l) or resistant (> 8 mg/l) to fluconazole and resistant to itraconazole and ketoconazole High dosed intravenous fluconazole treatment with 400 to 600 mg daily failed in 11 patients with fluconazole resistant C. albicans and in 3 (3/10) patients with les susceptible C. albicans isolates.

Azole resistance in Candida

Resistance of Candida to azoles is an increasing problem. Susceptibility testing of Candida against fluconazole and ketoconazole is now feasible and desirable. Good correlation of resistance in vitro with clinical failure of fluconazole therapy has now been shown in mucosal candidiasis. The relationship, if any, between resistance and clinical failure in the context of invasive candidiasis is not clear at present and additional correlative work needs to be done. Monitoring of resistance trends in Candida is clearly important now.

Multiple efflux mechanisms are involved in Candida albicans fluconazole resistance

Fluconazole-susceptible Candida albicans strains accumulated [3H]fluconazole at a rate of approximately 2 pmol/min per 10(9) cells. Fluconazole accumulation was not affected by the pretreatment of cells with sodium azide or with 2-deoxyglucose. The rate of fluconazole accumulation became saturated at high fluconazole concentrations and was not affected by the addition of ketoconazole, and there was no fluconazole accumulation in cells incubated at 4 degrees C. A fluconazole-resistant mutant of C. albicans SGY-243 was isolated following growth enrichment in fluconazole-containing medium. Cells of the mutant strain, designated FR2, showed a reduced rate of fluconazole accumulation compared with SGY-243 and were not resistant to other azole antifungal agents. The rates of fluconazole accumulation by C. albicans FR2 and the other azole-resistant strains, B59630, AD, and KB, were increased in the presence of sodium azide, suggesting that fluconazole resistance in these strains may be associated with an energy-dependent drug efflux. Fluconazole-resistant C. albicans strains all contained elevated amounts (2- to 17-fold) of mRNA encoding Cdr1, and an ATP-binding cassette-type transporter. In addition, C. albicans FR2 also contained increased amounts of mRNA encoding Benr, a major facilitator superfamily transporter. These results suggest that fluconazole enters C. albicans cells by facilitated diffusion and that fluconazole resistance may involve energy-dependent drug efflux associated with increased expression of Benr and/or Cdr1.

Susceptibility testing of fungi: current status of correlation of in vitro data with clinical outcome

In summary, it is clear that in vitro susceptibility testing can predict outcome in selected clinical situations. The clearest data are from the fluconazole-treated AIDS patients with oropharyngeal candidiasis. In this setting, the homogeneity of the underlying immune defect, combined with the ease of identification and monitoring of the infection, creates a near-perfect test situation. In more complex scenarios, such as the heterogeneous population of patients enrolled in a recent study of candidemia, no such clear-cut correlation was present. The importance of host factors in the correlation of the MIC with outcome cannot be overemphasized. Examples of these parameters include patient status (underlying disease, the presence of intravascular catheters, and CD4+ T-cell number), drug pharmacokinetics (absorption and distribution), patient compliance, and drug-drug interactions. Identification of relevant factors can substantially improve the degree of the MIC-outcome correlation and thus improve the clinical utility of in vitro testing. An important feature in this entire process is the role of standardized susceptibility testing procedures. While not without flaws, the proposed NCCLS reference method has been invaluable in allowing multiple investigators to contribute data that can be used to clarify the correlation between the fluconazole MIC and outcome. While the development of simplified second-generation methods is eagerly anticipated, the role of the reference method as a common touchstone is critical. Only by use of either the reference method itself or methods with a known relationship to the reference method can this broad collaborative process really proceed. Current work is focusing on defining interpretive breakpoints for fluconazole and Candida species, refinement of the in vitro procedures used to measure susceptibility to amphotericin B, ketoconazole, and itraconazole, and the acquisition of a broad base of data on the relationship between the MIC and outcome for these three drugs. Although considerable work remains to be done, the available data suggest that solutions to each of these problems are possible and that routine susceptibility testing of fungi will become meaningful for clinical decision making in the foreseeable future.

[Susceptibility testing of yeasts against fluconazole: comparison of the Etest method with microdilution and agar dilution]

In bacteriology, the Etest has a broad field of application in bacteriology and is recently available for the antimycotics fluconazole and itraconazole. By means of the presence of gradient concentrations of the active substance on the carrier material, it is possible to obtain reproducible MICs of the antimycotic substances. The results of susceptibility testing of 326 clinical yeast isolates with the Etest were compared to those MICs obtained by microdilution and agar dilution. A 100% concordance of the MIC markers (mode-, MIC50- and MIC90-value, standard deviation of the mean log2-MIC-dilution steps) was given when compared by a +/- 1 MIC-dilution step range with microdilution and by +/- 2 MIC-dilution steps with agar dilution; species dependent all strains were within 2 x standard deviation of the individual MIC-mean of the species. By comparison of the individual MIC-values maximum differences of +/- 6 MIC-dilution steps were obtained, where 70% of all results were within +/- 2 MIC-dilution steps, and more than 92% of all strains were within +/- 3 MIC-dilution steps. The Pearson's correlation coefficients show a good agreement of the Etest with microdilution (r = 0.92) resp., agar dilution (r = 0.88) demonstrate, however, clearly insufficient correlations (r < 0.65) to the reference methods, for species with difficult to read Etest inhibition zones (e.g., Candida glabrata, Candida krusei, Candida parapsilosis). The differences between the proposed test methods recommended by the NCCLS and the working group "Clinical Mycology" of the German Speaking Mycological Society (AG-KMYK) are tabled.

Testing susceptibility of Candida species to fluconazole and itraconazole using the microdilution assay

A microdilution system was established for testing the susceptibility of Candida species to fluconazole and itraconazole. The assay used a sodium phosphate-buffered (0.1 mol l-1) Casitone/glucose medium (pH 7.2) supplemented with potassium, iron, magnesium, trace elements and vitamins. Tests were read photometrically after 24 h, and the minimum inhibitory concentration (MIC) was defined as the IC90. In nearly all strains sharp end points were observed. The MICs against Candida species without any known pre-exposure to azoles were found to range from 0.2 to 1.56 micrograms ml-1 for fluconazole and from 2.3 to 12 ng ml-1 for intraconazole. For strains from candidosis patients refractory to treatment with fluconazole the MICs of fluconazole ranged from 6.25 to 100 micrograms ml-1, while those for itraconazole varied between 12 and > 450 ng ml-1. The strains did not respond congruent to both azoles. A similar disparity of the MICs was observed with Candida tropicalis and Candida krusei. The unusually low MICs of itraconazole were attained because (1) the dilution series was prepared from a preformed (concentrated) dilution series in 75% dimethylsulphoxide and not directly by serial dilution in the broth and (2) the incubation was performed in microtitre plates made of quartz glass rather than in the generally used polystyrene microtitre plates.

An evaluation of seven methods of testing in vitro susceptibility of clinical yeast isolates to fluconazole

Four commercially available in vitro test systems (Candifast, E-test, Mycototal and spiral-gradient end point method), agar diffusion with 25-micrograms fluconazole paper test discs and 15-micrograms test tablets and agar dilution were compared with the microbroth dilution method for fluconazole susceptibility testing of 145 clinical isolates. In addition, the culture media provided or recommended by the manufacturers of the test systems were compared with high-resolution (HR) antifungal test medium. With all currently available culture media, growth problems (inhibition or delayed growth of the clinical isolates) occurred with solid or semisolid media. With minor improvements, HR medium demonstrated the most reproducible and comparable results (supplementation with asparagine and deletion of sodium hydrogen carbonate). The best correlation with microdilution was obtained by the agar dilution method (> 95% concordance) followed by the spiral-gradient end point method (85%), Candifast (83%), Mycototal (81%) and the E-test (78%). Regression analysis demonstrated good correlation between agar diffusion and micro-/agar dilution (r > 0.9).

Antifungal susceptibility testing of yeasts: evaluation of technical variables for test automation

The technical parameters for antifungal susceptibility testing with Candida species were reexamined to determine the optimal conditions for testing with semiautomated preparations of broth microdilution cultures, automated spectrophotometric readings of the cultures, and dose-response and endpoint determinations by means of a computer spreadsheet. Tests were based on proposed standard method M27P of the National Committee for Clinical Laboratory Standards for antifungal agents. RPMI 1640 broth with extra glucose to a final concentration of 2% gave higher and more reproducible drug-free control readings without affecting susceptibility endpoint readings. An inoculum of 8 x 10(4) yeasts per ml prepared from a carbon-limiting broth culture without further standardization was found to give optimal control readings after 48 h of incubation at 37 degrees C. For flucytosine, fluconazole, itraconazole, and ketoconazole, endpoints based on 50% growth inhibition (50% inhibitory concentration) gave the minimum variation with inoculum size and the fewest endpoint differences with RPMI 1640 medium obtained from two different suppliers. The 50% inhibitory concentration was also the optimal endpoint for fluconazole and ketoconazole susceptibilities in comparison with broth macrodilution MICs determined by the method of the National Committee for Clinical Laboratory Standards. Intralaboratory reproducibility was determined by retrospective analysis of replicate results for isolates retested at random over a 2-year period. This approach showed less favorable reproducibility than has been reported from purpose-designed, prospective antifungal susceptibility studies, but it may better reflect real-life test reproducibility. Susceptibility data for 616 clinical isolates of yeasts, representing 16 Candida and Saccharomyces spp., confirmed the tendency of Candida lusitaniae isolates to show relatively low susceptibilities to amphotericin B, the tendency of Candida krusei isolates to show low flucytosine and fluconazole susceptibilities, and the presence of some isolates in the species Candida albicans, Candida glabrata, and Candida tropicalis with low susceptibilities to azole derivative antifungal agents. The study demonstrates the value of automation and standardization in all stages of yeast susceptibility testing, from plate preparation to data analysis.

[Comparison of seven methods of in vitro susceptibility testing of clinical yeast isolates against fluconazole]

Four commercially available in vitro test systems (Candifast, E-test, Mycototal, Spiral-Gradient Endpoint Method), agardiffusion with 25 micrograms fluconazole paper test discs and 15 micrograms test tablets, and agardilution were compared to the microbroth dilution method by fluconazole susceptibility testing of 145 clinical isolates. In addition, the culture media provided or recommended by the manufacturers of the test systems were compared to the high resolution (HR) antifungal test medium. With all currently available culture media growth problems (inhibition or delayed growth of the clinical isolates) occurred with solid or semi-solid media. With minor improvements, HR medium demonstrated the most reproducible and comparable results (supplementation with asparagine and deletion of sodium hydrogen carbonate). Best correlation to microdilution was obtained by the agardilution method > (95% concordance) followed by the spiral gradient endpoint method (85%), Candifast (83%), Mycototal (81%) and the E-test (78%). Regression analysis demonstrated good correlation between agardiffusion and micro-/agardilution(r > 0.9).

In vitro susceptibility of Candida species isolated from patients with haematological malignancies

Candida spp. (83 isolates including C. (Torulopsis) glabrata) were tested in vitro for their susceptibility to 5-fluorocytosine, amphotericin B, ketoconazole, itraconazole, fluconazole, and miconazole. The yeasts were isolated from clinical specimens, mostly from the lower respiratory tract, of 30 oncologic patients, 27/30 with haematological malignancies, during a 6-month period (December 1991-May 1992). Minimal inhibitory concentration (MIC) and minimal fungicidal concentration (MFC) values of the 6 drugs were obtained for each yeast using a microdilution broth method developed in our laboratory. Amphotericin B, and 5-fluorocytosine were active against the majority of the yeasts with MIC90/MFC90 values within achievable serum concentrations (3.12/6.25 micrograms ml(-1) and 0.625/0.625 micrograms ml(-1) respectively). Azole derivatives showed a species-specific activity. MFC values were two to four times higher than those of the MICs, confirming the fungistatic rather than fungicidal activity of azole derivatives. An interesting correlation was found when the in vitro susceptibility values of the isolates were compared with data of patients with or without antifungal prophylaxis or therapy during that period. In general, with respect to fluconazole, C. albicans strains isolated from patients who received no treatment showed MIC and MFC values lower than those obtained from patients who were under prophylaxis or treatment with this drug. Fluconazole administration appears to influence in vitro susceptibility testing.

New in vitro assay based on glucose consumption for determining intraconazole and amphotericin B activities against Aspergillus fumigatus

We developed a new in vitro method of evaluating antifungal molecules. Fungal growth was determined by measuring glucose consumption, the only carbon source in a synthetic medium. First, the growth of 12 Aspergillus fumigatus strains was studied. Glucose consumption was an excellent indicator of fungal growth. Second, the partial inhibition of growth was calculated in terms of the 90% or 50% inhibitory concentration for the 12 strains after treatment with itraconazole and amphotericin B. With a 3-day incubation time, the calculated 90% and 50% inhibitory concentrations agreed with those obtained by a macromethod and with those reported in previous publications. In each case the high degrees of efficacy of itraconazole and amphotericin B against A. fumigatus were confirmed. Partial inhibition induced by low concentrations of antifungal agents was quantifiable by this new method.

Comparison of broth macrodilution, broth microdilution, and E test antifungal susceptibility tests for fluconazole

A comparison of the E test, the broth microdilution test, and the reference broth macrodilution susceptibility test of the National Committee for Clinical Laboratory Standards for fluconazole susceptibility testing was performed with 238 clinical isolates of Candida species and Torulopsis (Candida) glabrata. An 80% inhibition endpoint MIC was determined by the reference broth macrodilution method after 48 h of incubation. The MICs obtained by the two study methods were read after 24 and 48 h of incubation. Overall, excellent agreement within 2 doubling dilutions was obtained between the broth microdilution and the broth macrodilution methods for the combined results for all species at both 24 h (93%) and 48 h (94%). The correlation of 24-h MIC endpoints between the E test and the broth macrodilution methods was 37% for T. glabrata, 56% for Candida tropicalis, 93% for Candida albicans, and 90% for other Candida species. The percent agreement at 48 h ranged from 34% for T. glabrata to 97% for Candida species other than C. albicans and C. tropicalis. These initial results support the further evaluation of the E test as an alternative method for fluconazole susceptibility testing of Candida species.

Emergence of fluconazole-resistant strains of Candida albicans in patients with recurrent oropharyngeal candidosis and human immunodeficiency virus infection

After repeated use of fluconazole for therapy of oropharyngeal candidosis, the emergence of in vitro fluconazole-resistant Candida albicans isolates (MIC, > or = 25 micrograms/ml) together with oral candidosis unresponsive to oral dosages of up to 400 mg of fluconazole were observed in patients with human immunodeficiency virus (HIV) infection. Antifungal susceptibility testing was done by broth microdilution and agar dilution techniques on C. albicans isolates recovered from a cohort of patients with symptomatic HIV infection who were treated repeatedly with fluconazole for oropharyngeal candidosis. In vitro findings did show a gradual increase in the MICs for C. albicans isolates recovered from selected patients with repeated episodes of oropharyngeal candidosis. Primary resistance of C. albicans to fluconazole was not seen. Cross-resistance in vitro occurred between fluconazole and other azoles (ketoconazole, itraconazole), but to a lesser extent. The results of the study suggest that the development of clinical resistance to fluconazole could be clearly correlated to in vitro resistance to fluconazole. Itraconazole may still serve as an effective antifungal agent in patients with HIV infection and oropharyngeal candidosis nonresponsive to fluconazole.

Fluconazole susceptibilities of Candida species and distribution of species recovered from blood cultures over a 5-year period

The distribution and fluconazole susceptibilities of Candida species isolated over a 5-year period were investigated. Susceptibilities were determined by using a new microtiter procedure and the National Committee for Clinical Laboratory Standards (NCCLS) proposed standard. The new method correlated well with the NCCLS proposed standard and gave very clear end points. Results indicate that there are species-related differences in MICs as reflected in the MICs for 90% of species tested. Candida albicans is most susceptible to fluconazole, while Candida glabrata is among the least susceptible. These findings coincided with the observation of a shift in distribution of yeast species recovered from blood cultures from 1987 to 1992. C. albicans was the predominant species (87%) in the pre- or early fluconazole years but decreased to only 31% of the isolates in 1992. Thus, Candida species for which MICs of fluconazole were higher have become more prominent in recent years. Significantly, throughout this period, MICs for each species did not change appreciably.

Antagonistic effects of fluconazole and 5-fluorocytosine on candidacidal action of amphotericin B in human serum

This study addressed the effects of fluconazole and 5-fluorocytosine on the candidacidal activity of amphotericin B in the presence of human serum. A Candida albicans isolate that was susceptible to all three agents according to standard testing procedures was employed. Fungicidal activity was estimated by using a flow cytometric procedure that exploited the fact that yeast cells killed by amphotericin B diminish in size and take up propidium iodide. The following findings were made. (i) Fluconazole and 5-fluorocytosine each failed to inhibit pseudohyphal formation and cell aggregation even when applied at 10 and 50 micrograms/ml, respectively, for up to 10 h. Hence, these agents were not fungistatic when tested in the presence of serum. (ii) Simultaneous application of 5-fluorocytosine had neither enhancing nor inhibitory effects on the fungicidal activity of amphotericin B. However, yeasts that were preincubated for 20 h with 5-fluorocytosine became less susceptible to killing by amphotericin B. (iii) Fluconazole exerted a frank antagonistic effect on the fungicidal activity of amphotericin B. Thus, under our in vitro conditions, both fluconazole and 5-fluorocytosine can overtly antagonize the candidacidal action of amphotericin B.

Candida albicans genotyping in studies with patients with AIDS developing resistance to fluconazole

We characterized Candida albicans strains responsible for recurrent oropharyngeal candidosis (OPC) in four patients with AIDS who developed clinical and mycological resistance to fluconazole (FCZ). Karyotype and restriction fragment length polymorphism analyses were performed on the clonal populations to differentiate relapse from reinfection, and the results were assessed with those of serotype and FCZ MICs. Despite the polymorphism in chromosomal bands larger than 2.2 Mbp related to an intraclonal variation, karyotype analysis showed a single strain type attributable to each patient. On the other hand, EcoRI and HinfI restriction fragments revealed a polymorphism for one patient between the first sample and the subsequent ones, relevant to the acquisition of a new strain causing the following episodes of OPC. This result coincided with switching of the serotype and with the acquisition of a resistance to FCZ. For the other three patients, the similarity of the DNA electrophoretic patterns and the serotype of the samples suggested that recurrence can be due to the initial strain that generates FCZ resistance. Although useful for epidemiological studies, molecular typing methods seem to be inadequate to detect the acquisition of FCZ resistance.

In vitro susceptibility of Candida albicans, Austria 1992

A total of 38 isolates of Candida albicans were collected from clinical specimens and tested for minimum inhibitory concentrations (MICs) and minimum fungicidal concentrations (MFCs) of amphotericin B, flucytosine and fluconazole. For amphotericin B and flucytosine all patients had isolates with MICs < or = 2 micrograms ml-1 and < or = 8 micrograms ml-1 respectively. For fluconazole all but two isolates had MICs < or = 16 micrograms ml-1. In vitro susceptibility testing remains problematic since standardized methods have not been established and the susceptibility and resistance ranges of MIC/MFC values still have to be defined in relation to in vivo data. Nevertheless, these results indicate that topical application of amphotericin B (more than 1 kg of amphotericin B per year at an intensive care unit performing selective digestive tract decontamination on certain trauma patients) had no major impact on the resistance patterns of C. albicans. In contrast, in this study both fluconazole-resistant isolates were from AIDS patients who were known to have received fluconazole therapy. Treatment with fluconazole--quite common in patients with AIDS--may produce resistant fungal isolates.

Antifungal susceptibility testing

Unlike antibacterial susceptibility testing, reliable antifungal susceptibility testing is still largely in its infancy. Many methods have been described, but they produce widely discrepant results unless such factors as pH, inoculum size, medium formulation, incubation time, and incubation temperature are carefully controlled. Even when laboratories agree upon a common method, interlaboratory agreement may be poor. As a result of numerous collaborative projects carried out both independently and under the aegis of the Subcommittee on Antifungal Susceptibility Testing of the National Committee for Clinical Laboratory Standards, the effects of varying these factors have been extensively studied and a standard method which minimizes interlaboratory variability during the testing of Candida spp. and Cryptococcus neoformans has been proposed. This review summarizes this work, reviews the strengths and weaknesses of the proposed susceptibility testing standard, and identifies directions for future work.

Two-site comparison of broth microdilution and semisolid agar dilution methods for susceptibility testing of Cryptococcus neoformans in three media

This study evaluated the inter- and intralaboratory agreement between results of the semisolid agar dilution and broth microdilution methods of antifungal susceptibility testing of Cryptococcus neoformans. Three media were tested in two laboratories. The drugs tested were amphotericin B, flucytosine, itraconazole, fluconazole, and Schering 39304. Analysis by kappa statistics revealed good agreement between the laboratories for the two methods. The highest level of inter- and intralaboratory agreement was observed in RPMI 1640 with L-glutamine followed by Eagle's minimum essential medium and yeast nitrogen broth. The broth microdilution method appears more suitable than the semisolid agar dilution method for testing cryptococci because of its ease in performance, cost, and simplicity.