Multicenter evaluation of a broth macrodilution antifungal susceptibility test for yeasts

In vitro susceptibilities of clinical and environmental isolates of Cryptococcus neoformans to five antifungal drugs

A total of 53 Cryptococcus neoformans strains, including clinical and environmental Brazilian isolates, were tested for their susceptibilities to amphotericin B, 5-flucytosine, ketoconazole, fluconazole, and itraconazole. The tests were performed according to the National Committee of Clinical Laboratory Standards recommendations (document M27-P). In general, there was a remarkable homogeneity of results for all strains, and comparable MICs were found for environmental and clinical isolates. This paper represents the first contribution in which susceptibility data for Brazilian C. neoformans isolates are provided.

[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.

Fungicidal effect of tyrothricin on Candida albicans

Tyrothricin, a polypeptide antibiotic, is active against yeast cells. Tyrothricin was rapidly fungicidal towards Candida albicans. Concentration of four times the minimum inhibitory (25 mg l-1) reduced the yeast numbers by more than 3 log10 within 1 h. Similar results were obtained in a flow cytometric antifungal activity assay using the new two-colour probe for yeast viability, FUN-1, which measures impairment of metabolic activity. The respiratory activity of Candida albicans, measured in a XTT kinetic assay, was significantly reduced in comparison with controls by 3.12 mg l-1 of the substance. Because fungicidal concentrations of tyrothricin are locally achievable in patients, an evaluation of the local effect of tyrothricin in patients suffering from mucosal infections with Candida species should be considered.

Comparison of two alternative microdilution procedures with the National Committee for Clinical Laboratory Standards reference macrodilution method M27-P for in vitro testing of fluconazole-resistant and -susceptible isolates of Candida albicans

The National Committee for Clinical Laboratory Standards has proposed a reference broth macrodilution method for in vitro antifungal susceptibility testing of yeasts (the M27-P method). This method is cumbersome and time-consuming and includes MIC endpoint determination by the visual and subjective inspection of growth inhibition after 48 h of incubation. Two alternative microdilution procedures for MIC endpoint determination, a spectrophotometric MIC endpoint test that evaluates 80% growth inhibition by the drug and a colorimetric method with an oxidation-reduction indicator (Alamar Blue), were compared with the M27-P method for fluconazole susceptibility testing of 45 susceptible and resistant isolates of Candida albicans. The spectrophotometric method was performed with RPMI 1640 medium with 2% glucose, and the other two tests were performed with plain RPMI 1640 medium. All tests were incubated at 35 degrees C. Excellent agreement was demonstrated between the M27-P method and both 24-h microdilution tests (97.7%) as well as between the two microdilution tests (95.5%). Also, there was agreement in the detection in vivo of fluconazole resistance by the three methods. These preliminary data indicate that both microdilution methods may serve as less subjective alternatives to the M27-P method for the determination of fluconazole MIC endpoints.

Synergy of fluconazole with macrophages for antifungal activity against Candida albicans

The possible between macrophages and fluconazole for antifungal activity against different isolates of C. albicans was studied. The susceptibility of C. albicans isolates to fluconazole (FCZ), when incubated in RPMI-1640 with 10% fetal bovine serum (FBS) and 10% fresh mouse serum (test medium, TM) was determined by using a quantitative culture methodology, Multiplication of isolate Sh27 was strongly inhibited by FCZ, even at 1.0 microgram/ml. However, FCZ even at 100 microgram/ml was not fungicidal. Resident murine peritoneal macrophages (MP) incubated for 48 h in RPMI-1640 + 10% FBS (tissue culture medium, TCM), then challenged with Sh27 in TM for 24 h, were fungistatic (20 +/ 9%, n = 4). Cultured macrophages synergized with FCZ (10 micrograms/ml) for fungicidal activity when co-cultured with sh27 in TM for 24 h (46 +/ 8%) and for 48 h (74 +/ 5%), n = 3. Macrophages and FCZ (10 micrograms/ml) could not synergize for significant killing of a less FCZ-sensitive C. albicans isolate 94-164. Multiplication of a FCZ-resistant isolate (94-20) was not inhibited by FCZ at 10 micrograms/ml TM; however, macrophages and FCZ (10 micrograms/ml) could synergize for fungistatic (64%), but not fungicidal, activity.

Transmission of the effect of an antifungal agent within a single hypha

A micro-compartment culture method was devised in which a single hypha of Rhizopus stolonifer growing on an agar section traversed an antifungal non-diffusible barrier to another agar section; thus the local environment of the distal or proximal part of the hypha could be controlled independently. The responses in terms of hyphal extension of the test fungus to local application of amphotericin B in this culture system were estimated by using an automatic analysing system. After hyphae had traversed the barrier, distal application of amphotericin B caused no appreciable effect on the proximal hyphae. In contrast, proximal application of amphotericin B caused inhibition of the extension of distal hyphae. The reversal of polarized cytoplasmic streaming also occurred during the inhibition of distal hyphal extension. The extents of inhibition of the distal hyphal extension and the cytoplasmic streaming were dependent upon the hyphal distance between the amphotericin B application site and the hyphal tip. These results show that the effect of an antifungal agent on a hypha depends on the region of the hypha exposed. Cytoplasmic streaming may play key role in the transmission of antifungal effects within a single hypha.

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.

Different components in human serum inhibit multiplication of Cryptococcus neoformans and enhance fluconazole activity

The inhibitory effect of human serum on the multiplication of Cryptococcus neoformans and the interaction with fluconazole were studied. Compared with cryptococcal multiplication in RPMI 1640 medium alone, 5% human serum in medium inhibited multiplication by 76% +/- 6% (n = 8). The inhibitory effect of human serum was donor independent, [corrected] heat stable (56 degrees C, 30 min), and not due to albumin or globulin. Bovine and murine sera were not inhibitory at that concentration. A fungistatic concentration of fluconazole (5.0 micrograms/ml) in medium plus 5% human serum resulted in 40% +/- 5% (n = 8) killing (reduction of inoculum CFU) in a 24-h assay. Bovine or murine sera did not have the enhancing effect, and this human serum activity was heat stable and donor independent. At 2.5 micrograms of fluconazole per ml, fungistasis by fluconazole plus human serum was significantly greater than with either alone. Higher serum concentrations [corrected] potentiated fluconazole more. At higher fluconazole concentrations (e.g., 20 micrograms/ml) fluconazole alone could kill, but serum potentiated this. A fluconazole-resistant isolate (MIC, 100 micrograms/ml) was not killed by fluconazole (5.0 micrograms/ml) in 5% human serum, but human serum potentiated the partial fluconazole inhibition. When human serum was dialyzed (molecular weight cutoff, 6,000 to 8,000) against phosphate-buffered saline, it lost the ability to synergize with fluconazole for killing Cryptococcus organisms but not the capacity to inhibit multiplication. Filtration of serum suggested the filtrate with a molecular weight of < 10,000 could interact synergistically with fluconazole for killing but could not inhibit cryptococcal multiplication. These findings indicate that human serum has two components, one (macromolecular) with a unique ability to inhibit C. neoformans and a low-molecular-weight component that enhances fluconazole anticryptococcal activity.

Cryptococcosis in the era of AIDS--100 years after the discovery of Cryptococcus neoformans

Although Cryptococcus neoformans and cryptococcosis have existed for several millennia, a century has passed since the discovery of this encapsulated yeast and its devastating disease. With the advent of the AIDS pandemic, cryptococcal meningitis has emerged as a leading cause of infectious morbidity and mortality and a frequently life-threatening opportunistic mycosis among patients with AIDS. Both basic and clinical research have accelerated in the 1990s, and this review attempts to highlight some of these advances. The discussion covers recent findings, current concepts, controversies, and unresolved issues related to the ecology and genetics of C. neoformans; the surface structure of the yeast; and the mechanisms of host defense. Regarding cell-mediated immunity, CD4+ T cells are crucial for successful resistance, but CD8+ T cells may also participate significantly in the cytokine-mediated activation of anticryptococcal effector cells. In addition to cell-mediated immunity, monoclonal antibodies to the major capsular polysaccharide, the glucuronoxylomannan, offer some protection in murine models of cryptococcosis. Clinical concepts are presented that relate to the distinctive features of cryptococcosis in patients with AIDS and the diagnosis, treatment, and prevention of cryptococcosis in AIDS patients.

Comparative evaluation of macrodilution and alamar colorimetric microdilution broth methods for antifungal susceptibility testing of yeast isolates

A comparative evaluation of the macrodilution method and the Alamar colorimetric method for the susceptibility testing of amphotericin B, fluconazole, and flucytosine was conducted with 134 pathogenic yeasts. The clinical isolates included 28 Candida albicans, 17 Candida tropicalis, 15 Candida parapsilosis, 12 Candida krusei, 10 Candida lusitaniae, 9 Candida guilliermondii, 18 Torulopsis glabrata, and 25 Cryptococcus neoformans isolates. The macrodilution method was performed and interpreted according to the recommendations of the National Committee for Clinical Laboratory Standards (document M27-P), and the Alamar colorimetric method was performed according to the manufacturer's instructions. For the Alamar colorimetric method, MICs were determined at 24 and 48 h of incubation for Candida species and T. glabrata and at 48 and 72 h of incubation for C. neoformans. The overall agreement within +/- 1 dilution for Candida species and T. glabrata against the three antifungal agents was generally good, with the values for amphotericin B, fluconazole, and flucytosine being 85.3, 77.9, and 86.2%, respectively, at the 24-h readings and 69.3, 65.2, and 97.2%, respectively, at the 48-h readings. Most disagreement was noted with fluconazole against C. tropicalis and T. glabrata. Our studies indicate that determination of MICs at 24 h by the Alamar colorimetric method is a valid alternate method for testing amphotericin B, fluconazole, and flucytosine against Candida species but not for testing fluconazole against C. tropicalis and T. glabrata. For flucytosine, much better agreement can be demonstrated against Candida species and T. glabrata at the 48-h readings by the Alamar method. Excellent agreement within +/- dilution can also be observed for amphotericin B, fluconazole, and flucytosine (80, 96, and 96%, respectively) against c. neoformans when the MICs were determined at 72 h by the Alamar method.

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.

Central venous catheter infection with Rhodotorula minuta in a patient with AIDS taking suppressive doses of fluconazole

A case of Rhodotorula minuta central venous catheter infection with fungaemia is described in a patient with advanced acquired immunodeficiency syndrome (AIDS), HIV nephropathy, end-stage renal disease requiring haemodialysis, and a permanent Quinton catheter in place for 6 months. At the time of fungaemia, the patient was taking 100 mg fluconazole per os daily for a previous episode of Candida oesophagitis. R. minuta central venous catheter infection with fungaemia was successfully treated with 455 mg total dose amphotericin B (0.6 mg kg-1 day-1) over 25 days without removal of the catheter. In vitro antifungal susceptibility testing for R. minuta revealed a minimum inhibitory concentration to fluconazole of > 100 micrograms ml-1 and to amphotericin B of 1.2 microgram ml-1. Clinically evident fungaemia, even with an unusual organism such as R. minuta, may occur in patients with intravenous catheters, and while the immunosuppressed patient is receiving azole therapy.

Three distinct genotypes within Candida parapsilosis from clinical sources

Three genetically distinct groups of Candida parapsilosis were detected among clinical isolates. These were distinguishable on the basis of isoenzyme profiles and DNA sequences of internally transcribed spacer (ITS) sequences flanking the 5.8S RNA gene. In an investigation of 45 strains, including 32 clinical isolates from Texas, C. parapsilosis group I composed the majority of the common clinical isolates. The type strain of C. parapsilosis was a member of this group. The 10 group II isolates were indistinguishable from group I strains when tested with the API 20C kit. The two group III isolates differed from those in groups I and II by being D-xylitol positive by the API 20C kit; however, isolates in all groups assimilated D-xylitol from broth. Isoenzyme profiles excluded the close relationship of any of these groups to Lodderomyces elongisporus, which is a teleomorphic yeast that has a physiological profile similar to that of C. parapsilosis. Although there were insignificant differences in the ITS2 rDNA sequences, comparisons of the ITS1 sequences revealed several differences. A sequence analysis of ITS1 in which missing bases were counted as mismatches showed the following similarities: group I versus group II, 87.7%; group I versus group III, 82.1%; group II versus group III, 84.5%. Also, the activity of secreted proteinase showed differences among the three groups, with many group I isolates having moderate to high activity. The degree of susceptibility to antifungal agents, amphotericin B, ketoconazole, and 5-fluorocytosine, could not be used to determine an isolate's group.

Effect of increasing inoculum sizes of pathogenic filamentous fungi on MICs of antifungal agents by broth microdilution method

Inoculum size is a critical variable in development of methods for antifungal susceptibility testing for filamentous fungi. In order to investigate the influence of different inoculum sizes on MICs of amphotericin B, 5-fluorocytosine, itraconazole, and miconazole, 32 clinical isolates (8 Aspergillus fumigatus, 8 Aspergillus flavus, 5 Rhizopus arrhizus, 8 Pseudallescheria boydii, and 3 Fusarium solani isolates) were studied by the broth microdilution method. Four inoculum sizes were studied: 1 x 10(2) to 5 x 10(2), 1 x 10(3) to 5 x 10(3), 1 x 10(4) to 5 x 10(4), and 1 x 10(5) to 5 x 10(5) CFU/ml. The National Committee for Clinical Laboratory Standards reference method for antifungal susceptibility testing in yeasts was modified and applied to filamentous fungi. The inoculum was spectrophotometrically adjusted, and all tests were performed in buffered medium (RPMI 1640) at pH 7.0 with incubation at 35 degrees C for 72 h. MICs were read at 24, 48, and 72 h. Amphotericin B showed a minimum effect of inoculum size on MICs for all species with the exception of P. boydii (P < 0.05). A significant effect of inoculum size on MICs was observed with 5-fluorocytosine, for which there was an increase of more than 10-fold in MICs against all Aspergillus spp. between inoculum concentrations of 10(2) and 10(4) CFU/ml (P < 0.001). For itraconazole, the results showed a more species-dependent increase of MICs, most strikingly for R. arrhizus and P. boydii. Miconazole, which was tested only with P. boydii, did not demonstrate a significant effect of inoculum size on MICs. In summary, the effect of inoculum size on MICs for filamentous fungi was dependent upon the organism and antifungal compound tested. Thus, among antifungal compounds, itraconazole and 5-fluorocytosine demonstrated significant inoculum effects, while amphotericin B and miconazole showed comparatively minimum inoculum effects against pathogenic filamentous fungi. Moreover, among filamentous fungi, P. boydii and R. arrhizus exhibited the greatest inoculum effect.

Comparison of visual and spectrophotometric methods of MIC endpoint determinations by using broth microdilution methods to test five antifungal agents, including the new triazole D0870

A study to compare three different methods for reading MIC endpoints tested by the broth microdilution modification of the National Committee for Clinical Laboratory Standards (Villanova, Pa.) reference method was conducted. MICs of amphotericin B, flucytosine, fluconazole, itraconazole, and a new triazole, D0870, were determined for five reference yeast strains and 100 clinical isolates of Candida spp. MICs were read visually according to National Committee for Clinical Laboratory Standards guidelines from microdilution trays that had been (VS) and had not been (V) shaken. MICs were also determined spectrophotometrically (SP) at 492 nm. SP endpoints were determined as the concentrations resulting in a > or = 50% inhibition of growth (flucytosine and azoles) and a > or = 90% inhibition of growth (amphotericin B) relative to control growth. The five reference strains were tested nine times each against all five antifungal agents, and the MIC results for each reading method were compared with a 3-log2 dilution reference range determined by the macrodilution (M27-P) method. Overall, 84 to 100% of the MICs determined by V, 93 to 100% of those determined by VS, and 89 to 100% of those determined by SP fell within the 3-log2 dilution reference range for each reference strain and antifungal agent. Reproducibility was 99% for V and SP and 98% for VS. Agreement among the three methods of reading ranged from 97 to 99%. Excellent agreement among reading methods was also observed for all antifungal agents when tested against 100 clinical isolates. Agreement between the standard V method (no agitation) and VS ranged from 99 to 100%, and that between V and SP ranged from 89 to 99%. The VS and SP reading methods provided more definitive endpoints than the V method, which does not involve shaking.

Quality control guidelines for National Committee for Clinical Laboratory Standards recommended broth macrodilution testing of amphotericin B, fluconazole, and flucytosine

Amphotericin B, fluconazole, and flucytosine (5FC) were tested in a multilaboratory study to establish quality control (QC) guidelines for yeast antifungal susceptibility testing. Ten candidate QC strains were tested in accordance with National Committee for Clinical Laboratory Standards M27-P guidelines against the three antifungal agents in each of six laboratories. Each laboratory was assigned a unique lot of RPMI 1640 broth medium as well as a lot of RPMI 1640 common to all of the laboratories. The candidate QC strains were tested 20 times each against the three antifungal agents in both unique and common lots of RPMI 1640. A minimum of 220 MICs per drug per organism were generated during the study. Overall, 95% of the MICs of amphotericin B, fluconazole, and 5FC fell within the desired 3 log2-dilution range (mode +/- 1 log2 dilution). Excellent performance with all three drugs was observed for Candida parapsilosis ATCC 22019 and C. krusei ATCC 6258. With these strains, on-scale 3 log2-dilution ranges encompassing 96 to 99% of the MICs of all three drugs were established. These two strains are recommended for QC testing of amphotericin B, fluconazole, and 5FC. Reference ranges were also established for an additional four strains for use in method development and for training. Four strains failed to perform adequately for recommendation as either QC or reference strains.

Detection of amphotericin B-resistant Candida isolates in a broth-based system

Because of the limited ability of the National Committee for Clinical Laboratory Standards proposed M27P methodology to detect resistance to amphotericin B by Candida isolates, we sought to identify alternative media and pH conditions that could reliably identify resistant isolates. Antibiotic Medium 3 broth (also known as Penassay broth) buffered to pH 5 or pH 7 produced superior results and readily identified a series of resistant isolates.

[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 susceptibilities of clinical yeast isolates to three antifungal agents determined by the microdilution method

A comparative evaluation of the in vitro susceptibilities of 597 clinical yeast isolates to amphotericin B, fluconazole, and 5-fluorocytosine (5FC) was conducted. The broth macrodilution reference method of the National Committee for Clinical Laboratory Standards (NCCLS, M27-P) was adapted to the microdilution method. Microdilution endpoints for amphotericin B were scored as the lowest concentration in which a score of 0 (complete absence of growth) was observed and for 5FC and fluconazole as the lowest concentration in which a score of 2 (prominent decrease in turbidity; MIC-2) was observed compared to the growth control. The MIC values were read after 24 and 48 h incubation. A broad range of MIC values was observed with each antifungal agent. Amphotericin B was very active (MIC90 < or = 1.0 microgram/ml) against all of the yeast isolates with the exception of C. lusitaniae (MIC90 > or = 2.0 micrograms/ml). Fluconazole was most active against C. parapsilosis (MIC90 of 1.0 microgram/ml) and least active against C. krusei (MIC90 of 32 micrograms/ml). 5FC was most active against C. albicans, C. parapsilosis, C. tropicalis, and T. glabrata (MIC90 < or = 1.0 microgram/ml) and was least active against C. krusei and C. lusitaniae (MIC90 > or = 16 micrograms/ml). These data indicate that the microdilution method, performed in accordance with M27-P, provides a means of testing larger numbers of yeast isolates against an array of antifungal agents and allows this to be accomplished in a reproducible and standardized manner. Given these results, it appears that the microdilution method may be a useful alternative to the macrodilution reference method for susceptibility testing of yeasts.

Itraconazole treatment of disseminated histoplasmosis in patients with the acquired immunodeficiency syndrome. AIDS Clinical Trial Group

PURPOSE

Amphotericin B has been the treatment of choice for disseminated histoplasmosis in patients with acquired immunodeficiency syndrome (AIDS). Oral antifungal agents would be welcome alternatives to standard treatment of disseminated histoplasmosis in less severe cases. The purpose of this study was to assess the efficacy and safety of itraconazole therapy in patients with AIDS and disseminated histoplasmosis.

PATIENTS AND METHODS

This was a multicenter, open-label, nonrandomized prospective trial conducted in university hospitals of the AIDS Clinical Trial Group. All patients had AIDS and first episodes of disseminated histoplasmosis. Patients with central nervous system involvement or with severe clinical manifestations were excluded. Patients were treated with itraconazole BID by mouth 300 mg for 3 days and then 200 mg BID for 12 weeks. Resolution of clinical findings, clearance of positive cultures, and drug tolerance were the main outcome measurements. A secondary objective was effect of therapy on Histoplasma capsulatum var capsulatum antigen levels.

RESULTS

Of 59 evaluable patients, 50 (85%) responded to therapy. Five patients withdrew because of progressive infection, 1 died of a presumed pulmonary embolus within the first week of therapy without improvement, 2 withdrew because of toxicity, and 1 was lost to follow-up after week 2 of therapy. Patients with moderately severe clinical (fever > 39.5 degrees C or Karnofsky score < 60) or laboratory abnormalities (alkaline phosphatase > 5 times normal or albumin < 3 g/dL) at baseline tended to respond more poorly than did other patients. Resolution of complaints of fever and improvement in fatigue occurred after a median of 3 and 6 weeks, respectively, and weight gain after 2 weeks. Fungemia cleared after a median of 1 week. H capsulatum var capsulatum antigen cleared from the urine and serum at rates of 0.2 and 0.3 units per week, respectively.

CONCLUSIONS

Itraconazole is safe and effective induction therapy for mild disseminated histoplasmosis in patients with AIDS, offering an alternative to amphotericin B in such cases. Patients with moderately severe or severe histoplasmosis should first be treated with amphotericin B and then may be switched to itraconazole after achieving clinical improvement.

Susceptibility testing of Candida albicans and Aspergillus species by a simple microtiter menadione-augmented 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide assay

We describe a simple microtiter method for determining the susceptibility of Candida albicans and hyphal forms of Aspergillus fumigatus against antifungal agents. The assay measures mitochondrial respiration by determining reduction of 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) to formazan, a process that is enhanced in the presence of menadione. C. albicans or conidial suspensions of A. fumigatus are seeded into microtiter plates. Hyphal outgrowth of Aspergillus spp. was achieved by a 12 to 14-h culture at 30 degrees C. Antifungal agents (amphotericin B, fluconazole, itraconazole) were added to the cultures for 24 h. Thereafter, incubations were continued for 3 h in the presence of MTT plus 0.1 mM menadione. Formazan formation was quantified photometrically after extraction of the formazan with acid isopropanol. Well-defined dose-response curves reflecting impairment of mitochondrial function by the antifungal agents were obtained. With C. albicans, the results correlated excellently with the MIC determinations performed according to the standard macrodilution procedure. In confirmation of a recent report, it was found that fluconazole was unable to exert its fungistatic action on a sensitive C. albicans strain in the presence of serum. The presented method can easily be integrated in the standard repertoire of a diagnostic microbiology laboratory and should prove useful as a means to assess the antifungal action of various agents on yeasts and filamentous fungi in the presence and absence of serum proteins or body fluids.

Comparison of Etest and National Committee for Clinical Laboratory Standards broth macrodilution method for azole antifungal susceptibility testing

The use of Etest strips for antimicrobial susceptibility testing is a new and promising method with broad applications in microbiology. Since these strips contain a predefined continuous gradient of a drug, it is possible to obtain a reproducible, quantitative MIC reading. We performed a prospective and double-blinded study to compare the Etest and National Committee for Clinical Laboratory Standards (Villanova, Pa.) broth macrodilution methods for determining the MICs of fluconazole, itraconazole, and ketoconazole for 100 clinical isolates (25 Candida albicans, 25 Cryptococcus neoformans var. neoformans, 20 Torulopsis [Candida] glabrata, 15 Candida tropicalis, and 15 Candida parapsilosis). The Etest method was performed according to the manufacturer's instructions, and the reference method was performed according to National Committee for Clinical Laboratory Standards document M27-P guidelines. Despite differences between results for some species-drug combinations, Etest and macrobroth MICs were, in general, in good agreement. The MIC agreement rates for the two methods, within +/- 1 dilution, were 71% for ketoconazole, 80% for fluconazole, and 84% for itraconazole. According to our data, Etest has potential utility as an alternative method.

Comparative and collaborative evaluation of standardization of antifungal susceptibility testing for filamentous fungi

The purpose of the study was to evaluate the interlaboratory agreement of broth dilution susceptibility methods for five species of conidium-forming (size range, 2 to 7 microns) filamentous fungi. The methods used included both macro- and microdilution methods that were adaptations of the proposed reference method of the National Committee for Clinical Laboratory Standards for yeasts (m27-P). The MICs of amphotericin B, fluconazole, itraconazole, miconazole, and ketoconazole were determined in six centers by both macro- and microdilution tests for 25 isolates of Aspergillus flavus, Aspergillus fumigatus, Pseudallescheria boydii, Rhizopus arrhizus, and Sporothrix schenckii. All isolates produced clearly detectable growth within 1 to 4 days at 35 degrees C in the RPMI 1640 medium. Colony counts of 0.4 x 10(6) to 3.3 x 10(6) CFU/ml (mean, 1.4 x 10(6) CFU/ml) were demonstrated in 90% of the 148 inoculum preparations. Overall, good intralaboratory agreement was demonstrated with amphotericin B, fluconazole, and ketoconazole MICs (90 to 97%). The agreement was lower with itraconazole MICs (59 to 79% median). Interlaboratory reproducibility demonstrated similar results: 90 to 100% agreement with amphotericin B, fluconazole, miconazole, and ketoconazole MICs and 59 to 91% with itraconazole MICs. Among the species tested, the MICs for S. schenckii showed the highest variability. The results of the study imply that it may be possible to develop a reference method for antifungal susceptibility testing of filamentous fungi.

Salivary concentrations of ketoconazole and fluconazole: implications for drug efficacy in oropharyngeal and esophageal candidiasis

OBJECTIVE

To determine whether salivary concentrations of ketoconazole and fluconazole may explain the apparent disparity between in vitro activity and clinical efficacy observed with these drugs.

DESIGN

Healthy subjects received a single oral dose of ketoconazole 400 mg or fluconazole 100 mg in a randomized, crossover fashion. Saliva was collected at 0, 1, 2, 3, 6, 12, and 24 hours. Blood samples were obtained at 2 and 24 hours. Salivary concentrations and plasma concentrations for each drug were determined by HPLC. Minimum inhibitory concentration (MIC) testing was determined in triplicate on 6 clinical isolates of Candida albicans, and times over the median MIC values were calculated.

PARTICIPANTS

Eight subjects completed the study.

RESULTS

The mean (+/- SD) peak salivary concentration for ketoconazole was 0.119 +/- 0.050 microgram/mL at 3 hours; no subject had a detectable ketoconazole salivary concentration at 24 hours. At 2 and 24 hours, mean ketoconazole plasma concentrations were 7.64 +/- 3.87 and 0.11 +/- 0.05 microgram/mL, respectively. The saliva to plasma concentration ratio at 2 hours was 0.01. The mean peak salivary concentration of fluconazole was 2.56 +/- 0.34 microgram/mL at 3 hours. At 24 hours, the mean salivary concentration was 1.44 +/- 0.33 microgram/mL. At 2 and 24 hours, mean fluconazole plasma concentrations were 4.39 +/- 3.33 and 3.72 +/- 2.83 micrograms/mL, respectively. The saliva to plasma concentration ratio at 2 hours was 0.55. Median MIC values were 0.0625 microgram/mL (range 0.0313-0.125) for ketoconazole and 0.25 microgram/mL (range 0.125-0.5) for fluconazole. Calculated times over which ketoconazole and fluconazole exceeded the median MICs in saliva were approximately 13 and greater than 24 hours, respectively.

CONCLUSIONS

After a single oral dose, fluconazole achieved higher salivary concentrations than did ketoconazole. This may explain the increased clinical efficacy of fluconazole in the treatment of oropharyngeal-esophageal candidiasis when compared with ketoconazole.

Antifungal susceptibility testing of isolates from a randomized, multicenter trial of fluconazole versus amphotericin B as treatment of nonneutropenic patients with candidemia. NIAID Mycoses Study Group and the Candidemia Study Group

The antifungal susceptibilities of 232 pathogenic blood stream Candida isolates collected during a recently completed trial comparing fluconazole (400 mg/day) with amphotericin B (0.5 mg/kg of body weight per day) as treatment for candidemia in the nonneutropenic patient were determined both by the National committee for Clinical Laboratory Standards M27-P macrobroth methodology and by a less cumbersome broth microdilution methodology. For amphotericin B, M27-P yielded a very narrow range of MICs (0.125 to 1 microgram/ml) and there were no susceptibility differences among species. For fluconazole, a broad range of MICs were seen (0.125 to > 64 micrograms/ml), with characteristic MICs seen for each species in the rank order Candida albicans < C. parapsilosis approximately equal to C. lusitaniae < C. glabrata approximately equal to C. krusei approximately equal to C. lipolytica. The MIC distribution for C. tropicalis was bimodal and could not be ranked. Both microdilution MICs were within one tube dilution of the M27-P MIC for > 90% of isolates with amphotericin B and for > or = 77% of isolates with fluconazole. For both methods, elevated MICs did not predict treatment failure. In the case of amphotericin B, the MIC range was too narrow to permit identification of resistant isolates. In the case of fluconazole, MICs for isolates associated with failure to clear the bloodstream consistently were equivalent to the median MIC for the given species. Successful courses of therapy were seen with four isolates from four patients despite MICs of > or = 32 micrograms/ml. As MICs obtained by M27-P and similar methods correlate with responsiveness to fluconazole therapy in animal models and in AIDS patients with oropharyngeal candidiasis, the lack of correlation in this setting suggests that the MICs for these isolates are at or below the relevant fluconazole breakpoint for this dose of fluconazole and patient setting and that host factors such as failure to exchange intravenous catheters were more important than MIC in predicting outcome.

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.

Turbidimetric and visual criteria for in vitro susceptibility testing of Cryptococcus neoformans clinical isolates

The drug concentration which inhibited 50% of growth (IC50), the lowest drug concentration at which growth was less than 30% of that in a positive control well (IC30), the visual minimal inhibitory concentration (MIC visual), were applied to study the effects of fluconazole, itraconazole, amphotericin B and flucytosine against 27 isolates of Cryptococcus neoformans by a broth microdilution technique. When the recommendations established by NCCLS Subcommittee on Antifungal Susceptibility Test were applied for the visual reading of the microplates, the results were comparable with those obtained by the turbidimetric method. No statistically significant differences between MIC visual and IC30 readings were observed with the azoles. There were, however, differences with amphotericin B and flucytosine. In absolute terms MICs of amphotericin B and flucytosine showed higher values than IC30s and IC50s.

Epidemiology of oral candidiasis in HIV-infected patients: colonization, infection, treatment, and emergence of fluconazole resistance

PURPOSE

To study the epidemiology of oral candidiasis and the effect of treatment of thrush in human immunodeficiency virus (HIV)-infected patients.

PATIENTS AND METHODS

We conducted a prospective observational study of 92 patients over 1 year, including a nonblinded, randomized treatment trial of thrush with clotrimazole troches or oral fluconazole. Oral sites were cultured monthly and when thrush occurred. Candida albicans strains were typed by contour-clamped homogeneous electric field (CHEF) electrophoresis. Changes in strains were evaluated over time and in regard to their associations with particular sites, episodes of thrush, relapse after treatment, and colonization of sexual partners. Susceptibility to fluconazole was tested and CHEF analysis was done on these strains to determine the epidemiology of fluconazole resistance.

RESULTS

Yeasts colonized 84% of patients. C albicans accounted for 81% of all isolates and was separated into 34 distinct strains. Most patients had persistent carriage of 1 or 2 dominant strains of C albicans. Three couples shared strains. Nineteen different C albicans strains caused 82 episodes of thrush in 45 patients. CD4 < 200/microL was associated with development of thrush. Clinical cure rates were similar with fluconazole (96%) and clotrimazole (91%), but mycologic cure was better with fluconazole (49%) than clotrimazole (27%). Following mycologic cure, colonization recurred with the same strain 74% of the time. Colonization with Torulopsis glabrata and Saccharomyces cerevisiae increased after treatment with either drug, but these organisms were never a sole cause of thrush. In a subset of 35 patients followed for over 3 months in whom fluconazole susceptibilities were performed, minimum inhibitory concentrations (MICs) to fluconazole increased only in those on fluconazole prophylaxis. Clinical failure of fluconazole was associated with an MIC > or = 64 micrograms/mL in 3 patients, and with an MIC of 8 micrograms/mL in 1 patient. In 2 of these 4 patients, the prior colonizing strain developed fluconazole resistance. In the other 2, new resistant strains were acquired.

CONCLUSIONS

Many different strains of C albicans colonize and cause thrush in patients infected with HIV. Patients are usually persistently colonized with a single strain, and recurrences following treatment are usually due to the same strain. Transmission of strains may occur between couples. Fluconazole and clotrimazole are equally effective in treating thrush, but mycologic cure occurs more often with fluconazole. Fluconazole resistance in C albicans occurs most often in patients who have low CD4 counts and are taking fluconazole prophylactically for recurrent thrush. Fluconazole resistance may occur through acquisition of a new resistant strain or by development of resistance in a previously susceptible strain.

Comparison of fluconazole minimum inhibitory concentrations in three different formulations of RPMI-1640

This study aimed to compare the susceptibilities of fluconazole, obtained in two laboratories, using three RPMI-1640 formulations (manufacturers') and inhibition standards (80%). A total of 39 selected Candida species (in vitro susceptible and resistant) and Cryptococcus neoformans isolates were tested in each medium by broth macro- and microdilution procedures following the National Committee for Clinical Laboratory Standards proposed reference method (M27-P). Macrodilution minimum inhibitory concentrations (MIC80%) were the lowest drug concentrations with turbidity (growth) less than or equal to that of the specific 80% inhibition standards (1:5 growth control). Microdilution MIC-2 were the lowest drug concentrations in which there was prominent decrease in growth. In general, the three formulations of RPMI-1640 medium provided similar MICs for most of the yeasts tested in both laboratories and by each test.

Comparative study of broth macrodilution and microdilution techniques for in vitro antifungal susceptibility testing of yeasts by using the National Committee for Clinical Laboratory Standards' proposed standard

A comparative study of broth macro- and microdilution methods for susceptibility testing of fluconazole, itraconazole, flucytosine, and amphotericin B was conducted with 273 yeasts. The clinical isolates included 100 Candida albicans, 28 Candida tropicalis, 25 Candida parapsilosis, 15 Candida lusitaniae, 15 Candida krusei, 50 Cryptococcus neoformans var. neoformans, 25 Torulopsis (Candida) glabrata, and 15 Trichosporon beigelii strains. Both methods were performed according to the National Committee for Clinical Laboratory Standards' (NCCLS) recommendations (document M27-P). For fluconazole, itraconazole, and flucytosine, the endpoint was the tube that showed 80% growth inhibition compared with the growth control for the macrodilution method and the well with slightly hazy turbidity (score 1) compared with the growth control for the microdilution method. For amphotericin B, the endpoint was the tube and/or well in which there was absence of growth. For the reference macrodilution method, the MICs were determined after 48 h of incubation for Candida spp., T. glabrata, and T. beigelii and after 72 h for C. neoformans var. neoformans. For the microdilution method, either the first-day MICs (24 h for all isolates other than C. neoformans and 48 h for C. neoformans var. neoformans) or the second-day MICs (48 and 72 h, respectively) were evaluated. The agreement within one doubling dilution of the macrodilution reference for all drugs was higher with the second-day MICs than with the first-day MICs for the microdilution test for most of the tested strains. General agreement was 92% for fluconazole, 85.7% for itraconazole, 98.3% for flucytosine, and 96.4% for amphotericin B. For C. neoformans var. neoformans and T. beigelii, the agreement of the first-day reading was higher than that of the second-day reading for fluconazole (94 versus 92%, respectively, for C. neoformans var. neoformans, and 86.7 versus 80%, respectively, for T. beigelii). Our studies indicate that the microdilution technique performed following the NCCLS guidelines with a second-day reading is a valid alternative method for testing fluconazole, itraconazole, flucytosine, and amphotericin B against these eight species of yeasts.

Evaluation of 80% inhibition standards for the determination of fluconazole minimum inhibitory concentrations in three laboratories

The purpose of this study was to evaluate 1:5 growth control dilutions (80% inhibition standards) to determine fluconazole minimum inhibitory concentration (MIC) end points in three laboratories. We tested 39 selected Candida species (in vitro susceptible: fluconazole MIC of < or = 1 microgram/ml, and resistant: fluconazole MIC of > or = 8 micrograms/ml) and Cryptococcus neoformans isolates by broth macro- and microdilution procedures following the National Committee for Clinical Laboratory Standards proposed reference method for yeasts (M27-P). Macrodilution MIC80% were the lowest drug concentrations with turbidity (growth) less than or equal to that of specific 1:5 dilutions of the growth control. Microdilution MICs-2 were the lowest drug concentrations in which there was prominent decrease of growth. A total of 1608 MICs were evaluated. C. krusei, C. parapsilosis, and C. tropicalis strains had reproducible fluconazole MICs by both tests (24 and 48 h). Fluconazole MIC80% and MIC-2 end points were consistent at 24 h (with C. albicans) and more variable at 48 h. MICs of C. neoformans were more reproducible at 72 h than at 48 h by both tests. This study suggests that the determination of fluconazole MICs is dependent on the length of incubation and the yeast being tested, and that antifungal testing of the yeasts may be performed by either test.

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.

Evaluation of a novel colorimetric broth microdilution method for antifungal susceptibility testing of yeast isolates

A comparative evaluation of two broth microdilution methods for antifungal susceptibility testing of 600 clinical yeast isolates (Candida spp., Torulopsis glabrata, and Cryptococcus neoformans) against amphotericin B, fluconazole, and flucytosine (5FC) was conducted. Microdilution testing was performed according to National Committee for Clinical Laboratory Standards (NCCLS) recommendations (NCCLS document M27-P). By using the growth control for comparison, reference microdilution MIC endpoints for amphotericin B were scored as the lowest concentration at which a score of 0 (complete absence of growth) was observed, and those for 5FC and fluconazole were scored at the lowest concentration at which a score of 2 (prominent decrease in turbidity) (MIC-2) was observed. The second microdilution method employed a colorimetric endpoint using an oxidation-reduction indicator (Alamar Biosciences, Inc., Sacramento, Calif.) and was assessed independently of the reference microdilution MICs. The MICs for the two microdilution test systems were read after 24 and 48 h of incubation. Excellent agreement between the reference and colorimetric microdilution MICs was observed. Overall agreement was > or = 95% for all three drugs at 24 h. At 48 h, agreement was > or = 98% for amphotericin B and 5FC but dropped to 84% for fluconazole. Given these results it appears that the colorimetric microdilution approach to antifungal susceptibility testing may be viable alternative to the NCCLS reference method for testing yeasts.

Etest for antifungal susceptibility testing of yeasts

Comparison [+/- 1 dilution between minimum inhibitory concentrations (MICs)] of fluconazole and flucytosine Etest MICs of 10 candidate quality control yeast isolates and 78 clinical isolates demonstrated good agreement (> or = 90%) with the reference method (National Committee for Clinical Laboratory Standards document M27-P). This study suggests that the Etest is a promising alternative method for antifungal susceptibility testing in the clinical laboratory, but that further evaluations are needed.

Selection of candidate quality control isolates and tentative quality control ranges for in vitro susceptibility testing of yeast isolates by National Committee for Clinical Laboratory Standards proposed standard methods

The National Committee for Clinical Laboratory Standards has developed a proposed standard method for in vitro antifungal susceptibility testing of yeast isolates (National Committee for Clinical Laboratory Standards, document M27-P, 1992). In order for antifungal testing by the M27-P method to be accepted, reliable quality control (QC) performance criteria must be developed. In the present study, five laboratories tested 10 candidate QC strains 20 times each against three antifungal agents: amphotericin B, fluconazole, and 5-fluorocytosine. All sites conformed to the M27-P standards and used a common lot of tube dilution reagents and RPMI 1640 broth medium. Overall, 98% of MIC results with amphotericin B, 95% with fluconazole, and 92% with 5-fluorocytosine fell within the desired 3-log2 dilution range (mode +/- 1 log2 dilution). Excellent performance with all three antifungal agents was observed for six strains: Candida albicans ATCC 90028, Candida parapsilosis ATCC 90018, C. parapsilosis ATCC 22019, Candida krusei ATCC 6258, Candida tropicalis ATCC 750, and Saccharomyces cerevisiae ATCC 9763. With these strains, 3-log2 dilution ranges encompassing 94 to 100% of MICs for all three drugs were established. Additional studies with multiple lots of RPMI 1640 test medium will be required to establish definitive QC ranges.

Morphological effects of lipopeptides against Aspergillus fumigatus correlate with activities against (1,3)-beta-D-glucan synthase

The lipopeptide antifungal agents, echinocandins, papulacandins, and pneumocandins, kill Candida albicans by inhibiting glucan synthesis. For this fungus, there is a good correlation of in vitro enzyme inhibition with in vitro assays of MICs. Semisynthetic lipopeptides such as cilofungin, LY303366, L-693,989, and L-733,560 have activity in vivo against Aspergillus infections but appear to be inactive in broth dilution in vitro tests (MICs, > 128 micrograms/ml). To understand how compounds which lack activity in vitro can have good in vivo activity, we monitored the effect of pneumocandins on the morphology of Aspergillus fumigatus and A, flavus strains by light microscopy and electron microscopy and related the changes in growth to inhibition of glucan synthesis. Pneumocandin B0 caused profound changes in hyphal growth; light micrographs showed abnormally swollen germ tubes, highly branched hyphal tips, and many cells with distended balloon shapes. Aspergillus electron micrographs confirmed that lipopeptides produce changes in cell walls; drug-treated germlings showed very stubby growth with thick walls and a conspicuous dark outer layer which was much thicker in the subapical regions. The rest of the hyphal tip ultrastructure was unaffected by the drug, indicating considerable specificity for the primary target. The drug-induced growth alteration produced very compact clumps in broth dilution wells, making it possible to score the morphological effect macroscopically. The morphological changes could be assayed quantitatively by using conventional broth microdilution susceptibility assay conditions. We defined the endpoint as the lowest concentration required to produce the morphological effect and called it the minimum effective concentration to distinguish it from the no-growth endpoints used in MIC determinations. The minimum effective concentration assay was related to inhibition of glucan synthase activity in vitro and may provide a starting point for development of susceptibility testing methods for lipopeptides.

Multisite reproducibility of colorimetric broth microdilution method for antifungal susceptibility testing of yeast isolates

MICs of fluconazole and amphotericin B were determined independently for 100 coded yeast isolates by each of six laboratories to determine reproducibility of results by using a colorimetric oxidation-reduction-based broth microdilution test. In addition, each site tested five quality control isolates on at least four different occasions during the study. Results agreed within a three-dilution range (mode +/- 1 log2 dilution) for 96.2% of fluconazole tests and 92.7% of amphotericin B tests. Agreement among tests with the quality control isolates was 99.4% with fluconazole and 98.6% with amphotericin B. These results indicate that the colorimetric microdilution method is reproducible among laboratories.

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.

Comparative evaluation of alternative methods for broth dilution susceptibility testing of fluconazole against Candida albicans

A comparative evaluation of methods for broth macro- and microdilution susceptibility testing of fluconazole was conducted with 119 clinical isolates of Candida albicans. Macro- and microdilution testing were performed according to National Committee for Clinical Laboratory Standards recommendations. For reference macrodilution testing, an 80% inhibition endpoint (MIC 80%) was determined after 48 h of incubation in accordance with National Committee for Clinical Laboratory Standards proposed standard M27-P. Microdilution endpoints were scored as the first tube or well in which a prominent reduction in turbidity (score 2 out of a possible 4) was observed compared with the growth control (Micro MIC-2). Alternative endpoint criteria were assessed independently of the reference MIC 80% and Micro MIC-2 values and included a colorimetric microdilution endpoint determined by using an oxidation-reduction indicator (Alamar Blue; Alamar Bio-sciences Inc., Sacramento, Calif.). The MICs for the two microdilution test systems were read after 24 and 48 h of incubation. The percentage of fluconazole MICs within 2 doubling dilutions of the macrodilution reference values was 94% for both microdilution tests read at 24 h. Agreement was slightly lower at 48 h and ranged from 91 to 93%. Comparison of Micro MIC-2 and colorimetric microdilution MICs resulted in agreements of 97 and 93% at 24 and 48 h, respectively. These results show excellent agreement among alternative methods for fluconazole susceptibility testing.

Variations in fluconazole susceptibility and electrophoretic karyotype among oral isolates of Candida albicans from patients with AIDS and oral candidiasis

DNA subtyping by pulsed-field gel electrophoresis and in vitro susceptibility testing were used to study strain variation and fluconazole resistance in Candida albicans isolates from patients with AIDS undergoing azole (fluconazole and clotrimazole) therapy for oropharyngeal candidiasis. A total of 29 patients suffered 71 episodes of oropharyngeal candidiasis. Overall, 121 isolates of C. albicans recovered throughout the course of treatment of each infection were available for further characterization. DNA subtyping revealed a total of 61 different DNA subtypes. In vitro susceptibility testing of the 121 isolates by using proposed standard methods of the National Committee for Clinical Laboratory Standards revealed MICs of fluconazole ranging from < or = 0.125 to > 64 micrograms/ml. The MIC for 50% of isolates tested was 0.25 microgram/ml, and the MIC for 90% of isolates tested was 8.0 micrograms/ml. MICs were > or = 64 micrograms/ml for only 7.4% of the isolates tested. The majority (62%) of the patients with oropharyngeal candidiasis and undergoing azole therapy were infected or colonized with more than one DNA subtype, and the introduction or selection of strains with a more resistant DNA subtype during the course of fluconazole therapy was not uncommon. With one exception, this did not appear to have an adverse effect on clinical outcome. In contrast, for patients with AIDS and oropharyngeal candidiasis infected with a single DNA subtype of C. albicans, an increase in fluconazole MICs for the infecting strain was rarely demonstrated over the course of therapy.

Efficacy of oral amphotericin B in AIDS patients with thrush clinically resistant to fluconazole

Reports of thrush clinically refractory to azoles in AIDS patients are increasing with the more widespread use of these agents. We studied our own oral preparation of amphotericin B in the treatment of two AIDS patients who developed oral thrush due to Candida glabrata after prolonged fluconazole use. Improvement occurred in both in less than 1 week, with eventual clearing and absence of side effects. Oral amphotericin B may have advantages over alternatives for this increasing problem.

Fluconazole-resistant Candida in AIDS patients. Report of two cases

Oropharyngeal candidiasis develops in up to 95% of patients with acquired immunodeficiency syndrome. Oral fluconazole is frequently prescribed for persons who are human immunodeficiency virus-seropositive as initial or suppressive therapy for oropharyngeal and esophageal candidiasis or as suppressive therapy for cryptococcal meningitis. We report two cases of oropharyngeal candidiasis, caused by Candida albicans, which developed in two patients with acquired immunodeficiency syndrome who had taken fluconazole for extended periods. In addition to the clinical resistance we observed, isolates of the organism appeared to be resistant in vitro to fluconazole and ketoconazole.

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.

Turbidimetric and visual criteria for determining the in vitro activity of six antifungal agents against Candida spp. and Cryptococcus neoformans

The drug concentration which inhibited 50% of growth (IC50), the lowest drug concentration at which growth was less than 30% of that in a positive control well (IC30), the visual minimal inhibitory concentration (MIC), and the minimum fungicidal concentration (MFC), were applied to study the effects of fluconazole, itraconazole, ketoconazole, miconazole, flucytosine, and amphotericin B against 36 isolates of Candida spp. and Cryptococcus neoformans by a broth microdilution technique. When the recommendations established by the NCCLS Subcommittee on Antifungal Susceptibility Tests were applied for the visual reading of the microplates, the results were comparable with those obtained by the turbidimetric methods. Differences between MICs and IC30s were observed with miconazole against strains of C. glabrata (p = 0.014) and with flucytosine against strains of C. neoformans (p = 0.041). Differences between MICs and IC50s were observed with fluconazole against strains of C. albicans (p = 0.027), C. tropicalis (p = 0.046), and C. neoformans (p = 0.041); with miconazole against strains of C. glabrata (p = 0.014); and with amphotericin B against strains of C. parapsilosis (p = 0.025). Ten additional isolates of C. albicans from AIDS patients suffering from recurrent episodes of oral candidiasis and clinically resistant to fluconazole also were included in this study. The MICs of fluconazole of these strains were significantly higher than those of the control group (p = 0.003). When the turbidimetric parameters were applied for testing the in vitro activity of fluconazole against the above isolates, both IC30 and IC50 were capable of discriminating the strains of the two groups (p = 0.002, p = 0.001, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro studies of a new antifungal triazole, D0870, against Candida albicans, Cryptococcus neoformans, and other pathogenic yeasts

We investigated the effects of various assay conditions on the activity of D0870 against seven species of fungi in the broth macrodilution testing procedure proposed by the National Committee for Clinical Laboratory Standards (NCCLS). Multivariate analysis demonstrated that endpoint definition, starting inoculum size, medium composition, type of buffer, and length of incubation, but not pH or temperature, had significant effects on results. Increasing the inoculum from 10(2) to 10(5) yeast cells/ml raised the MICs for all isolates up to > 75,000 fold. This effect was greatest when endpoints corresponded to a 90% reduction in visually determined turbidity (MIC90), was less prominent with an 80% inhibition visual endpoint (MIC80), and was nearly absent with a 50% endpoint measured by a spectrophotometer (IC1/2). Differences due to medium composition were attributable to antibiotic medium 3 with RPMI and yeast nitrogen base media performing nearly identically. Under standardized conditions as specified in NCCLS document M27-P (Reference Method for Broth Dilution Antifungal Susceptibility Testing of Yeasts; Proposed Standard, 1992), 79 strains (5 to 25 strains for each species) demonstrated median MIC80s of 0.0037 and 0.0075 microgram/ml for Candida albicans and Cryptococcus neoformans, respectively. In contrast, Candida krusei and Torulopsis glabrata had a median MIC80 of 1.0 microgram/ml. Our studies indicate that the pathogenic yeasts C. albicans and C. neoformans are more susceptible to D0870 than other pathogenic yeasts.