Fluconazole susceptibility testing of Candida albicans: microtiter method that is independent of inoculum size, temperature, and time of reading

Approaches for identifying and measuring heteroresistance in azole-susceptible Candida isolates

Heteroresistance to antifungal agents poses a significant challenge in the treatment of fungal infections. Currently, the absence of established methods for detecting and measuring heteroresistance impedes progress in understanding this phenomenon in fungal pathogens. In response to this gap, we present a comprehensive set of new and optimized methods designed to detect and quantify azole heteroresistance in Candida albicans. Here, we define two primary assays for measuring heteroresistance: population analysis profiling, based on growth on solid medium, and single-cell assays, based on growth in liquid culture. We observe good correlations between the measurements obtained with liquid and solid assays, validating their utility for studying azole heteroresistance. We also highlight that disk diffusion assays could serve as an additional tool for the rapid detection of heteroresistance. These methods collectively provide a versatile toolkit for researchers seeking to assess heteroresistance in C. albicans. They also serve as a critical step forward in the characterization of antifungal heteroresistance, providing a framework for investigating this phenomenon in diverse fungal species and in the context of other antifungal agents. Ultimately, these advancements will enhance our ability to effectively measure antifungal drug responses and combat fungal infections.IMPORTANCEHeteroresistance involves varying antimicrobial susceptibility within a clonal population. This phenomenon allows the survival of rare resistant subpopulations during drug treatment, significantly complicating the effective management of infections. However, the absence of established detection methods hampers progress in understanding this phenomenon in human fungal pathogens. We propose a comprehensive toolkit to address this gap in the yeast Candida albicans, encompassing population analysis profiling, single-cell assays, and disk diffusion assays. By providing robust and correlated measurements through both solid and liquid assays, this work will provide a framework for broader applications across clinically relevant Candida species. These methods will enhance our ability to understand this phenomenon and the failure of antifungal therapy.

Candida albicans Biofilm Formation and Growth Optimization for Functional Studies Using Response Surface Methodology

AIM

Optimization of Candida growth and biofilm formation is essential for understanding the recalcitrance of this pathogen to advance functional analysis on hospital tools and material surfaces. Optimization and quantification of biofilm have always been a challenge using the conventional One Variable at a Time (OVAT) method. The present study uses Central Composite Design-based Response Surface Methodology for optimization of conditions to induce growth and biofilm formation in Candida albicans on polystyrene microtitre plates.

METHODS & RESULTS

The variables considered in the design matrix were pH, temperature, incubation period, shaker speed, and inoculum size. A four-pronged quantification approach with XTT assay (cell viability), crystal violet assay (biofilm), calcofluor white assay, and wet/dry weight measurements (cell mass) were used to understand different aspects of biofilm. Heterogeneity in growth conditions for local strains of C. albicans clinical isolates were observed. Cell viability and cell mass were inversely related; however, biofilm was independent of these two factors. The study also highlighted the fact that Foetal Bovine Serum does not significantly contribute to cell adhesion and biofilm formation in vitro.

CONCLUSIONS

A high throughput optimization of C. albicans growth and biofilm formation on polystyrene microplate has been developed & validated.

SIGNIFICANCE AND IMPACT OF STUDY

The microtiter plate-based approach can be used for future screening of therapeutics for the control of C. albicans.

Management of Candida guilliermondii joint infection in a dog

Background

Candida spp. are dimorphic fungi in the family Cryptococcaceae. Infections with Candida spp. are usually rare conditions in dogs, but immunocompromised patients have a higher risk for developing invasive candidal infections.

Case presentation

A 5-year-old male Boxer, positive to Leishmania infantum, was referred to the Veterinary Teaching Hospital of the Department of Veterinary Medicine, University of Perugia, Italy for examination of a non-weight bearing left hind limb lameness of a duration of at least 3 months. During this period, treatment involved systemic anti-inflammatory medications and intra-articular corticosteroid administration. On presentation, clinical examination and radiographic findings were suggestive of cranial cruciate ligament deficiency. To support this diagnosis a stifle arthroscopy was performed: it confirmed a partial rupture of cranial cruciate ligament. Samples culture of synovial fluid and membrane was routinely collected as well, and revealed Candida guilliermondii joint infection. Treatment for the C. guilliermondii joint infection involved systemic anti-fungal therapy, joint lavage and intra-articular administration of antifungal drugs. Lameness improved markedly during this treatment, but lameness did not resolve completely, probably due to cranial cruciate ligament deficiency. Tibial tuberosity advancement (TTA) was chosen in order to treat stifle instability and was performed 4 weeks following cessation of treatment of the C. guilliermondii joint infection. Six month after TTA the dog showed a completely recovery with no lameness.

Conclusions

To the authors’ knowledge, this is the first case of Candida spp. joint infection reported in dogs. The cause of the progression of the joint C. guilliermondii infection remains unclear but it may be associated with leishmaniasis or intra-articular corticosteroid injections. Treatment with systemic and intra-articular anti-fungal therapies was successful. In the evaluation of hind limb lameness in a chronically immunocompromised dog, it would be advisable to consider also an intra-articular Candida spp. infection.

Effect of Ethiopian multiflora honey on fluconazole-resistant Candida species isolated from the oral cavity of AIDS patients

This study aimed to determine the antifungal effect of Ethiopian multiflora honey against Candida species isolated from the oral cavity of AIDS patients. Oral rinses were obtained from 13 AIDS patients and cultured on CHROMagar plates at 37°C for 48 hours. Candida species were identified by microbiological and molecular techniques. The antifungal effect of the honey sample on Candida was investigated by an agar dilution technique. Susceptibility of the Candida species to fluconazole was tested following a semi-modified microdilution method. Growth of both fluconazole-susceptible and -resistant Candida species was inhibited with a minimum fungicidal concentration (MFC) of 35–40% (v/v) honey. The MFC of different Candida species was not significantly different ( P > 0.05). From the total of 25 Candida isolates tested for susceptibility, 11 (44%), eight (32%) and six (24%) of the isolates were sensitive (minimum inhibitory concentrations [MICs] < 8 µg/mL), susceptible (dose-dependent: MICs 16–32 µg/mL) and resistant (MICs > 64 µg/mL) to fluconazole, respectively. Ethiopian multiflora honey has antifungal activity against fluconazole-resistant Candida species isolated from the oral cavity of AIDS patients. This supports the existing folkloric practice of using honey to treat oral lesions. Nevertheless, identification of the bioactive agents in honey, their clinical evaluation and pharmacological standardization are crucial.

Susceptibility testing of Cryptococcus diffluens against amphotericin B, flucytosine, fluconazole, itraconazole, voriconazole and posaconazole

Cryptococcus diffluens is a recently re-established species that shares several phenotypic features with Cryptococcus neoformans. We evaluated the application of the Clinical Laboratory Standards Institute (CLSI, formerly NCCLS) macro- and microbroth dilution methods and the E-test agar diffusion method to determine the in vitro susceptibilities of known strains of C. diffluens against amphotericin B (AMB), flucytosine (5-FC), fluconazole (FLC), itraconazole (ITC) and the novel triazoles, voriconazole (VRC) and posaconazole (PSC). Seven strains were found to be resistant in vitro to AMB (MICs >/=2 microg/ml), five were resistant to 5-FC (MICs of >/=32 microg/ml), four were resistant to FLC (MICs of FLC >/=32 microg/ml) and nine were resistant to ITC (MICs of ITC >1 microg/ml). In contrast, VRC and PSC showed good in vitro activity against C.diffluens strains, even those with elevated MICs to amphotericin B and/or established azoles. Most of the isolates were inhibited by 0.5 microg/ml of both VRC and PSC. A clinical isolate showing phenotypic switching exhibited elevated MICs to both agents, i.e., VRC (>16 microg/ml) and PSC (>8 microg/ml).

Role for cell density in antifungal drug resistance in Candida albicans biofilms

Biofilms of Candida albicans are less susceptible to many antifungal drugs than are planktonic yeast cells. We investigated the contribution of cell density to biofilm phenotypic resistance. Planktonic yeast cells in RPMI 1640 were susceptible to azole-class drugs, amphotericin B, and caspofungin at 1 x 10(3) cells/ml (standard conditions) using the XTT [2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide sodium salt] assay. As reported by others, as the cell concentration increased to 1 x 10(8) cells/ml, resistance was observed with 10- to 20-fold-greater MICs. Biofilms that formed in microtiter plate wells, like high-density planktonic organisms, were resistant to drugs. When biofilms were resuspended before testing, phenotypic resistance remained, but organisms, when diluted to 1 x 10(3) cells/ml, were susceptible. Drug-containing medium recovered from high-cell-density tests inhibited low-cell-density organisms. A fluconazole-resistant strain showed greater resistance at high planktonic cell density, in biofilm, and in resuspended biofilm than did low-density planktonic or biofilm organisms. A strain lacking drug efflux pumps CDR1, CDR2, and MDR1, while susceptible at a low azole concentration, was resistant at high cell density and in biofilm. A strain lacking CHK1 that fails to respond to the quorum-sensing molecule farnesol had the same response as did the wild type. FK506, reported to abrogate tolerance to azole drugs at low cell density, had no effect on tolerance at high cell density and in biofilm. These observations suggested that cell density has a role in the phenotypic resistance of biofilm, that neither the drug efflux pumps tested nor quorum sensing through Chk1p contributes to resistance, and that azole drug tolerance at high cell density differs mechanistically from tolerance at low cell density.

Inflammatory response and clinical course of adult patients with nosocomial bloodstream infections caused by Candida spp

Candida spp. are an important cause of nosocomial bloodstream infection (nBSI) and are associated with significant morbidity and mortality. An historical cohort study was performed to evaluate the clinical course of 60 randomly selected adult patients with nBSIs caused by Candida spp. Patients with BSI caused by Candida albicans (n = 38) and non-albicans spp. (n = 22) were compared with 80 patients with Staphylococcus aureus BSI by serial systemic inflammatory response syndrome (SIRS) and APACHE II scores. The patients had a mean age of 52 years, the length of hospital stay before BSI averaged 21 days, and 57% of patients required care in an intensive care unit before BSI. The mean APACHE II score was 17 on the day of BSI, and 63% of BSIs were caused by C. albicans. Antifungal therapy within the first 24 h of onset of BSI was appropriate in 52% of patients. Septic shock occurred in 27% of patients, and severe sepsis in an additional 8%. Overall mortality was 42%, and the 7-day mortality rate was 27%. The inflammatory response and clinical course were similar for patients with BSI caused by C. albicans and non-albicans spp. In univariate analysis, progression to septic shock was correlated with high overall mortality, as was an APACHE II score >25 at the onset of BSI. In multivariate analysis, the APACHE II score at the onset of BSI and a systemic inflammatory response independently predicted overall mortality, but the 7-day mortality rate was only predicted independently by the APACHE II score. Clinical course and mortality in patients with Candida BSI were predicted by systemic inflammatory response and APACHE II score, but not by the infecting species.

Susceptibility testing of fungi--current status and open questions

The increase of fungal infections and the improvement of therapeutical options demand reliable antifungal susceptibility testing. In vitro susceptibility testing of fungi--in contrast to bacteria--is not yet established as a routine method. The NCCIS (National Committee for Clinical Laboratory Standards) guidelines for susceptibility testing of yeasts (and proposed for hyphomycetes) are most important for standardization. Meanwhile, essential parts of this test procedure are accepted, but it should still be improved. The concept of using only one test medium for all drugs and test organisms is not realized so far. There are also some test situations that prevent the NCCLS standard from being applied. Based on our experience, this article describes the NCCLS methods and their modifications. It places emphasis on lipophilic drugs showing controversies despite standardization. Furthermore, the prediction of MICs on the clinical outcome is discussed. Since there are some pitfalls in testing antifungals, this should be done in experienced laboratories only. The MIC has to be regarded as only one, but an important, factor in the management of fungal diseases. Host-, drug-, and pathogen-specific data should be considered simultaneously.

Evaluation of semisolid agar screening tests for determining fluconazole and amphotericin B susceptibilities of Candida strains by using three different media

The susceptibilities of 164 Candida isolates against fluconazole and amphotericin B were determined by semisolid agar screening tests and the microdilution method according to NCCLS M27-A standards. The semisolid agar screening tests were performed with three different media containing 0.5% agar and 2, 8, and 40 microg/ml of fluconazole or 0.5 and 2.0 microg/ml of amphotericin B. These media were MOPS buffered RPMI 1640, brain-heart infusion and 1/3 diluted Sabouraud dextrose agar. The results of both methods were interpreted as susceptible, dose dependent susceptible and resistant for fluconazole and susceptible and resistant for amphotericin B. The agreement rates of semisolid agar screening tests using RPMI 1640, brain-heart infusion and Sabouraud dextrose media with the reference microdilution method were found to be 71.4%, 51.2%, and 57.3% for fluconazole and 79.3%, 53.7%, and 56.7% for amphotericin B, respectively. Overall, we conclude that semisolid agar screening tests using RPMI 1640 can be used for determining the susceptibilities of Candida isolates against fluconazole and amphotericin B in clinical microbiology laboratories.

Susceptibility testing of Malassezia pachydermatis using the urea broth microdilution method

The in vitro susceptibility of 24 isolates of Malassezia pachydermatis to four antifungal drugs in combination with lysozyme was determined using a urea broth microdilution method. The antifungal activities of each drug alone against 24 isolates of M. pachydermatis were determined as the mean minimal inhibitory concentrations (MICs). MICs of bifonazole, itraconazole, amorolfine and terbinafine were 3.2 microg ml(-1), 1.6 microg ml(-1), 25 microg ml(-1) and 3.2 microg ml(-1), respectively. Lysozyme alone inhibited the growth of M. pachydermatis in a dose-dependent manner, although the lysozyme was unable to kill the cells of M. pachydermatis at the highest concentration of 20 microg ml(-1). Furthermore, the mean MICs of bifonazole, itraconazole, amorolfine and terbinafine in combination with lysozyme were the same as the results for each drug alone. Although the activity of antifungal drugs in combination with lysozyme is enhanced for other fungi. These results suggested that M. pachydermatis might not be affected by the host's natural defences.

Homogeneous cell suspension of Malassezia pachydermatis obtained with an ultrasonic homogenizer

It is difficult to produce homogeneous cell suspensions of Malassezia pachydermatis, since yeast cells paste up and form many clumps. However, homogeneous fungal suspensions are required for susceptibility examinations and biochemical analyses. Although several types of trials have been carried out using glass homogenizers and many types of agents to obtain homogeneous fungal suspension. They have not yielded good results. We therefore attempted to use an ultrasonic homogenizer to separate clumps of yeast cells into separate individual cells. We succeeded in this fashion in producing homogeneous cell suspensions of M. pachydermatis. These results indicate that an ultrasonic homogenizer can be used to prepare homogeneous fungal suspensions of M. pachydermatis.

Antifungal susceptibility testing. New technology and clinical applications

The state of the art for susceptibility testing of yeasts is comparable with that of bacteria. Standardized methods for performing antifungal susceptibility testing are reproducible, accurate, and available in clinical laboratories. The development of quality control limits and interpretive criteria for a limited number of antifungal agents provides a basis for the application of this testing in the clinical laboratory. A proficiency testing program is available as a quality assurance measure for laboratories and has documented steady improvement among laboratories using the NCCLS method. As with antibacterial agents, surveillance programs are now in place using reference quality testing methods to monitor antifungal resistance trends on a global scale. It is clear that antifungal susceptibility testing can predict outcome in several clinical situations. Susceptibility testing is most helpful in dealing with infection caused by non-albicans species of Candida, and susceptibility testing of azoles is increasingly important in the management of candidiasis in critically ill patients. Susceptibility testing also has been standardized for filamentous fungi that cause invasive infections. Studies are ongoing to further refine this approach and evaluate the in vivo correlation with the in vitro data for molds. Future efforts must be directed toward establishing and validating interpretive break-points for licensed antifungals such as amphotericin B, and for new antifungals that are not yet licensed. Finally, procedures must be optimized for testing non-Candida yeasts (e.g., C. neoformans) and molds.

In vitro antifungal susceptibility testing

With the rising frequency of fungal infections, as well as increasing reports of resistance to antifungal agents, it is imperative that clinically applicable antifungal susceptibility testing be available. In 1997 the National Committee for Clinical Laboratory Standards published standard guidelines for antifungal susceptibility testing of Candida sp and Cryptococcus neoformans with amphotericin B, flucytosine, fluconazole, itraconazole, and ketoconazole. Although the methods are standard, they are time consuming, can be difficult to interpret, and are approved only for testing limited organisms and drugs. Modifications to the methods and alternative approaches have been proposed to make these tests more convenient and efficient, applicable to a greater number of species, and appropriate for performing in the clinical laboratory.

Disk diffusion method for fluconazole susceptibility testing of Candida albicans strains

This study evaluated the usefulness of the disk diffusion method by using different media for the susceptibility testing of fluconazole against Candida albicans strains. The susceptibility of 108 clinical isolates of C. albicans against fluconazole were determined by microdilution and disk diffusion methods by using RPMI 1640 agar and 25 microg disks. 93 of these isolates were also tested by disk diffusion technique on four different media (yeast nitrogen base agar, Sabouraud dextrose agar, Mueller Hinton agar and Mueller Hinton methylene blue agar). The results of the microdilution method were evaluated visually and optically. The disk diffusion results were determined after 24 and 48 hours of incubation. When the 24-hour zone diameters were compared to the minimal inhibitory concentrations determined visually and optically, the best results were obtained for RPMI 1640 agar and yeast nitrogen base agar. The correlation coefficients were r=-0.34, -0.41 and r=-0.33, -0.32 for the first and second media, respectively. The best values (r=-0.29, -0.39) were obtained for Mueller Hinton methylene blue agar when the 48-hour zone diameters were considered. Agreement between the disk diffusion and microdilution methods was best for RPMI 1640, yeast nitrogen base and Mueller Hinton methylene blue agar after 24 hours of incubation (87-89%, 88-90%, 93-96%, respectively) and for Mueller Hinton methylene blue agar after 48 hours of incubation (89-96%). Disk diffusion method using RPMI 1640, yeast nitrogen base and Mueller Hinton methylene blue agar appears to be a useful, rapid and reliable screening technique for testing the susceptibility of C. albicans strains to fluconazole.

Susceptibility testing of voriconazole, fluconazole, itraconazole and amphotericin B against yeast isolates in a Turkish University Hospital and effect of time of reading

Voriconazole is a promising azole effective against a variety of fungi, including yeasts. In this study, we tested in vitro activities of voriconazole, fluconazole, itraconazole and amphotericin B against some ATCC and reference strains and 250 clinical yeast isolates. We also evaluated the effect of time of reading on MIC results. Voriconazole was the most active agent against Candida and Trichosporon isolates, including the putatively fluconazole-resistant C. krusei (MIC(90) 0.25 microg/ml) and C. glabrata (MIC(90) 0.5 microg/ml). Amphotericin B MICs were scattered in a considerably narrow range in both RPMI 1640 and Antibiotic Medium 3. MICs at 24 hours and 48 hours were similar in general for all antifungals tested. The highest percentage of strains that showed 24-hour and 48-hour MICs within +/-1-log(2) dilution was observed for amphotericin B tested in RPMI (99%), and the lowest for amphotericin B tested in Antibiotic Medium 3 (80%). In conclusion, voriconazole is very effective against a wide spectrum of Candida species and 24-hour readings could substitute 48-hour MIC evaluation.

Susceptibility testing of Malassezia species using the urea broth microdilution method

A urea broth microdilution method to assay the susceptibilities of seven Malassezia species was developed. This method indicated the same sensitivities as the agar plate dilution method for isolates of Malassezia furfur, M. pachydermatis, M. slooffiae, and M. sympodialis.

Comparison of three methods of determining MICs for filamentous fungi using different end point criteria and incubation periods

Three different methods were used to determine the in vitro activities of amphotericin B, ketoconazole, itraconazole, and flucytosine against 30 isolates of different genera of filamentous fungi. MICs were determined visually, with or without agitation, and spectrophotometrically by using a broth microdilution method. For amphotericin B, there was one end point reading criterion (the minimum concentration of antifungal that inhibited 100% of growth), but for azoles and flucytosine there were two (the minimum concentrations that inhibited 50 and 75% of fungal growth, respectively) after 48 and 72 h of incubation. All tests were performed in triplicate. An intraclass correlation coefficient (ICC) was used to evaluate the reproducibility of each of the methods and the correlation among them. The reproducibility of the three methods was very high (ICC of 0.808 to 0.992), particularly in the case of azoles and flucytosine. In general, the degree of reproducibility was highest for azoles and amphotericin B after 72 h of incubation and for flucytosine after 48 h of incubation. The degree of correlation among the three methods was very high (ICC of >0.98) with all of the antifungals under all the conditions tested. The end point reading criteria and the time of incubation affected neither the reproducibility of the methods nor their correlation, and their effect on MICs was statistically significant.

Determination of antifungal MICs by a rapid susceptibility assay

A novel microtiter assay for antifungal susceptibility testing was developed. This method has several potential advantages over the M27-A assay of the National Committee for Clinical Laboratory Standards. These include provision of MIC results within 6 to 19 h, graphical display of data, and the availability of objective quantitative endpoints. We refer to the method as the rapid susceptibility assay (RSA). RSA is based on substrate utilization by fungi in the presence of antifungal drugs. Substrate uptake is determined by a colorimetric method, which can be scored by analysis of data obtained from a microplate reader. Variables evaluated in the development of the RSA included inoculum size, incubation period, and efficacy with different classes of antifungal drugs and different yeast isolates. With the rapidly available and quantitative endpoints of the RSA, correlation of MICs and therapeutic drug doses can be evaluated more successfully than they can be evaluated by existing assays.

Optimizing voriconazole susceptibility testing of Candida: effects of incubation time, endpoint rule, species of Candida, and level of fluconazole susceptibility

Voriconazole is a new triazole antifungal agent that has potent activity against many isolates of Candida, including Candida krusei and Candida glabrata. In this work, we studied the impact of glucose supplementation, incubation time, agitation of the plates prior to reading, endpoint determination rule, visual versus spectrophotometric reading, Candida species, and fluconazole MIC on the MIC of voriconazole for Candida isolates tested by using the microdilution format assay of the National Committee for Clinical Laboratory Standards (NCCLS) M27-A antifungal susceptibility testing methodology. For both voriconazole and fluconazole, a spectrophotometric endpoint of 50% reduction in turbidity relative to the growth control correlated most closely with the NCCLS-defined visual endpoint of "prominent decrease in turbidity." Correlation was generally better after 24 h of incubation than after 48 h. Supplementation of the medium to contain 20 g of glucose/liter did not alter the MIC significantly but did enhance growth and simplify visual readings. All Candida species appeared potentially susceptible to voriconazole, including isolates of C. krusei. For some isolates for which fluconazole MICs were markedly elevated voriconazole MICs were also elevated, but the clinical significance of these observations remains to be determined.

The trailing end point phenotype in antifungal susceptibility testing is pH dependent

The interpretation of end points in azole antifungal drug susceptibility testing is problematic, in part due to incomplete growth inhibition of Candida species. Such trailing growth can cause the MICs of fluconazole for some isolates to be low (<1 microg/ml) after 24 h of growth but much higher (>64 microg/ml) after 48 h. Isolates having this type of growth have been described as having a low-high phenotype. Although these isolates would be considered resistant by current National Committee of Clinical Laboratory Standards definitions, growing evidence suggests that they are susceptible in vivo. To further characterize these isolates in vitro, microdilution susceptibility testing comparing the complex defined medium RPMI 1640 to a defined minimal medium (yeast nitrogen broth) was performed. Isolates having trailing growth in MOPS (morpholinepropanesulfonic acid)-buffered RPMI 1640 (pH 7.0) were found to have clear end points in the minimal medium at its native pH of 4.5. The pH of the medium influenced the low-high phenotype, as these same isolates trailed in minimal medium adjusted to a pH of >/=6.0 but did not trail in RPMI 1640 adjusted to a pH of </=5.0. This pH effect was independent of the medium buffering capacity, as trailing was decreased in both minimal medium and RPMI 1640 (pH 4.5) buffered in citrate. Adjustment in the pH of MOPS-buffered RPMI 1640 reduced trailing in multiple strains of Candida albicans without affecting the MICs for isolates having known susceptible (low-low) and resistant (high-high) phenotypes. Adjustment of the medium pH could be considered to eliminate trailing in azole drug susceptibility testing.

Colorimetric MTT assessment of antifungal activity of D0870 against fluconazole-resistant Candida albicans

The in vitro antifungal activity of D0870 against eight isolates of fluconazole-resistant Candida albicans was compared with that of itraconazole, ketoconazole and miconazole. The colorimetric MTT [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide] assay was used to assess the antifungal activities. The 50% minimum inhibitory concentration (MIC50) of D0870 was below 0.031 microgram ml-1 for seven isolates and 0.25 microgram ml-1 for one isolate. The activity of D0870 was superior to that of the other azoles. Ketoconazole was the most effective azole next to D0870. Therefore, the new bis-triazole, D0870, is expected to be promising for the therapy of fluconazole-resistant candidosis. The present data also confirmed that the MTT assay may be useful for evaluation of resistance and detection of resistant C. albicans.

Clinical evaluation of the ASTY colorimetric microdilution panel for antifungal susceptibility testing

A method using a commercially prepared colorimetric microdilution panel (ASTY; Kyokuto Pharmaceutical Industrial Co., Ltd.) was compared in four different laboratories with the National Committee for Clinical Laboratory Standards (NCCLS) reference microdilution method by testing 802 clinical isolates of Candida spp. (C. albicans, C. glabrata, C. tropicalis, C. parapsilosis, C. krusei, C. lusitaniae, C. guilliermondii, C. lipolytica, C. rugosa, and C. zeylanoides) against amphotericin B, 5-fluorocytosine (5FC), fluconazole, and itraconazole. Reference MIC endpoints were established after 48 h of incubation, and ASTY endpoints were established after 24 and 48 h of incubation. ASTY endpoints were determined to be the time at which the color of the first well changed from red (indicating growth) to purple (indicating growth inhibition) or blue (indicating no growth). Excellent agreement (within 2 dilutions) between the reference and colorimetric MICs was observed. Overall agreement was 93% at 24 h and 96% at 48 h. Agreement ranged from 90% with itraconazole and 5FC to 96% with amphotericin B at 24 h and from 92% with itraconazole to 99% with amphotericin B and 5FC at 48 h. The ASTY colorimetric microdilution panel method appears to be comparable to the NCCLS reference method for testing the susceptibilities of Candida spp. to a variety of antifungal agents.

Enhanced activity of antifungal drugs by lysozyme against Cryptococcus neoformans

The in vitro susceptibility of 16 isolates of Cryptococcus neoformans to three antifungal drugs and lysozyme in combination was determined using an urea broth microdilution method. The antifungal activities of each drug alone against 16 isolates of Cr. neoformans were determined as mean minimal inhibitory concentrations (MICs). MICs of fluconazole, itraconazole and terbinafine were 2.0 micrograms ml-1, 0.004 microgram ml-1 and 0.25 microgram ml-1, respectively. Lysozyme alone inhibited the growth of Cr. neoformans in a dose-dependent manner, although the lysozyme was unable to kill the cells of Cr. neoformans at the highest concentration of 20 micrograms ml-1. The mean MICs of fluconazole, itraconazole and terbinafine in combination with lysozyme were 0.13 microgram ml-1, 0.004 microgram ml-1 and 0.03 microgram ml-1 respectively. The antifungal activity of fluconazole and terbinafine in combination with lysozyme against Cr. neoformans was greatly enhanced compared with that of each drug alone. Itraconazole was unable to enhance the antifungal activity, as it demonstrated higher activity against Cr. neoformans when alone rather than in combination. Lysozyme was confirmed to enhance the antifungal activity of fluconazole and terbinafine in vitro.

Susceptibility testing of Cryptococcus neoformans using the urea broth microdilution method

An urea broth microdilution method to assay the susceptibility of Cryptococcus neoformans to antifungal drugs was newly developed. Using this method, urease activity of the fungus was measured instead of the viability by checking colony development. The urease activities were indicated by colour changes in optical density at 545 nm. The end point in this assay was considered as 99% inhibitory concentration. When we measured antifungal activities of the three drugs against 16 isolates of Cr. neoformans using this assay method, mean minimum-inhibitory concentrations (MICs) of fluconazole, itraconazole and terbinafine were 2.0 micrograms ml-1, 0.008 microgram ml-1 and 0.25 microgram ml-1 respectively. This assay method resulted in higher sensitivity in MICs of the three antifungal drugs than the broth microdilution method recommended by the Committee for Laboratory Standards of the Japanese Society for Medical Mycology. The results obtained using this assay method support the more effective evaluation of antifungal substances in susceptibility testing of Cr. neoformans.

Optimizing the correlation between results of testing in vitro and therapeutic outcome in vivo for fluconazole by testing critical isolates in a murine model of invasive candidiasis

The trailing growth phenomenon seen when determining the susceptibilities of Candida isolates to the azole antifungal agents makes consistent endpoint determination difficult, and the M27-A method of the National Committee for Clinical Laboratory Standards addresses this problem by requiring an 80% reduction in growth after 48 h of incubation. For some isolates, however, minor variations of this endpoint criterion can produce up to 128-fold variations in the resulting MIC. To investigate the significance of this effect, isolates of Candida that exhibited various forms of trailing growth when tested against fluconazole were identified. The isolates were examined in a murine model of invasive candidiasis and were ranked by their relative response to fluconazole by using both improvement in survival and reduction in fungal burden in the kidney. The resulting rank order of in vivo response did not match the MICs obtained by using the M27-A criterion, and these MICs significantly overestimated the resistance of three of the six isolates tested. However, if the MIC was determined after 24 h of incubation and the endpoint required a less restrictive 50% reduction in growth, MICs which better matched the in vivo response pattern could be obtained. Minor variations in the M27-A endpoint criterion are thus required to optimize the in vitro-in vivo correlation for isolates that demonstrate significant trailing growth when tested against fluconazole.

Comparison of broth macrodilution, broth microdilution and E-test susceptibility tests of Cryptococcus neoformans for fluconazole

Forty Cryptococcus neoformans strains isolated from cerebral spinal fluid specimens collected from 39 patients were included in the study. The MICs for fluconacole were determined by YNB macrodilution test, microdilution tests using both RPMI1640 and YNB medium and E-tests on solidified RPMI1640 medium, Casitone and YNB agar. In comparison with the reference macrodilution method NCCLS M27-P both the microdilution as well as the E-test techniques can be used for fluconacole susceptibility testing of Cr. neoformans.

Comparison of Etest, microdilution and colorimetric dilution with reference broth macrodilution method for antifungal susceptibility testing of clinically significant Candida species isolated from immunocompromised patients

Amphotericin B and fluconazole macrodilution minimum inhibitory concentration values of 101 Candida strains isolated from 91 immunocompromised patients were comparatively evaluated with the results of Etest, microdilution and colorimetric microdilution methods. The overall agreement rates of Etest, microdilution and colorimetric microdilution methods with the reference macrodilution method were found to be acceptably high after an incubation period of 24 and 48 h (varying from 86 to 93% for amphotericin B and from 84 to 89% for fluconazole). In addition, the results pointed out relatively high minimum inhibitory concentration values of fluconazole for Candida krusei and Candida glabrata isolates. These methods are not only reliable alternatives to the present reference macrodilution method but are also easy-to-perform and less time-consuming.

In vitro activities of terbinafine in combination with fluconazole and itraconazole against isolates of Candida albicans with reduced susceptibility to azoles

A checkerboard microdilution method was applied to study the in vitro interaction of terbinafine with either fluconazole and itraconazole against 30 strains of Candida albicans. Synergy was observed in 40% of the terbinafine-fluconazole interactions and in 43% of the terbinafine-itraconazole interactions, while antagonism was not observed. Even when only additivity was achieved, the combinations still showed beneficial effects since at least twofold reductions in the MICs of both drugs were found in 100% of the terbinafine-fluconazole interactions and in 76% of the terbinafine-itraconazole interactions.

In vitro activity of a new echinocandin, LY303366, compared with those of amphotericin B and fluconazole against clinical yeast isolates

The in vitro activity of LY303366, a new echinocandin derivative, was evaluated with 191 yeast isolates by a broth microdilution method. The MICs at which 50% of the isolates were inhibited were 0.125 microg/ml for Candida albicans and C. tropicalis, 0.25 microg/ml for C. krusei, C. kefyr, and C. glabrata, and 2.0 microg/ml for C. parapsilosis.

Effects of incubation temperature, inoculum size, and time of reading on broth microdilution susceptibility test results for amphotericin B fgainst Fusarium

In vitro antifungal susceptibility testing for filamentous fungi remains unstandardized and is unreliable for determining adequate therapy. A study was performed to evaluate the effect of inoculum size (10(2), 10(3), 10(4), and 10(5) conidia/ml), incubation time (48 and 72 h), and temperature (25, 30, and 35 degrees C) on MICs of amphotericin B for Fusarium spp. (20 strains). The inoculum size showed the clearest effect: when the inoculum was varied from 10(2) to 10(5) conidia/ml, the geometric mean MICs showed increases of between 10- and 19-fold in all the combined conditions of temperature and incubation time assayed. Time of incubation had less effect (increases of between two- and threefold in approximately half of the geometric mean MICs), and temperature especially had little effect (the increases were no higher than twofold). The effects of interaction between inoculum size and temperature on MICs were not statistically significant, while the combined effects of inoculum size and time of reading and of time of reading and temperature produced systematic variation in MICs.

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.

Candida tropicalis arthritis in a patient with acute myeloid leukemia successfully treated with fluconazole: case report and review of the literature

The case of a 77-year-old woman with acute myeloid leukemia who developed Candida tropicalis septic arthritis of the knee after remission-inducing chemotherapy is reported. A literature review of C. tropicalis non-prosthetic arthritis is included. The isolate was susceptible to fluconazole (MIC 0.25 mg/l). She was treated with fluconazole (400 mg orally) and frequent relieving synovial aspirations. After 1 month of antifungal therapy the synovial fluid became culture negative. Fluconazole concentration in the synovial fluid and serum were 20 mg/l and 19.4 mg/l, respectively. The patient was treated for a total of 7 months and made a full recovery. This is the first report of the successful use of fluconazole in the treatment of septic arthritis due to C. tropicalis.

An ATP bioluminescence assay applicable to rapid fluconazole susceptibility testing of dermatophytes

An ATP bioluminescence assay as a rapid reference method for fluconazole (FLCZ) susceptibility testing of dermatophytes, as well as yeasts, was developed and evaluated by comparing it with viability, turbidity and fungal protein content-based conventional methods. FLCZ susceptibility results obtained with strains of Candida albicans and dermatophytes by the bioluminescence method in high-resolution medium were well correlated with those obtained by conventional methods currently used in clinical microbiology laboratories or reported previously, including a broth dilution method by the National Committee for Clinical Laboratory Standards (NCCLS). Thus, ATP bioluminescence assay can be used to monitor fungal growth in liquid culture media. The procedure has considerable potential for the rapid testing of FLCZ susceptibility of dermatophytes and other fungi.

Efficacy of continuous flucytosine infusion against Candida lusitaniae in experimental hematogenous murine candidiasis

Candida lusitaniae may cause life-threatening infections in the immunocompromised host and may be resistant to amphotericin B. Flucytosine (5-FC) is very active against C. lusitaniae isolates in vitro, while the in vivo response of murine infection to 5-FC is not as good. To evaluate the hypothesis that this discrepancy may be primarily due to the short half-life of 5-FC in mice, we compared the same total dosage of 75 mg of 5-FC per kg of body weight per day given by bolus injections or infused continuously via a subcutaneously implanted pump in immunosuppressed CF1 mice infected with C. lusitaniae. The fungal titers in the kidneys of mice treated with the continuous 5-FC infusion were significantly lower (P < or = 0.05) than those in the kidneys of mice that received bolus injections once or thrice daily. The antifungal activity of 5-FC against murine candidiasis is best evaluated when the drug is administered by continuous infusion.

Microdilution antifungal susceptibility testing of Candida albicans and Cryptococcus neoformans with and without agitation: an eight-center collaborative study

The growth patterns observed in the trailing wells when fluconazole is being tested may give rise to readings that suggest resistance or increased MICs for known susceptible strains. We conducted a multicenter study to evaluate the intralaboratory and interlaboratory reproducibilities of a method that uses agitation to disperse these types of growth. Ten strains of Candida albicans and five strains of Cryptococcus neoformans were tested against fluconazole, flucytosine, and amphotericin B by using a microdilution adaptation of the proposed reference method of the National Committee for Clinical Laboratory Standards for yeasts (M27-T). The endpoint criterion used before agitation was consistent with the M27-T recommendation, while a criterion of 50% or more reduction of growth compared with the control was used after agitation. The results of this study showed that use of agitation and the modified endpoint criterion both improved intralaboratory and inter-laboratory agreement and increased the frequency of interpretable MICs. The MICs obtained by this method were comparable to those obtained by the broth macrodilution M27-T method. Like M27-T, this method was not able to definitely distinguish amphotericin B-susceptible from -resistant strains, although the MICs for the resistant strains were consistently higher than those for the susceptible ones. The findings imply that agitation should be seriously considered when antifungal agents, particularly fluconazole, are tested in a microdilution format.

Comparison of a spectrophotometric microdilution method with RPMI-2% glucose with the National Committee for Clinical Laboratory Standards reference macrodilution method M27-P for in vitro susceptibility testing of amphotericin B, flucytosine, and fluconazole against 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 visual and subjective inspection of growth inhibition after 48 h of incubation. An alternative microdilution procedure was compared with the M27-P method for determination of the amphotericin B, flucytosine, and fluconazole susceptibilities of 8 American Type Culture Collection strains (6 of them were quality control or reference strains) and 50 clinical isolates of candida albicans. This microdilution method uses as culture medium RPMI 1640 supplemented with 18 g of glucose per liter (RPMI-2% glucose). Preparation of drugs, basal medium, and inocula was done by following the recommendations of the National Committee for Clinical Laboratory Standards. The MIC endpoint was calculated objectively from the turbidimetric data read at 24 h. Increased growth of C. albicans in RPMI-2% glucose and its spectrophotometric reading allowed for the rapid (24 h) and objective calculation of MIC endpoints compared with previous microdilution methods with standard RPMI 1640. Nevertheless, good agreement was shown between the M27-P method and this microdilution test. The MICs obtained for the quality control or reference strains by the microdilution method were in the ranges published for those strains. For clinical isolates, the percentages of agreement were 100% for amphotericin B and fluconazole and 98.1% for flucytosine. These data suggest that this microdilution method may serve as a less subjective and more rapid alternative to the M27-P method for antifungal susceptibility testing of yeasts.

Comparative resistance of Candida albicans clinical isolates to fluconazole and itraconazole in vitro and in vivo in a murine model

Relationships between azole susceptibility and in vivo response to antifungal therapy in a murine model of candidiasis were investigated for Candida albicans isolates sampled from human immunodeficiency virus type 1-positive patients with oropharyngeal candidiasis. The susceptibilities of seven clinical isolates and two reference strains to fluconazole (FCZ) and itraconazole (ITZ) were determined in vitro by the broth microdilution method. Four isolates were resistant to FCZ and ITZ, two were susceptible to both azoles, and three were resistant to FCZ and susceptible to ITZ (dissociated resistance). CD1 mice were inoculated with each isolate and treated with either FCZ or ITZ (drug regimen, 5 mg/kg of body weight twice daily for 5 days). Quantitative cultures of kidneys were performed at the end of the treatment. On the other hand, the survival rates of the mice were followed daily. These two parameters were clearly correlated with in vitro susceptibility. Thus, the phenomenon of a dissociation of resistance to FCZ and ITZ may be found in vivo as well as in vitro.

In vitro activities of semisynthetic pneumocandin L-733,560 against fluconazole-resistant and -susceptible Candida albicans isolates

Lipopeptide L-733,560 is a water-soluble derivative of pneumocandin B0 that exhibits enhanced anti-Candida activity. We investigated the in vitro activity of L-733,560 compared with those of amphotericin B, flucytosine, and itraconazole, against fluconazole-resistant (n = 44) and fluconazole-susceptible (n = 46) Candida albicans isolates. Tests were performed with a photometer-read broth microdilution method with RPMI-2% glucose and National Committee for Clinical Laboratory Standards reference strains. Except for those of itraconazole, MICs were not significantly different between the two groups of isolates, as expected for agents with different mechanisms of action. L-733,560 was the most active agent against C.albicans, with MICs for 50 and 90% of the strains tested of 0.01 and 0.06 microgram/ml, respectively.

Interlaboratory evaluation of Etest method for testing antifungal susceptibilities of pathogenic yeasts to five antifungal agents by using Casitone agar and solidified RPMI 1640 medium with 2% glucose

An interlaboratory evaluation (two centers) of the Etest method was conducted for testing the antifungal susceptibilities of yeasts. The MICs of amphotericin B, fluconazole, flucytosine, itraconazole, and ketoconazole were determined for 83 isolates of Candida spp., Cryptococcus neoformans, and Torulopsis glabrata. Two buffered (phosphate buffer) culture media were evaluated: solidified RPMI 1640 medium with 2% glucose and Casitone agar. MIC endpoints were determined after both 24 and 48 h of incubation at 35 degrees C. Analysis of 3,420 MICs demonstrated higher interlaboratory agreement (percentage of MIC pairs within a 2-dilution range) with Casitone medium than with RPMI 1640 medium when testing amphotericin B (84 to 90% versus 1 to 4%), itraconazole (87% versus 63 to 74%), and ketoconazole (94 to 96% versus 88 to 90%). In contrast, better interlaboratory reproducibility was determined between fluconazole MIC pairs when RPMI 1640 medium rather than Casitone medium was used (96 to 98% versus 77 to 90%). Comparison of the flucytosine MICs obtained with RPMI 1640 medium revealed greater than 80% reproducibility. The study suggests the potential value of the Etest as a convenient alternative method for testing the susceptibilities of yeasts. It also indicates the need for further optimization of medium formulations and MIC endpoint criteria to improve interlaboratory agreement.

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.

Fluconazole treatment of Candida albicans infection in mice: does in vitro susceptibility predict in vivo response?

A series of fluconazole-susceptible and-fluconazole resistant Candida albicans fungal isolates were used to infect mice intravenously. Mice were treated with varying doses of fluconazole beginning one day after infection. For all of the 6 fluconazole-susceptible isolates, fluconazole was highly effective at <0.25 mg/kg of body weight twice daily. By contrast, fluconazole was less effective in at least 6 of 10 fluconazole-resistant isolates and was ineffective at > or = 40 mg/kg twice daily in 4 fluconazole-resistant isolates. Although the correlation is not precise, in vitro susceptibility testing of C. albicans can predict in vivo response to fluconazole.

Fluconazole- and itraconazole-resistant Candida albicans strains from AIDS patients: multilocus enzyme electrophoresis analysis and antifungal susceptibilities

Multilocus enzyme electrophoresis and in vitro susceptibility testing with a broth microdilution method were used to analyze Candida albicans strain diversity in four AIDS patients with recurrent oropharyngeal candidiasis who successively developed clinical resistance to fluconazole (FCZ) and itraconazole (ITZ). One to ten colonies per sample were randomly chosen from oral washings collected before the initial FCZ treatment and just before every other antifungal treatment; a total of 98 isolates were analyzed. Multilocus enzyme electrophoresis analysis revealed 14 different electrophoretic types (ETs). Statistical analysis of genetic distances showed that C. albicans isolates clustered into five subpopulations (I to V). In each subpopulation, isolates are closely related, and genetic distances between subpopulations I to IV are short. In contrast, subpopulation V, which contained isolates typed as ET8 and ET14, is strongly divergent from the others; these isolates may represent atypical C. albicans isolates. Only one patient was infected with a single strain during the course of azole therapy; for the three remaining patients, variants of the same strain and different strains were concurrently isolated. Clinical FCZ resistance was clearly correlated with in vitro data for three patients. Moreover, MICs of ITZ increased during FCZ therapy, and MICs of ITZ which were > or = 1.56 micrograms/ml were found when clinical ITZ resistance occurred; isolates from subpopulation V showed the highest MICs of ITZ. Because of the emergence of clinical ITZ resistance after clinical FCZ resistance, the feasibility of long-term azole therapy for mucosal candidiasis in AIDS patients is questioned.

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.