ANTIFUNGAL SUSCEPTIBILITY TESTING

Drosophila melanogaster as a model organism for invasive aspergillosis

Mammalian hosts have traditionally been considered the "gold standard" models for studying pathogenesis and antifungal drug activity in invasive aspergillosis (IA). Nevertheless, logistical, economical, and ethical constraints make these host systems difficult to use for high-throughput screening of putative Aspergillus virulence factors and novel antifungal compounds. Here, we present Drosophila melanogaster, a heterologous non-vertebrate host with conserved innate immunity and genetic tractability, as an alternative, easy-to-use, and inexpensive pathosystem for studying Aspergillus pathogenesis and antifungal activity. We describe three different infection protocols (i.e., injection, rolling, ingestion) that introduce Aspergillus conidia at different anatomical sites of Toll-deficient Drosophila flies. These reproducible assays can be used to (1) determine the virulence of various Aspergillus strains and to (2) assess the anti-Aspergillus activity of orally absorbed antifungal agents in vivo. These methods can also be adapted to study pathogenesis and antifungal drug activity against other medically important human fungal pathogens.

Rapid susceptibility testing of medically important zygomycetes by XTT assay

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

Fruit flies as a minihost model for studying drug activity and virulence in Aspergillus

Invasive aspergillosis (IA) is a significant cause of morbidity and mortality in profoundly immunosuppressed patients. The mediocre efficacy of antifungals for IA in clinical practice and an incomplete understanding of the pathogenesis of IA contribute to its overall poor prognosis. Although logistically difficult for large scale use, conventional animal models of IA provide valuable information regarding both antifungal drug efficacy and Aspergillus mutant virulence. However, in the era of introduction of molecular biology techniques for studying Aspergillus and increasing antifungal options, the existing in vivo models of IA might be well complemented by nonvertebrate minihost models such as the Drosophila melanogaster (fruit fly). Drosophila may offer the distinct advantage of performing fast, inexpensive high-throughput screening of compounds for anti-Aspergillus activity and putative Aspergillus mutants for their role in Aspergillus virulence.

Comparative evaluation of a new fluorescent carboxyfluorescein diacetate-modified microdilution method for antifungal susceptibility testing of Candida albicans isolates

This report presents a fluorescent carboxyfluorescein diacetate (CFDA)-modified microdilution method used for the susceptibility testing of Candida albicans to amphotericin B, fluconazole, ketoconazole, itraconazole, voriconazole, and flucytosine. Four different broth microdilution susceptibility testing methods were simultaneously evaluated at 24 and 48 h. The MICs determined using the CFDA-modified method (MIC(cfda)) were compared to those obtained by the standard broth microdilution method (MIC(visual)) and a procedure employing the indicator Alamar blue (MIC(alamar)). The reference MIC was determined visually as recommended by the NCCLS M27-A protocol, and then quantified spectrophotometrically following agitation (MIC(spec)). The CFDA-modified microdilution method was demonstrated to effectively determine the MICs for all the antifungal drugs tested at both 24 and 48 h. The results from both the MIC(spec) and MIC(cfda) methods yielded >80% agreement within +/-1 dilution and >90% agreement within +/-2 dilutions at 24 h in comparison to the reference MIC(visual) method, respectively. The trailing growth phenomenon that occurs with azole antifungal drugs and many strains of C. albicans did not inhibit the effectiveness of the MIC(spec) and MIC(cfda) methods. The MIC(spec) and MIC(cfda) methods shared 92.8% agreement within +/-1 dilution at 24 h and 87.6% agreement within +/-1 dilution at 48 h.

Comparison of spectrophotometric and visual readings of NCCLS method and evaluation of a colorimetric method based on reduction of a soluble tetrazolium salt, 2,3-bis [2-methoxy-4-nitro-5-[(sulfenylamino) carbonyl]-2H-tetrazolium-hydroxide], for antifungal susceptibility testing of Aspergillus species

The susceptibilities of 25 clinical isolates of various Aspergillus species (Aspergillus fumigatus, A. flavus, A. terreus, A. ustus, and A. nidulans) to itraconazole (ITC) and amphotericin B (AMB) were determined using the standard proposed by NCCLS for antifungal susceptibility testing of[filamentous fungi, a modification of this method using spectrophotometric readings, and a colorimetric method using the tetrazolium salt 2,3-bis [2-methoxy-4-nitro-5-[(sulfenylamino) carbonyl]-2H-tetrazolium-hydroxide] (XTT). Five MIC end points for ITC (MIC-0, no visible growth or <or=5% the growth control value [GC]; MIC-1, slight growth or 6 to 25% the GC; MIC-2, prominent reduction in growth or 26 to 50% the GC; MIC-3, slight reduction in growth or 51 to 75% the GC; and MIC-4, no reduction in growth or 76 to 100% the GC) and one for AMB (MIC-0) were determined visually by four observers and spectrophotometrically. The intraexperimental (between the observers) and interexperimental (between the experiments) levels of agreement of the NCCLS and XTT methods exceeded 95% for MIC-0 of AMB and MIC-0 and MIC-1 of ITC. The MIC-2 of ITC showed lower reproducibility, although spectrophotometric reading and/or incubation for 48 h increased the interexperimental reproducibility from 85 to >93%. Between visual and spectrophotometric readings, high levels of agreement were found for AMB (approximately 97%) and MIC-1 (approximately 92%) and MIC-2 (approximately 88%) of ITC. Poor agreement was found for MIC-0 of ITC (51% after 24 h), since the spectrophotometric readings resulted in higher MIC-0 values than the visual readings. The agreement was increased to 98% by shifting the threshold level of MIC-0 from 5 to 10% relative optical density and by establishing an optical density of greater than 0.1 for the GC as the validation criterion. No statistically significant differences were found between the NCCLS method and the XTT method, with the levels of agreement exceeding 97% for MIC-0 of AMB and 83% for MIC-0, MIC-1, and MIC-2 of ITC. The XTT method and spectrophotometric readings can increase the sensitivity and the precision, respectively, of in vitro susceptibility testing of Aspergillus species.

Colorimetric assay for antifungal susceptibility testing of Aspergillus species

A colorimetric assay for antifungal susceptibility testing of Aspergillus species (Aspergillus fumigatus, Aspergillus flavus, Aspergillus terreus, Aspergillus nidulans, and Aspergillus ustus) is described based on the reduction of the tetrazolium salt 2,3-bis(2-methoxy-4-nitro-5-[(sulphenylamino)carbonyl]-2H-tetrazolium-hydroxide (XTT) in the presence of menadione as an electron-coupling agent. The combination of 200 microg of XTT/ml with 25 microM menadione resulted in a high production of formazan within 2 h of exposure, allowing the detection of hyphae formed by low inocula of 10(2) CFU/ml after 24 h of incubation. Under these settings, the formazan production correlated linearly with the fungal biomass and less-variable concentration effect curves for amphotericin B and itraconazole were obtained.

Novel fluorescent broth microdilution method for fluconazole susceptibility testing of Candida albicans

A comparative evaluation of the reference National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution method with a novel fluorescent carboxyfluorescein diacetate (CFDA)-modified microdilution method for the susceptibility testing of fluconazole was conducted with 68 Candida strains, including 53 Candida albicans, 5 Candida tropicalis, 5 Candida glabrata, and 5 Candida parapsilosis strains. We found trailing endpoints and discordant fluconazole MICs of < 8 microg/ml at 24 h and of > or =64 microg/ml at 48 h for 12 of the C. albicans strains. These strains satisfy the definition of the low-high MIC phenotype. All 12 low-high phenotype strains were correctly shown to be susceptible at 48 h with the CFDA-modified microdilution method. For the 41 non-low-high phenotype C. albicans strains, the CFDA-modified microdilution method yielded 97.6% (40 of 41 strains) agreement within +/-1 dilution at 24 h compared with the reference method and 92.7% (38 of 41 strains) agreement within +/-1 dilution at 48 h compared with the reference method. The five strains each from C. tropicalis, C. glabrata, and C. parapsilosis that were tested showed 100% agreement within +/-2 dilutions for the two methods being evaluated.

Systemic antifungal agents

Anti-fungal agents are classified under two major headings, systematic and topical agents. Only systematic anti-fungal agents will be discussed in this chapter. Since the discovery in 1955, amphotericin B has been the cornerstone of anti-fungal treatment. It is active against most species of fungi. However, Candida lusitaniae, Pseudallescheria boydii, and fusarium spp have primary resistance to amphotericin B. Recently, new liposomal preparations of amphotericin B have been developed. They are less nephrotoxic. The azole family of anti-fungal includes two broad classes: the imidazoles (clotrimazote, ketoconazote, miconazole) and the triazoles (flucouazole and itracouazole). Imidazoles are still widely used for the treatment of superficial mycoses and vaginal candidiasis. The systematic triazoles are more slowly metabolized and have less effect on human synthesis than imidazoles, hence they are preferred for systemic therapy. Flucytosine is a fluorinated pyrimidine. Clinically, the principal use of flucytosine is as adjunctive therapy with amphotericin B in the treatment of candidial or cryptococcal diseases, Griseofuluin is derived from penicillium. It is fungistatic in vitro for species of dermatophytes. It is useful for the treatment of tinea capitis and tinea unginum.

Comparison of Etest with the broth microdilution method in susceptibility testing of yeast isolates against four antifungals

A comparative evaluation of the Etest and the broth microdilution methods for antifungal susceptibility testing of 102 clinical yeast isolates against amphotericin B, fluconazole, itraconazole, and ketoconazole was conducted. The agreements between the Etest and the broth microdilution methods were 93.1% for amphotericin B 85.2% for ketoconazole, 82.3% for itraconazole and 79.4% for fluconazole. These results suggest that the Etest approach to antifungal susceptibility testing may be a viable alternative to the NCCLS reference methods for testing yeasts, but that further evaluations are needed.

Quantitation of Candida albicans ergosterol content improves the correlation between in vitro antifungal susceptibility test results and in vivo outcome after fluconazole treatment in a murine model of invasive candidiasis

MIC end point determination for the most commonly prescribed azole antifungal drug, fluconazole, can be complicated by "trailing" growth of the organism during susceptibility testing by the National Committee for Clinical Laboratory Standards approved M27-A broth macrodilution method and its modified broth microdilution format. To address this problem, we previously developed the sterol quantitation method (SQM) for in vitro determination of fluconazole susceptibility, which measures cellular ergosterol content rather than growth inhibition after exposure to fluconazole. To determine if SQM MICs of fluconazole correlated better with in vivo outcome than M27-A MICs, we used a murine model of invasive candidiasis and analyzed the capacity of fluconazole to treat infections caused by C. albicans isolates which were trailers (M27-A MICs at 24 and 48 h, </=1.0 and >/=64 microg/ml, respectively; SQM MIC, </=1.0 microg/ml), as well as those which were fluconazole sensitive (M27-A and SQM MIC, </=1.0 microg/ml) and fluconazole resistant (M27-A MIC, >/=64 microg/ml; SQM MIC, 54 microg/ml). Compared with the untreated controls, fluconazole therapy increased the survival of mice infected with a sensitive isolate and both trailing isolates but did not increase the survival of mice infected with a resistant isolate. These results indicate that the SQM is more predictive of in vivo outcome than the M27-A method for isolates that give unclear MIC end points due to trailing growth in fluconazole.

Comparison of NCCLS and 3-(4,5-dimethyl-2-Thiazyl)-2, 5-diphenyl-2H-tetrazolium bromide (MTT) methods of in vitro susceptibility testing of filamentous fungi and development of a new simplified method

The susceptibility of 30 clinical isolates belonging to six different species of filamentous fungi (Aspergillus fumigatus, Aspergillus flavus, Scedosporium prolificans, Scedosporium apiospermum, Fusarium solani, and Fusarium oxysporum) was tested against six antifungal drugs (miconazole, voriconazole, itraconazole, UR9825, terbinafine, and amphotericin B) with the microdilution method recommended by the National Committee for Clinical Laboratory Standards (NCCLS) (M38-P). The MICs were compared with the MICs obtained by a colorimetric method measuring the reduction of the dye 3-(4,5-dimethyl-2-thiazyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) to formazan by viable fungi. The levels of agreement between the two methods were 96 and 92% for MIC-0 (clear wells) and MIC-1 (75% growth reduction), respectively. The levels of agreement were always higher for Aspergillus spp. (97% +/- 2.5%), followed by Scedosporium spp. (87% +/- 10.3%) and Fusarium spp. (78% +/- 7.8%). The NCCLS method was more reproducible than the MTT method: 98 versus 95% for MIC-0 and 97 versus 90% for MIC-1. However, the percentage of hyphal growth as determined visually by the NCCLS method showed several discrepancies when they were compared with the percentages of MTT reduction. A new simplified assay that incorporates the dye MTT with the initial inoculum and in which the fungi are incubated with the dye for 48 h or more was developed, showing comparable levels of agreement and reproducibility with the other two methods. Furthermore, the new assay was easier to perform and more sensitive than the MTT method.

In vitro susceptibility of yeasts for fluconazole and itraconazole. Evaluation of a microdilution test

In vitro susceptibilities were determined for a total of 159 clinical isolates and 12 reference strains of yeasts belonging to different Candida species including 94 Candida albicans strains, and further genera such as Cryptococcus, Trichosporon, Geotrichum and Saccharomyces. Minimum inhibitory concentration (MIC) values for fluconazole and itraconazole were assessed using a microdilution technique with the semisynthetic high resolution (HR) medium supplemented with glucose and asparagine but without sodium hydrogen carbonate (pH 7.0), according to a proposal of the working group 'Clinical Mycology' of the German Speaking Mycological Society. Fluconazole MIC values for C. albicans were between 0.125 and > or = 128 micrograms ml-1. Thus, the median of 1 microgram ml-1 showed that the overall fluconazole susceptibility was good. As expected, Candida krusei (seven strains) exhibited diminished in vitro susceptibility with MIC values for fluconazole of 8 to 128 micrograms ml-1 with a median of 64 micrograms ml-1. Some Candida kefyr strains seemed to be less susceptible against fluconazole which was indicated by a MIC90 of 64 micrograms ml-1. Surprisingly, no Candida glabrata isolate exhibited a MIC value greater than 16 micrograms ml-1. Other Candida species, Trichosporon cutaneum, Geotrichum candidum and Saccharomyces cerevisiae showed low MICs to fluconazole. In vitro susceptibility testing of itraconazole revealed that all Candida species except C. albicans, but also Trichosporon cutaneum, Geotrichum candidum, and Saccharomyces cerevisiae exhibited acceptable low MIC values against itraconazole (0.03-2 micrograms ml-1). Their MIC90 values for itraconazole were in the close range between 0.125 and 2 micrograms ml-1. MIC values between 0.125 and 2 micrograms ml-1 were obtained, even for C. krusei strains. On the other hand, the range of C. albicans MICs was between 0.0125 and > or = 16 micrograms ml-1 with MIC50 and MIC90 values of 0.125 and > or = 16 micrograms ml-1, respectively, indicating that a considerable number of yeast strains have high MICs. The comparative evaluation of different experimental conditions revealed that there exists a marked influence both of inoculum size and incubation time on the results of susceptibility testing. Therefore, for routine usage 10(2) CFU ml-1 and 18-24 h incubation time for this microdilution method with HR medium are recommended.

The Role of Resistance Testing in Clinical Trial Design and Product Labelling: A Regulatory Perspective

Assays that attempt to characterize HIV susceptibility or resistance are among the latest technologies that are likely to impact HIV clinical trial design, antiretroviral drug development and patient management. However, at present the Food and Drug Administration (FDA) have yet to approve any phenotypic or genotypic HIV resistance assay and the role of resistance testing in clinical management of patients and in drug development is ill defined. In November 1999, the Division of Antiviral Drug Products at the FDA convened a meeting of its advisory committee to consider the available information about HIV resistance testing, and to generate some recommendations about how these assays could be utilized in anti-retroviral drug development. In addition, the committee was presented with several hypothetical regulatory scenarios in order to illustrate how HIV resistance testing might be incorporated in antiretroviral drug development and drug labelling. In this article, we discuss the regulatory history of resistance testing in antimicrobial drug development, the current use of resistance testing for antiretrovirals, as well as a summary of the hypothetical scenarios that were presented to the committee and the discussion of the committee members regarding those scenarios.

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.

Quantitation of ergosterol content: novel method for determination of fluconazole susceptibility of Candida albicans

MIC end points for the most commonly prescribed azole antifungal drug, fluconazole, can be difficult to determine because its fungistatic nature can lead to excessive "trailing" of growth during susceptibility testing by National Committee for Clinical Laboratory Standards broth macrodilution and microdilution methods. To overcome this ambiguity, and because fluconazole acts by inhibiting ergosterol biosynthesis, we developed a novel method to differentiate fluconazole-susceptible from fluconazole-resistant isolates by quantitating ergosterol production in cells grown in 0, 1, 4, 16, or 64 microg of fluconazole per ml. Ergosterol was isolated from whole yeast cells by saponification, followed by extraction of nonsaponifiable lipids with heptane. Ergosterol was identified by its unique spectrophotometric absorbance profile between 240 and 300 nm. We used this sterol quantitation method (SQM) to test 38 isolates with broth microdilution end points of </=8 microg/ml (susceptible), 16 to 32 microg/ml (susceptible dose-dependent [SDD]), or >/=64 microg/ml (resistant) and 10 isolates with trailing end points by the broth microdilution method. No significant differences in mean ergosterol content were observed between any of the isolates grown in the absence of fluconazole. However, 18 susceptible isolates showed a mean reduction in ergosterol content of 72% after exposure to 1 microg of fluconazole/ml, an 84% reduction after exposure to 4 microg/ml, and 95 and 100% reductions after exposure to 16 and 64 microg of fluconazole/ml, respectively. Ten SDD isolates showed mean ergosterol reductions of 38, 57, 73, and 99% after exposure to 1, 4, 16, and 64 microg of fluconazole/ml, respectively. In contrast, 10 resistant isolates showed mean reductions in ergosterol content of only 25, 38, 53, and 84% after exposure to the same concentrations of fluconazole. The MIC of fluconazole, by using the SQM, was defined as the lowest concentration of the drug which resulted in 80% or greater inhibition of overall mean ergosterol biosynthesis compared to that in the drug-free control. Of 38 isolates which gave clear end points by the broth microdilution method, the SQM MIC was within 2 dilutions of the broth microdilution MIC for 33 (87%). The SQM also discriminated between resistant and highly resistant isolates and was particularly useful for discerning the fluconazole susceptibilities of 10 additional isolates which gave equivocal end points by the broth microdilution method due to trailing growth. In contrast to the broth microdilution method, the SQM determined trailing isolates to be susceptible rather than resistant, indicating that the SQM may predict clinical outcome more accurately. The SQM may provide a means to enhance current methods of fluconazole susceptibility testing and may provide a better correlation of in vitro with in vivo results, particularly for isolates with trailing end points.

Endophthalmitis

Endophthalmitis is an inflammatory reaction of intraocular fluids or tissues. Infectious endophthalmitis is one of the most serious complications of ophthalmic surgery. Occasionally, infectious endophthalmitis is the presenting feature of an underlying systemic infection. Successful management of infectious endophthalmitis depends on timely diagnosis and institution of appropriate therapy. Recognition of the different clinical settings in which endophthalmitis occurs and awareness of the highly variable presentation it may have facilitate timely diagnosis. Biopsy of intraocular fluid/tissue is the only method that permits reliable diagnosis and treatment. The different presenting clinical settings, a rational approach to diagnosis (i.e., when, what, and how to biopsy), and the treatment of infectious endophthalmitis are reviewed.

Comparison of three methods for testing azole susceptibilities of Candida albicans strains isolated sequentially from oral cavities of AIDS patients

Three susceptibility testing procedures were compared to determine fluconazole, itraconazole, and ketoconazole MICs against 47 Candida albicans strains isolated sequentially from the oral cavities of five AIDS patients undergoing azole therapy. They included the broth microdilution method (BM), performed according to the National Committee for Clinical Laboratory Standards' tentative standard, the agar dilution method (AD), and the Etest; the latter two tests were performed both in Casitone agar (AD-Cas and Etest-Cas) and in RPMI (AD-RPMI and Etest-RPMI). Twenty-four- and 48-h MICs obtained by AD and Etest were compared with 48-h MICs obtained by BM. The MICs of all the azoles determined by BM were usually lower than those obtained by the other methods, mainly due to different reading criteria. In order to assess the most appropriate way of evaluating the agreement of MICs obtained by different methods with those produced by the proposed reference method (BM), we used the mean differences calculated according to Bland and Altman's method. Comparison of fluconazole MICs obtained by BM and AD-Cas yielded a mean difference of 3, and the percentages of agreement within +/-2 dilutions were 98 and 100% at 24 and 48 h, respectively. For ketoconazole and itraconazole MICs, lower mean differences were noted, and agreement ranged from 96 to 100%. Agreement between the AD-RPMI and BM results was poor for all azoles, and an increase in MICs was always observed between the 1st- and 2nd-day readings. Similarly, Etest-Cas gave better agreement with BM than did Etest-RPMI for all the azoles. BM, AD-Cas, and Etest-Cas each demonstrated a progressive increase in fluconazole MICs against strains isolated sequentially from a given patient, in accordance with the decreased clinical response to fluconazole.

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

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

In vitro susceptibility testing of Fonsecaea pedrosoi to antifungals

Based on the difficulties experienced in the treatment of chromoblastomycosis, 12 primary human isolates of F. pedrosoi, were tested for their in vitro susceptibility to various antimycotics. We adapted the recommendations of the NCCLS for yeasts and followed the indications for mold testing from other authors in order to determine their MIC's and the MLC's. It was found that a significant proportion of the isolates were resistant to 3 of the 4 antimycotics tested, as revealed by high MIC values, as follows: 33% were resistant to amphotericin B (AMB), 58.3% to 5 fluocytosine (5 FC) and 66.7% to fluconazole (FLU). Contrarywise, none of the isolates proved resistant to itraconazole (ITZ). Determination of the MLC's revealed that a larger proportion of the isolates were not killed by AMB, 5 FC (91.7%), FLU (100%) or even, ITZ (41.7%). These data indicate that it would be desirable to determine the susceptibility of F. pedrosoi before initiating therapy, in order to choose the more effective antifungal and avoid clinical failure.

Trichoderma longibrachiatum infection in a pediatric patient with aplastic anemia

Trichoderma longibrachiatum infection of the skin in an 11-year-old child with severe aplastic anemia and prolonged neutropenia is reported. The patient received systemic antifungal therapy and underwent bone marrow transplantation. To our knowledge, this is the first description of T. longibrachiatum infection in a pediatric patient. It also is the first case successfully treated with medical therapy. A review of the literature suggests that Trichoderma spp. are recognized as human pathogens with increasing frequency, particularly for immunocompromised patients, and should be considered in the differential diagnosis of fungal infections in the pediatric population.

Plasma and tissue concentrations of fluconazole and their correlation to breakpoints

The prediction of clinical outcome during antifungal therapy is an issue of paramount importance in clinical research, because no consistently reliable parameters are available. Minimum inhibitory concentration (MIC) values and breakpoint interpretation may serve as surrogate criteria until standardized in vitro antifungal susceptibility testing is developed, especially for fluconazole. With reproducible susceptibility testing methods for Candida species now available, tentative fluconazole interpretive breakpoints derived from MIC values determined by the National Committee on Clinical Laboratory Standards (NCCLS) M27-T broth macrodilution method are open for public comment. Besides the in vitro susceptibility of the fungus, clinical response to antifungal therapy with fluconazole depends to a great extent on the daily dose and corresponding plasma and tissue concentrations. Promising results from a few dose-finding studies in non-neutropenic patients show that fluconazole doses of up to 1000 mg day-1 result in higher clinical response rates than lower dosages. Therapeutic success depends substantially on achieving fluconazole plasma and tissue levels that are sufficiently higher than MIC values indicated by in vitro testing. However, this simplified concept must be related to the clinical situation and it is essential to consider the immunological status and underlying disease of the patient. Misinterpretation of MIC values may result in selection of an antifungal agent for life-threatening infections that does not provide optimal efficacy or toleration.

Intracranial hypertension and cryptococcal meningitis in a girl with AIDS

A girl with HIV infection acquired at birth by blood transfusion, was admitted at the age of 10 years for diplopia, vomiting, headache and papilledema. CT scan was negative. A lumbar puncture revealed clear CSF, protein 0.40 g/l, glucose 2 mmol/l, 5 mononuclear cells/mm3. The Indian ink preparation and the latex agglutination antigen test were positive for Cryptococcus n. Treatment with amphotericin B and flucytosine was started. After 10 days, since the in vitro susceptibility testing of the isolates showed resistence to both drugs, fluconazolo (400 mg/day) was started. Acetazolamide, furosemide and spironolactone were then added to the antifungal therapy for the persistence of severe intracranial hypertension. Diuretics were maintained for 10 weeks. The patient returned to school two and half months after the admission to the hospital. After 19 months, she is doing well and she is on maintenance of fluconazole (200 mg/day). We hypothesized that the increased intracranial pressure would be due to an impaired CSF reabsorption probably as a consequence of a direct cryptococcal infiltration of the villi.

Amphotericin B responsive Scedosporium apiospermum infection in a patient with acute myeloid leukaemia

A 71 year old man with newly diagnosed acute myelomonocytic leukaemia developed a soft tissue infection of his foot during his first course of chemotherapy. Scedosporium apiospermum was isolated from the lesion, which resolved rapidly on treatment with intravenous amphotericin B despite being resistant in vitro to this agent. This observation suggests that sensitivity testing of S apiospermum should be interpreted with caution and that clinical response may be a better indicator of outcome.

[Plasma and tissue concentrations of fluconazole: discussion of the breakpoint problem]

The reliable prediction of the clinical outcome during antifungal therapy is an issue of paramount priority in clinical research, since there are no appropriate parameters available. MIC values and in the future breakpoints may serve as surrogate criteria if further standardization of in vitro antifungal susceptibility testing especially for fluconazole leading to improved interlaboratory reproducibility of the test results can be provided. With reproducible susceptibility testing methods for Candida species now being available, tentative fluconazole interpretative breakpoints derived from MICs determined by the NCCLS M27-T broth macrodilution methodology are now open for public commentary. Besides the proven susceptibility of the fungus, clinical response to antifungal therapy with fluconazole also depends to a great extent on the daily dosage and the corresponding plasma and tissue concentrations as well as the immunological status and the underlying disease of the patient. The promising results of a relatively small number of dose finding studies with fluconazole in non-neutropenic patients indicate that with daily doses up to 1000 mg/die higher clinical response rates compared to those under lower dosages can be achieved. In view of future valid breakpoints, higher corresponding plasma and tissue levels of fluconazole should be achieved on which the therapeutic success depends substantially. However, this simplified concept needs several adjustments. Misinterpretation of MIC values and breakpoints may have as a consequence that patients with often life-threatening fungal infections may not be treated with an efficacious and better tolerated agent.

Comparison of D0870 and fluconazole in the treatment of murine cryptococcal meningitis

Cryptococcal meningitis is a common infection in patients with AIDS. Using a murine cryptococcosis model, we compared treatment with a new triazole, D0870, and fluconazole. Groups of ICR (Institute for Cancer Research) mice were infected intracerebrally with eight different isolates of Cryptococcus neoformans variety neoformans with different in vitro susceptibilities to fluconazole. For survival studies mice were challenged with two to four times LD(50) or six to nine times LD(50). Treatment was given for 10 days. Mice were observed through to day 30. To assess the effect of treatment on fungal tissue burden, mice received a three to five times LD(50) inoculum and treatment for 10 days. They were sacrificed on day 12 and serial dilutions of brain homogenates were cultured. Fluconazole prolonged survival primarily in isolates which were susceptible in vitro. D0870 prolonged survival in all isolates except one, which was also resistant in vitro to D0870 and fluconazole. Both drugs reduced colony counts of all isolates. D0870 warrants further development for use in cryptococcosis, and appears effective for isolates relatively resistant to fluconazole. There is a relative correlation of in vivo and in vitro susceptibility to D0870 as well as fluconazole.

Initial use of a broth microdilution method suitable for in vitro testing of fungal isolates in a clinical microbiology laboratory

Antifungal susceptibility testing methods currently lack a standardized procedure. Many factors, such as inoculum preparation, inoculum density, medium selection, pH, incubation time and temperature, and endpoint determination, affect results. We developed a workable procedure for fungal susceptibility testing, with a microtiter method based upon modifications of the proposed guidelines from the National Committee for Clinical Laboratory Standards, using two different growth media. For this procedure, the microtiter tray is prepared as a panel of 6 drugs (amphotericin B, flucytosine, fluconazole, ketoconazole, miconazole, and itraconazole) alone and in combination with amphotericin B. Eagle's minimal essential medium and RPMI 1640 are the two growth media. Two separate susceptibility trays are inoculated for each sensitivity test, with one tray incubated at 30 degrees C and the other incubated at 35 degrees C. After 48 h of growth, results for both temperatures and both media are recorded and interpreted. The four test environments (two media each at two temperatures) provided growth for 100 of the first 104 organisms that were submitted for testing. This approach provides a workable methodology for routine antifungal susceptibility testing in a clinical microbiology laboratory setting.

Resistance to antifungal agents

The marked increase in the number of patients with AIDS and other forms of immunocompromise has resulted in the emergence of fungi as predominant pathogens in many institutions. Unfortunately, with the widespread use of antifungal agents to combat these infections, reports of resistance to antifungal agents have proliferated. In the present environment, the occurrence of resistance to antifungal agents is neither rare nor of negligible clinical importance. The expanding demand for antifungal agents mandates a new sense of vigilance for resistance. Although newly proposed standards for in vitro susceptibility testing should help to remove the ambiguity surrounding quantitative measurement of fungal resistance, lessons learned in the treatment of bacteria clearly now apply to fungi also: prolonged use of an antimicrobial agent will result in the selection of resistant organisms. The enlarging spectrum of resistance to antifungal agents must prompt aggressive searches for new modes of therapy. Strategies to inhibit fungal colonization, to augment host defenses, or to develop novel antifungal agents from Pseudomonas syringae or from peptide nanotubes are helping to solve this pressing clinical need.

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.

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.

Discrepancies between MIC and MLC values of amphotericin B against isolates of Aspergillus species

There is little information addressing the phenomena of discrepancy between minimal inhibitory concentrations (MIC) and minimal lethal concentrations (MLC) values of amphotericin B (AMB) to clinical isolates of fungi. This study assessed in vitro activity of AMB against 70 clinical isolates of aspergilli: 30 strains of Aspergillus fumigatus, 20 strains of Aspergillus flavus and 20 strains of Aspergillus niger. Susceptibility tests were accomplished using a macro broth dilution procedure, with special emphasis on ascertainment of MLCs. AMB exhibited low MIC values against all clinical isolates. While we did not identify any AMB resistant isolates among 70 Aspergillus spp. studied as judged by MIC levels, analysis of the data demonstrated a clear discrepancy between the MIC and MLC levels of AMB obtained against clinical isolates of Aspergillus spp. The MLC values of AMB were significantly higher than the MIC values with MIC 50 and MIC 90 of 0.29 and 0.5 microgram/ml, respectively, at the second reading time, and MLC 50 and MLC 90 of 2.31 and 9.24 micrograms/ml, respectively (p < 0.001). Additionally, minimal lethal concentrations in 36/70 (51%) of aspergillal isolates studied produced drug concentrations above those which can usually be sustained in patient plasma or tissue.

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.

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.