Detection of resistance to amphotericin B in Candida isolates by using Iso-Sensitest broth

A major limitation of the National Committee for Clinical Laboratory Standards M27-A methodology is reliable detection of amphotericin B (AMB) resistance. The results obtained by using Iso-Sensitest, a synthetic medium, to detect AMB resistance were analyzed and compared with those obtained with RPMI and antibiotic medium 3 (AM3). The ability to detect AMB resistance with RPMI is not enhanced by using a higher inoculum, glucose supplementation at a final concentration of 20 g/liter, spectrophotometric reading, or 24 h of incubation time. Testing using AM3 and an inoculum of 10(3) CFU/ml detects resistance. Identification of resistant isolates is not improved by glucose supplementation, changes in reading method, or changes in incubation time. However, the use of AM3 as assay medium and an inoculum of 10(5) CFU/ml did not allow detection of AMB resistance. Testing using Iso-Sensitest medium appears to be similar to AM3 in detecting resistance. The most pronounced discrimination is achieved by testing in Iso-Sensitest supplemented with glucose and spectrophotometric reading after 24 h of incubation. The reproducibility of MIC testing was greatest for Iso-Sensitest-based procedures. Use of Iso-Sensitest produces both highly reproducible MICs and reliable identification of AMB-resistant Candida isolates.

Standardization of antifungal susceptibility variables for a semiautomated methodology

Recently, the methodology that will serve as a basis of the standard for antifungal susceptibility testing of fermentative yeasts of the European Committee on Antibiotic Susceptibility Testing has been described. This procedure employs a spectrophotometric method for both inoculum adjustment and endpoint determination. However, the utilization of a spectrophotometer requires studies for standardization. The present work analyzes the following parameters: (i) accuracy of inoculum preparation, (ii) correlation between optical density and CFU per milliliter, (iii) influence of the wavelength on the endpoint determination, and (iv) influence of the dimethyl sulfoxide concentration on the growth kinetics. The main results can be summarized as follows: (i) inoculum preparation following the methodology recommended by the National Committee for Clinical Laboratory Standards is an exact procedure; (ii) the relationship between optical density and CFU per milliliter is linear (coefficient of determination, r(2) = 0.84); (iii) MICs obtained by means of spectrophotometric readings at different wavelengths are identical (for amphotericin B, an intraclass correlation coefficient of 0.98 was obtained; for fluconazole, the intraclass correlation coefficient was 1); and (iv) a 2% concentration of dimethyl sulfoxide produces a significantly slower and lower growth curve of Candida spp. than other concentrations.

Flucytosine primary resistance in Candida species and Cryptococcus neoformans

The in vitro activity of flucytosine (5FC) against 1,140 clinical isolates of Candida spp. and Cryptococcus neoformans was evaluated and compared with the activity of amphotericin B, fluconazole and itraconazole. Overall, 87.72% (1,000/1,140) of yeasts were susceptible to 5FC. This agent showed less potent in vitro activity against Candida glabrata, Candida krusei, Candida guilliermondii and Cryptococcus neoformans (MIC90s, 8-16 microg/ml) and intermediate activity or resistance to 6.5% of Candida albicans, 5.1% of Candida tropicalis and 0.8% of Candida parapsilosis strains. Amphotericin B showed potent activity against isolates with an MIC of 5FC > or = 8 microg/ml. A total of 112 of 140 strains that were SFC-intermediate or -resistant showed decreased susceptibility to azoles (P < 0.01).

Inoculum standardization for antifungal susceptibility testing of filamentous fungi pathogenic for humans

Two methods of inoculum preparation for filamentous fungi were compared: counting with a hematocytometer and spectrophotometric adjustment. One hundred eighty-two filamentous fungi pathogenic for humans were used. Colony counts were done for all inoculum preparations. The agreement between the hematocytometer counts and the colony counts (CFU per milliliter) was 97.2%. The reproducibility between the hematocytometer counts and the colony counts by means of an intraclass correlation coefficient was 0.70. Pearson's correlation index for hematocytometer counts versus colony counts was 0.56, whereas that for optical density versus colony counts was 0.008. Both methods can be used for inoculum size adjustment. However, the use of the spectrophotometric method requires that each species be standardized separately.

Influence of glucose supplementation and inoculum size on growth kinetics and antifungal susceptibility testing of Candida spp

The influences of inoculum size and glucose supplementation on the growth kinetics of 60 Candida spp. clinical isolates (Candida albicans, Candida tropicalis, Candida parapsilosis, Candida glabrata, Candida krusei, and Candida lusitaniae [10 isolates each]) are assessed. The combined influence of growth and reading method (visual or spectrophotometric) on the determination of the MICs of amphotericin B, flucytosine, fluconazole, itraconazole, ketoconazole, and voriconazole is also analyzed, and the MICs are compared with those determined by the National Committee for Clinical Laboratory Standards standard microdilution method (NCCLS document M27-A). Glucose supplementation and inoculum size had a significant influence on the growth cycles of these yeasts, and a statistically significant denser growth (optical density at 540 nm) was seen for both incubation periods, 24 and 48 h (P < 0.01). A longer exponential phase and shorter lag phase were also observed. The A540 values at 24 h of incubation with medium containing glucose and an inoculum of 10(5) CFU/ml were >0.4 U for all species, with the exception of that for C. parapsilosis (A540 = 0.26 +/- 0.025). The MICs at 24 h determined by testing with 2% glucose and an inoculum of 10(5) CFU/ml showed the strongest agreement (96.83%) with MICs determined by the reference method. MICs were not falsely elevated, and good correlation indexes were obtained. The reproducibility of results with this medium-inoculum combination was high (intraclass correlation coefficient, 0.955). The best agreement and reproducibility of results for spectrophotometric readings were achieved with endpoints of 50% growth inhibition for flucytosine and azoles and 95% for amphotericin B. Supplementation of test media with glucose and an inoculum size of 10(5) CFU/ml yielded a reproducible technique that shows elevated agreement with the reference procedures and a shorter incubation period for obtaining reliable MIC determinations. The spectrophotometric method offers an advantage over the visual method by providing a more objective and automated MIC determination.

Susceptibility of fluconazole-resistant clinical isolates of Candida spp. to echinocandin LY303366, itraconazole and amphotericin B

The in vitro activity of LY303366 was compared with those of itraconazole and amphotericin B against 156 fluconazole-resistant (MIC > or = 16 mg/L) clinical isolates of Candida spp. An adaptation of the NCCLS reference method was employed for determination of MICs. LY303366 was more potent than either itraconazole or amphotericin B against Candida albicans, Candida glabrata, Candida krusei and Candida tropicalis, even against isolates with itraconazole MICs > or = 1 mg/L. LY303366 was less potent in vitro against Candida parapsilosis and Candida guilliermondii isolates. LY303366 has promising antifungal activity and warrants further investigation.

Sustained gastrointestinal colonization and systemic dissemination by Candida albicans, Candida tropicalis and Candida parapsilosis in adult mice

The ability of nine clinical isolates of Candida species (three C. albicans, three C. tropicalis and three C. parapsilosis) to colonize and invade the gastrointestinal (GI) tract of adult male CD-1 (ICR) mice was determined. The effect of dietary tetracycline plus glucose supplementation on colonization was evaluated. Strains were intragastrically inoculated. Tetracycline and glucose altered substantially aerobic flora, especially streptococci (average fall 1.1 +/-0.3 log(10) CFU/g, p<0.01 by the Student's t test). At two weeks after oral challenge, sustained and high colonization of GI tract by Candida (mean 5,28 +/- 0.18 log(10) CFU/g, p<0.01) was achieved in all mice receiving glucose-tetracycline supplementation, excepting in animals inoculated with one of C. tropicalis isolates. Histologic sections of the stomachs revealed multiple intraepithelial micro-abscesses associated with hyphae in the region of the cardial-atrium fold. Under immunosuppression, systemic spread of C. albicans and C. tropicalis was observed in 62% and 24% of animals receiving dietary supplementation respectively. Dissemination was not noted for C. parapsilosis isolates. We have developed a simple and inexpensive murine model of sustained colonization of GI tract. This model could be useful for analyzing prophylaxis, treatment and diagnosis of systemic Candida infections and for evaluating virulence of strains.

Influence of shaking on antifungal susceptibility testing of Cryptococcus neoformans: a comparison of the NCCLS standard M27A medium, buffered yeast nitrogen base, and RPMI-2% glucose

Cryptococcus neoformans is a nonfermentative yeast that requires oxygen for growth. The shaking of culture media achieves good oxygenation, promoting the growth of cryptococci. In this study, three test media (RPMI 1640, RPMI 1640-2% glucose, and buffered yeast nitrogen base ¿BYNB) recommended in the National Committee for Clinical Laboratory Standards M27A standard were examined. Growth abilities and minimum inhibitory concentrations (MICs) in microplates incubated at 35 degrees C for 48 h were determined. The results indicated that shaking and an inoculum size of 10(5) CFU/ml yielded optimal growth of this yeast. Compared to RPMI 1640, supplementation of RPMI 1640 with 2% glucose did not significantly improve growth of C. neoformans and resulted in an 8.7-h delay of exponential growth. Cryptococcal growth in RPMI 1640 at 24 h was notably better than that in RPMI-2% glucose, although by 48 h the growths were comparable. The MIC range of amphotericin B observed for the C. neoformans strains grown in RPMI 1640 with or without glucose was too narrow to allow the separation of susceptible and resistant strains based on clinical outcome. The widest ranges of MICs of flucytosine and fluconazole were obtained with BYNB. This work demonstrates the need for a new antifungal susceptibility test for C. neoformans.

In-vitro comparative activity of UR-9825, itraconazole and fluconazole against clinical isolates of Candida spp

UR-9825 is a new broad-spectrum triazole antifungal agent with a good pharmacokinetic profile and excellent bioavailability. It shows high in-vitro activity and efficacy in models of systemic candidosis in rats and rabbits, comparing favourably with fluconazole. The purpose of this study was to evaluate the in-vitro activity of UR-9825 and to compare it with that of fluconazole and itraconazole against 283 clinical isolates of Candida spp. UR-9825 was more potent against Candida spp. than both fluconazole and itraconazole, even against some Candida albicans and Candida krusei isolates with decreased susceptibility to fluconazole (MIC 16 mg/L).

[A multicenter study on fungemia caused by yeasts in Spain (April-June, 1997). A Work Group to Study Fungemia)]

Yeast fungemia has increased markedly in the last few years. Currently, Candida spp. is one of the microorganisms recovered most frequently from blood cultures. To better know the prevalence of yeast fungemia in our country we conducted a cross-sectional study with yeasts recovered from blood cultures obtained during a 3-month period from 39 hospitals in the Spanish public health hospital net. A total of 153 yeasts were recovered (59 were C. albicans, 53 C. parapsilosis, and 41 other species of yeasts). The percentage of males was 64.1. Mean age was 50.8 years. Factors associated with the appearance of fungemia were previous antibiotic therapy (83.7%), venous catheter (66%), previous bacterial infections (47.7%), and parenteral nutrition (40.5%). The recovery of C. albicans was independently associated with the admission to ICU, PDA and presence of urological/renal disease; C. parapsilosis was independently associated with the presence of hematological disease. The susceptibility studies of these strains recovered from blood cultures showed that in vitro resistance to amphotericin B and 5-fluorocytosine are practically nonexistent and that resistance to azole compounds is low. Multicentric epidemiologic studies are still necessary in this field of Microbiology.

Comparative in vitro activity of voriconazole and itraconazole against fluconazole-susceptible and fluconazole-resistant clinical isolates of Candida species from Spain

The in vitro activity of voriconazole was compared with that of itraconazole against 299 fluconazole-susceptible (MIC < or = 8 microg/ml) and 130 fluconazole-resistant (MIC > or = 16 microg/ml) clinical isolates of Candida spp. An adaptation of the National Committee for Clinical Laboratory Standards reference method was employed for determination of MICs. Voriconazole showed more potent activity than either fluconazole and itraconazole, even against some Candida albicans, Candida glabrata, and Candida krusei isolates resistant to fluconazole. However, for fluconazole-resistant isolates, the MICs of itraconazole and voriconazole were proportionally higher than for fluconazole-susceptible isolates. These data may indicate cross-resistance.

Comparative in-vitro activity of voriconazole (UK-109,496) and six other antifungal agents against clinical isolates of Scedosporium prolificans and Scedosporium apiospermum

We report the in-vitro susceptibility of 27 clinical isolates of Scedosporium apiospermum and 43 of Scedosporium prolificans. S. apiospermum was resistant to fluconazole and flucytosine, with variable susceptibility to amphotericin B, itraconazole, ketoconazole and susceptible to miconazole. Voriconazole was much more active than fluconazole and flucytosine, more active than amphotericin B, itraconazole and ketoconazole and was as active as miconazole against S. apiospermum isolates. Voriconazole and the other six antifungal agents showed low activity against S. prolificans isolates.

Comparison of the in-vitro activity of voriconazole (UK-109,496), itraconazole and amphotericin B against clinical isolates of Aspergillus fumigatus

Voriconazole was compared with amphotericin B and itraconazole by a modification of the NCCLS microdilution reference method for yeasts against 62 clinical isolates of Aspergillus fumigatus. MICs of voriconazole were slightly lower than those of amphotericin B and itraconazole. The MIC of voriconazole at which 90% of isolates were inhibited was 1 mg/L and the MIC range was 0.25-2 mg/L. Voriconazole is a new antifungal agent with potential for use in the treatment of A. fumigatus infections.

Use of random amplification of polymorphic DNA (RAPD) and PCR-fingerprinting for genotyping a Scedosporium prolificans (inflatum) outbreak in four leukemic patients

Four isolates of the pathogenic fungus Scedosporium prolificans (inflatum), causing a previously reported nosocomial outbreak in four leukemic patients, were typed by random amplification of polymorphic DNA (RAPD) with two different 10-mer primers and PCR-fingerprinting with the core sequence of phage M13 as a single primer. Both techniques allowed 10 additional clinical isolates of Scedosporium prolificans from different areas of Spain, including Scedosporium prolificans NCPF 2884, to be classified into 10 different molecular types. The four outbreak isolates consisted of three molecular types with two patients sharing a similar strain, and the remaining two patients infected by two different strains.

Deep infections caused by Scedosporium prolificans. A report on 16 cases in Spain and a review of the literature. Scedosporium Prolificans Spanish Study Group

Scedosporium prolificans, a mold morphologically similar to Scedosporium apiospermum, may cause asymptomatic colonization or localized or disseminated infection following trauma, surgery, and immunosuppression. S. prolificans is normally resistant to available antifungal agents, and prognosis depends largely on the host's immune status, extent of infection, and feasibility of surgical debridement. We report on 16 patients with deep S. prolificans infections, focusing on predisposing factors, clinical characteristics, outcome, postmortem findings, and antifungal susceptibility testing to 6 antifungal agents. Between 1989 and 1994, 16 cases of deep infections by S. prolificans were documented in 6 clinical centers in Spain (15 adults and 1 child: male/female = 0.77). Fifteen patients had underlying hematologic malignancy (14 with neutropenia) and 1 had a prosthetic cardiac valve. Syndromes included disseminated infection in 14 patients (1 with prosthetic valve endocarditis) and fungal pneumonia and meningoencephalitis in 1 patient each. S. prolificans was isolated from 2 specimens in 14 patients and from 1 specimen in 2 patients (blood, n = 12; respiratory tract, n = 4; CNS, n = 4; and skin biopsy, n = 3). Antifungal susceptibility testing by a micromethod with RPMI-2% glucose medium was performed in 8 isolates, all of which were resistant to amphotericin B, flucytosine, ketoconazole, fluconazole, itraconazole, and miconazole. All patients received antifungal therapy (amphotericin B, n = 9; amphotericin B+ flucytosine, n = 1; amphotericin B+ itraconazole, n = 2; liposomal amphotericin B+ itraconazole, n = 1; amphotericin B+ fluconazole, n = 1 and 2 underwent surgical procedures. Two patients survived coinciding with hematologic recovery and 14 (87.5%) patients died in a median time of 4 days after the first positive culture (range, 0-60 d). Necropsy was performed in 10 patients, and disseminated infection was found in 9. In conclusion, S. prolificans is an emerging multiresistant fungal pathogen that may cause asymptomatic colonization, localized infection related to trauma or surgery, and rapidly fatal disseminated infection in immunocompromised hosts, particularly those with neutropenia. This mycosis underscores the urgent need for new antifungal agents.

Comparison of four molecular typing methods for evaluating genetic diversity among Candida albicans isolates from human immunodeficiency virus-positive patients with oral candidiasis

Candida albicans strain delineation by karyotyping. NotI restriction pattern analysis, hybridization with specific probe 27A, and PCR fingerprinting with the phage M13 core sequence were performed with 30 isolates from the oral cavities of 30 human immunodeficiency virus (HIV)-infected patients and 8 reference strains. Within the panel of clinical isolates, 20 were geographically related, although 10 isolates were susceptible to fluconazole and 10 isolates were resistant to fluconazole. The remaining isolates used in this study were fluconazole resistant and geographically unrelated. A composite DNA type was defined for each of the strains as the combination of types obtained by the four molecular methods. By this procedure, a great diversity of DNA types was found among isolates from the oropharynges of HIV-infected individuals with oral candidiasis. This diversity was not reduced when isolates were evaluated on the basis of whether they came from the same geographical locale and whether they were fluconazole resistant. These data refute the idea of a clonal origin for fluconazole-resistant strains among HIV-positive patients. Karyotyping was the least discriminatory method, yielding 19 DNA types among the 38 strains analyzed. Conversely, hybridization with the 27A probe showed a unique DNA pattern for each of the strains examined in this study. Our results demonstrate that at least two different molecular methods are needed for Candida albicans typing and that there is a great deal of strain variation within the species, irrespective of place of origin or antifungal resistance patterns.

Patterns of fluconazole susceptibility in isolates from human immunodeficiency virus-infected patients with oropharyngeal candidiasis due to Candida albicans

We evaluated 119 episodes of oropharyngeal candidiasis due to C. albicans to study the patterns of fluconazole susceptibility of the isolates and the characteristics of the patients and to confirm the correlation between fluconazole susceptibility of isolates and therapeutic outcome. Sixty-one isolates were considered susceptible to fluconazole (MICs, < or = 0.5 microg/mL), 33 were intermediate (MICs, 1.0-8.0 microg/mL), and 25 were resistant (MICs, > or = 16.0 microg/mL). Patients infected with resistant strains had significantly lower CD4+ cell counts and a less recent AIDS diagnosis than patients infected with intermediate or susceptible strains. Previous fluconazole therapy and prophylaxis were significantly more frequent for patients infected with resistant and intermediate strains (P < .001). Decreased susceptibility to ketoconazole and itraconazole was observed in resistant and intermediate strains. Fluconazole treatment was ineffective for patients infected with resistant isolates; however, high doses of ketoconazole or itraconazole were successful for nine (81%) of them. Different patterns of fluconazole susceptibility among C. albicans strains are correlated with patients' characteristics and with therapeutic outcomes.

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.

Mixed oropharyngeal candidiasis due to Candida albicans and non-albicans Candida strains in HIV-infected patients

In order to determine the clinical significance of mixed oropharyngeal candidiasis (Candida albicans plus a non-albicans strain of Candida) in patients infected with HIV-1, a retrospective chart review was done in 12 HIV-1-infected patients with a clinical episode of oropharyngeal candidiasis, in whom a mixed culture of Candida albicans (found to be fluconazole-sensitive) plus a non-albicans species of Candida was obtained from their oral cavities. This group was compared with 26 HIV-positive patients (control group) with oropharyngeal candidiasis due to Candida albicans (found to be fluconazole-sensitive). Antifungal susceptibility testing was performed by a broth microdilution test with RPMI-2% glucose. A fungal strain was considered fluconazole-sensitive if its MIC was < 0.5 micrograms/ml. Both the study and control groups had similar clinical and demographic characteristics. All the patients were severely immunocompromised, with a mean CD4+ lymphocyte count of 63/mm3 (95% CI 41-84) and 80/mm3 (95% CI 25-135) in the study and control groups, respectively. In the study group, seven patients had Candida albicans and Candida krusei in their oral cavity, four had Candida albicans and Candida glabrata, and one had Candida albicans and Candida tropicalis. Antifungal therapy consisted of ketoconazole (5 patients in the study group, 14 in the control group) or fluconazole (7 patients in the study group, 12 in the control group); no statistically significant difference in clinical outcome was observed. Fungal strain persistence after therapy was frequently observed in both groups. It is concluded that non-albicans strains of Candida, less sensitive to azole drugs than their Candida albicans counterparts, are not clinically relevant in episodes of mixed oropharyngeal candidiasis in HIV-1-infected patients.

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

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