Collaborative study of antibiotic medium 3 and flow cytometry for identification of amphotericin B-resistant Candida isolates

Current knowledge of Trichosporon spp. and Trichosporonosis

Trichosporon spp. are basidiomycetous yeast-like fungi found widely in nature. Clinical isolates are generally related to superficial infections. However, this fungus has been recognized as an opportunistic agent of invasive infections, mostly in cancer patients and those exposed to invasive medical procedures. It is possible that the ability of Trichosporon strains to form biofilms on implanted devices, the presence of glucuronoxylomannan in their cell walls, and the ability to produce proteases and lipases are all factors likely related to the virulence of this genus and therefore may account for the progress of invasive trichosporonosis. Disseminated trichosporonosis has been increasingly reported worldwide and represents a challenge for both diagnosis and species identification. Phenotypic identification methods are useful for Trichosporon sp. screening, but only molecular methods, such as IGS region sequencing, allow the complete identification of Trichosporon isolates at the species level. Methods for the diagnosis of invasive trichosporonosis include PCR-based methods, Luminex xMAP technology, and, more recently, proteomics. Treating patients with trichosporonosis remains a challenge because of limited data on the in vitro and in vivo activities of antifungal drugs against clinically relevant species of the genus. Despite the mentioned limitations, the use of antifungal regimens containing triazoles appears to be the best therapeutic approach.

Relationship between susceptibility of Candida spp. isolates to amphotericin B and death or survival of patients with candidemia episodes

In this study, the susceptibility to amphotericin B of Candida spp. isolates obtained from patients with candidemia was related to their respective clinical outcomes. The susceptibility tests were carried out in three culture media: RPMI 1640, Antibiotic medium 3 and Yeast Nitrogen Base dextrose. We have found that minimal inhibitory concentrations and minimal fungicidal concentrations obtained using AM3 and YNBd media were significantly higher for Candida spp. from patients who died than for those from patients who survived the candidemia (P < 0.05). The assays with RPMI 1640 medium did not show these differences.

Antifungal susceptibility testing

Antifungal susceptibility testing has been in routine use now for more than 15 years and has become a useful tool for clinicians who are faced with difficult treatment decision. Although most clinicians order susceptibility testing, much confusion still exists regarding the use of the results. Sufficient data have been generated to determine susceptibility trends for specific fungi against specific agents, but correlation data are minimal. Despite the lack of correlation data, antifungal susceptibility testing continues to provide useful information to assist with patient care.

Fluconazole susceptibility testing of Candida species by flow cytometry

OBJECTIVE

Currently, antifungal drug susceptibility testing is labor-intensive, limited by delays in obtaining results and high costs. The purpose of this study was to determine the usefulness of flow cytometry (FCM) antifungal drug susceptibility testing as a routine laboratory procedure.

METHODS

A total of 24 clinical isolates of Candida spp. and reference strains were tested for susceptibility to fluconazole by FCM using propidium iodide (PI) as an indicator of viability. The minimum inhibitory concentration (MIC) was defined as the lowest concentration of fluconazole that resulted in an increase of 30% in mean channel fluorescence (MCF), compared to the growth control. FCM results were compared with MIC results as determined by the Clinical and Laboratory Standards Institute (CLSI) method.

RESULTS

An 8h incubation was sufficient for determination of the MICs. The results by FCM at 8h and the NCCLS methods at 24h showed 87.5% agreement to within two drug dilutions. However, the FCM method is labor-intensive in proportion to the larger number of samples. For Candida lusitaniae, MICs by the FCM method showed poor correlation with the CLSI method.

CONCLUSIONS

Further evaluation is necessary to assess the usefulness of FCM as a technique for routine antifungal MIC testing in the clinical laboratory.

Candida pseudorugosa sp. nov., a novel yeast species from sputum

Two yeast strains, strains XH 1026 and XH 1164, isolated from the sputum of an intensive care unit patient with acute pneumonia, were originally identified as Candida albicans and C. tropicalis, respectively. Sequence analysis of the 26S rRNA gene D1/D2 domain and the internal transcribed spacer (ITS) region indicated that the two strains represent a novel yeast species closely related to C. rogusa. The name Candida pseudorugosa sp. nov. is therefore proposed (type strain, AS 2.3107 [CBS 10433]). The new species is able to grow at 42 degrees C and is resistant or insusceptible to amphotericin B (MIC, 2 microg/ml), caspofungin (MIC, 64 microg/ml), itraconazole (MIC, 1 microg/ml), and nystatin (MIC, 16 microg/ml); dose-dependent susceptible to fluconazole (MIC, 16 microg/ml); and susceptible to flucytosine (MIC, 0.125 microg/ml) and voriconazole (MIC, 0.125 to 0.25 microg/ml). The code for C. pseudorugosa sp. nov. provided by the API 20C AUX system is identical to that for C. rugosa. The colonies of the new species on CHROMagar Candida appear blue-green, similar to those of C. albicans. In addition to the molecular method based on D1/D2 domain or ITS region sequencing, use of the combination of the API system and CHROMagar Candida is helpful for the correct identification of C. pseudorugosa sp. nov.