A simple xylose-based agar medium for the differentiation of Candida dubliniensis and Candida albicans

Evaluation of various phenotypic methods for differentiation of Candida dubliniensis from Candida albicans

INTRODUCTION

Candida dubliniensis was first identified by Sullivan et al. (1995) in Dublin, Ireland. Its clinical significance is associated with development of fluconazole-resistance and invasive diseases in immunocompromised hosts. C. dubliniensis share many features with C. albicans so has been overlooked and misidentified for a long time.

AIMS

Evaluation of various phenotypic tests with polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) as a gold standard to find out the best method/methods for identifying C. dubliniensis.

MATERIALS AND METHODS

First PCR-RFLP was performed on 186C. albicans and 14C. dubliniensis strains and then five phenotypic tests were performed simultaneously on all the strains.

RESULTS

The results of salt tolerance test at 48 h, colony color on HiCrome candida differential agar (HCDA) at 72 h, heat tolerance test at 48 h, xylose assimilation using discs at 72 h and growth on xylose based agar medium (XAM) at 48 h are completely concordant with PCR-RFLP. Colony color on Tobacco agar could differentiate accurately 100% test strains while peripheral hyphal fringes and chlamydosporulation on this agar was seen in only 86% and 87% respectively. Our routine methods proved to be cost effective than PCR-RFLP but the turnaround time was same or more than PCR-RFLP.

CONCLUSION

For routine identification of C. dubliniensis we recommend use of colony color on HCDA and growth on XAM as simple, reliable and inexpensive method.

Hypertonic xylose agar medium: A novel medium for differentiation of Candida dubliniensis from Candida albicans

BACKGROUND

Candida dubliniensis is a pathogenic Candida species which shares many phenotypic features with Candida albicans. These similarities have caused significant problems in the identification of C. dubliniensis in an average clinical mycology laboratory. Several phenotypic-based tests have been developed to distinguish C. albicans from C. dubliniensis but none has been demonstrated being sufficient alone for accurate differentiation of the two species.

AIM

To facilitate the differentiation of these species, we evaluated the utility of a novel medium 'Hypertonic Xylose Agar Medium' (HXAM).

MATERIALS AND METHODS

A total of 200 Candida spp. were tested in this study which included 186 stock strains of C. albicans and 14 strains of C. dubliniensis. Identification of all these strains was confirmed by polymerase chain reaction-restriction fragment length polymorphism using Bln I (Avr II) enzyme. All isolates were inoculated on HXAM, incubated at 28°C and examined for visible growth every day up to 7 days.

RESULTS

On this medium at 28°C, all 186 C. albicans isolates showed visible growth at 48 h of incubation whereas none of the 14 C. dubliniensis isolates did so even on extending the incubation period up to 7 days.

CONCLUSION

Hence, we propose HXAM as a sole phenotypic method for identifying C. dubliniensis from germ-tube-positive isolates or from stock collections of known C. albicans.

Current methods for identifying clinically important cryptic Candida species

In recent years, the taxonomy of the most important pathogenic Candida species (Candida albicans, Candida parapsilosis and Candida glabrata) has undergone profound changes due to the description of new closely-related species. This has resulted in the establishment of cryptic species complexes difficult to recognize in clinical diagnostic laboratories. The identification of these novel Candida species seems to be clinically relevant because it is likely that they differ in virulence and drug resistance. Nevertheless, current phenotypic methods are not suitable to accurately distinguish all the species belonging to a specific cryptic complex and therefore their recognition still requires molecular methods. Since traditional mycological techniques have not been useful, a number of molecular based methods have recently been developed. These range from simple PCR-based methods to more sophisticated real-time PCR and/or MALDI-TOF methods. In this article, we review the current methods designed for discriminating among closely related Candida species by highlighting, in particular, the limits of the existing phenotypic tests and the development of rapid and specific molecular tools for their proper identification.