Old and Modern Identification Techniques of medically important yeasts

Abstract

Yeasts are unicellular fungal elements. Yeasts may exist as such, or may be found at 37 C in thermally dimorphic fungi like Histoplasma capsulatum and Talaromyces marneffei. They are often encountered in the clinical microbiology setting from various kind of clinical specimens like urine, cerebrospinal fluid (CSF), and blood. Medically important yeasts are Candida albicans, Candida auris, Malassezia furfur and Cryptococcus spp. They are incriminated in various infections like oral thrush, fungemia, meningitis and cystitis. Many a times they are also encountered as normal inhabitants of the human skin and mucosa. Various degrees of immunosuppression in people, in the form of neutropenia or low CD4 T cell count may predispose a susceptible host to develop infections caused by yeasts. They need to be identified correctly for epidemiological purposes and also for institution of correct empirical antifungal therapy. Many old and new techniques have hence been described in the scientific literature for yeast identification. Old techniques like observation of shape and size, Germ tube test using pooled human serum, observing microscopic morphology by Dalmau slit culture, thermotolerance, CHROM agar, lecithinase and lipase production on egg yolk agar, sugar fermentation and sugar assimilation may need a lot of time and expertise, and may yield non-specific results, too. Modern technologies for yeast identification, like MALDI-TOF and Whole genome sequencing, are, hence fast replacing existing ones in this regard, by dint of their rapidity, low cost at times and precision. In this review we have tried to collate, assimilate and present available techniques for yeast identification.