MICROBIOLOGY Study Guide - 2012

CHAPTER 5. FUNGI

5.4. YEASTS

Yeasts are eukaryotic, single-celled microscopic Fungi. Based on the presence and characteristics of their sexual process, yeasts are classified into the three aforementioned classes of higher fungi: Ascomycetes, Basidiomycetes, and Deuteromycetes.

Yeasts are classified primarily by their mode of sexual reproduction, i.e., sporulation. Spores of ascomycetous yeasts are formed via the conjugation of Cells of opposite mating types, meiotic division of the diploid Cell Nucleus, and The formation of an ascus. The Class Ascomycetes includes three Yeast families: Saccharomycetaceae, Schizosaccharomycetaceae, and Saccharomycodaceae.

In basidiomycetous yeasts, a complex sexual cycle culminates in the formation of basidiospores, which adopt the shape of budding yeast cells. Among basidiomycetous yeasts, sporulation is rare. Furthermore, sexual processes remain entirely unknown for nearly half of all described yeast species.

Imperfect yeasts are either anamorphs of known ascomycetous or basidiomycetous yeasts, or species in which a sexual reproductive stage is absent or has not yet been discovered. Their affinity can be determined through a set of characteristics that provide insight into the phylogenetic relationships of these yeasts with Ascomycetes or Basidiomycetes. On this basis, the family Candidaceae is distinguished by traits of ascomycetous affinity, alongside two families exhibiting basidiomycetous affinity: Cryptococcaceae (non-ballistospore-forming) and Sporobolomycetaceae (ballistospore-forming).

In the food and microbiological industries, yeasts of the genus Saccharomyces from the family Saccharomycetaceae are widely utilized. Saccharomyces species are employed in The production of ethanol, wine, beer, kvass, bread, and baker's yeast. Brewing utilizes various yeast strains of the genus Saccharomyces, whereas baking employs yeast species of Saccharomyces cerevisiae and Saccharomyces minor. Certain members of the family Saccharomycodaceae (genera Nadsonia, Saenko) participate in the Fermentation processes used for sherry and champagne production. All other yeast species are regarded as extraneous ("wild") contaminants that can compromise the biotechnological process at various stages.

Among yeasts, there are opportunistic pathogens capable of causing severe infections in immunocompromised individuals. For instance, Representatives of the yeast genus Cryptococcus cause cryptococcosis, a serious Brain infection, while yeasts of the genus Candida cause candidiasis. The clinical manifestations of candidiasis vary widely, ranging from Skin, nail, and mucosal lesions to extensive infections of Internal Organs and Tissues.

Cell shape and size. Cells of yeasts belonging to the genus Saccharomyces are round, oval, ovoid, or elliptic. Yeasts of the genus Hansenula feature cylindrical and lemon-shaped (apiculate) cells. The genus Schizosaccharomyces comprises rod-shaped yeast species. Other observed cell morphologies include cylindrical, sickle-shaped, spear-shaped, and triangular forms. Yeast cell dimensions typically range from 2.5–10 µm in width and 4.5–21 µm in length.

The shape and size of yeast cells can vary significantly depending on cultivation conditions and cell age (Fig. 15). Young cells are smaller, round, and frequently budding, whereas older cells are larger and oval or elliptic in shape.

Fig. 15. Yeasts of the species Saccharomyces cerevisiae:

a — young cells; b — mature cells; c — old cells