Yeasts are eukaryotic, unicellular organisms that are round to oval and range in size from 2 to 60 µm. The microscopic morphologic features have limited usefulness in helping to differentiate or identify these organisms. The microscopic morphology on cornmeal agar is most useful when considered in conjunction with the biophysical profile (i.e., a combination of the biochemical and physical characteristics used in the identification of a microorganism) obtained using a commercial system. Differentiation of yeasts in direct microscopic and histopathologic examination of clinical specimens is often impossible, but sometimes particular characteristics are seen that suggest the identification or are pathognomonic (i.e., unique) for a particular organism. Important morphologic characteristics that are useful in differentiating yeasts include the size of the yeasts, the presence or absence of a capsule, and broad-based or narrow-necked budding. For example, variability in size with evidence of a capsule and narrow-necked budding are features that can be helpful for distinguishing Cryptococcus spp. from Candida spp. The medically important yeasts and yeastlike organisms belong to different taxonomic groups, including the Ascomycota, Basidiomycota, and Deuteromycota.

In general, the yeasts reproduce asexually by blastoconidia formation (budding) (Figure 1) and sexually by the production of ascospores or basidiospores. The process of budding begins with a weakening and subsequent outpouching of the yeast cell wall. This process continues until the bud, or daughter cell, is completely formed. The cytoplasm of the bud is contiguous with the cytoplasm for the original cell. Finally, a cell wall septum is created between mother and daughter yeast cells. The daughter cell often eventually detaches from the mother cell, and a residual defect occurs at the budding site (i.e., a bud scar).

Fig1. Blastoconidia (budding cells [arrow]) characteristic of  the yeasts. 

With certain environmental stimuli, yeast can produce different morphologies. An outpouching of the cell wall that becomes tubular and does not have a constriction at its base is called a germ tube; it represents the initial stage of true hyphae formation (Figure 2). Alternatively, if buds elongate, fail to dissociate, and form subsequent buds, pseudohyphae are formed; to some, these resemble links of sausage (Figure 3). Pseudohyphae have cell wall constrictions rather than true intracellular septations delineating the fungal cell borders.

Fig2. Germ tube test for C. albicans showing yeast cells with  germ tubes. 

Fig3. Pseudohyphae consisting of elongated cells (arrow)  with constrictions at attachment. 

The number of fungal infections caused by yeasts and yeastlike fungi has increased significantly during recent years. Most of these infections have been caused by various Candida spp. However, other yeasts also cause significant disease, particularly in immunocompromised hosts, as yeasts are the cause of many opportunistic infections. In addition to causing disease in immunocompromised patients, infections also are common in post-surgical patients, trauma patients, and patients with long-term indwelling venous catheters. Some of these yeasts are resistant to commonly used antifungal agents, which emphasizes the need for prompt, appropriate identification and, in some cases, antifungal susceptibility testing.

The extent to which the laboratory should identify all yeast species is the subject of debate. Each laboratory director must decide how much time, effort, and expense is to be spent on the identification of yeasts in the laboratory.

The development of commercially available yeast identification systems has provided laboratories of all sizes with accurate, standardized methods. However, these methods should be used in conjunction with corn meal agar morphology to prevent misidentifications. Some commercial systems have extensive computer data bases that include biochemical profiles of a large number of yeasts. Variations in the reactions of carbohydrates and other substrates utilized are considered in the identification of yeasts provided by these systems.

Commercially available systems are recommended for all laboratories. They may be used in conjunction with some less expensive and rapid screening tests that provide presumptive identification of C. neoformans and definitive identification of Candida albicans.

More recent diagnostic tools that have been introduced for quicker characterization of yeasts include CHROMagar Candida (BD Diagnostics, Sparks, Maryland) and C. albicans PNA FISH (AdvanDX, Woburn, Massachusetts). Because some laboratories might prefer to use conventional systems, the information presented in this section discusses rapid screening methods for presumptive identification of yeasts, commercially available systems, and a conventional schema for identifying commonly encountered yeast species.

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مواضيع ذات صلة

Conventional Yeast identification methods

Commercially available yeast identification systems

Approach to identification of the Yeasts

Specimen collection, transport, and processing of The Yeasts

Cryptococcus neoformans: pathogenesis and spectrum of disease

Candida albicans: pathogenesis and spectrum of disease

Laboratory diagnosis of Pneumocystis jiroveci

Pathogenesis and spectrum of disease of Pneumocystis jiroveci

General Characteristics of Pneumocystis jiroveci

Approach to Identification of Systemic Mycoses

Pathogenesis and spectrum of disease of Systemic Mycoses

Epidemiology of Systemic Mycoses