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The fungal kingdom encompasses a diverse and rich group of organisms ranging from microscopic yeasts to mushrooms. Most fungi are free-living in nature where they function as decomposers in the energy cycle. Of the more than 90,000 known fungal species, fewer than 200 have been reported to produce disease in humans. Once considered clinical rarities, human fungal infections are becoming increasingly common, especially among immunocompromised patients. Therefore, it is important to understand the unique clinical and microbiological features of these diseases.


A “yeast” is identified growing from a patient’s blood culture.

  1. What are fungi, and do they commonly cause important human diseases?

  2. How does the lab technologist identify the species of this fungus?

  3. How does the clinician know if this microorganism is relevant to the care of the patient?


Fungi are eukaryotes with a higher level of biologic complexity than bacteria. Fungi may be unicellular or may differentiate and become multicellular by the development of long, branching filaments. They lack the chlorophyll of plants and therefore need to acquire nutrients from the external environment. The diseases caused by fungi are called mycoses. These infections vary greatly in their manifestations but tend to present with subacute or chronic features, often relapsing over time. Acute disease, such as that produced by many viruses and bacteria, is less common with fungal infections.

❋ Fungal cell organization is eukaryotic


The fungal cell has many typical eukaryotic features, including a nucleus with a nucleolus, nuclear membrane, and linear chromosomes (Figure 42–1). The cytoplasm contains a cytoskeleton with actin microfilaments and tubulin-containing microtubules. Ribosomes and organelles, such as mitochondria, endoplasmic reticulum, and the Golgi apparatus, are also present. Fungal cells have a rigid cell wall external to the cytoplasmic membrane, which differentiates them from mammalian cells. In addition to the cell wall, another important difference from mammalian cells is the sterol composition of the cytoplasmic membrane. In mammalian cells, the dominant membrane sterol is cholesterol; in fungi, it is ergosterol. Fungi are usually haploid in their DNA content, although diploid nuclei are formed through nuclear fusion in the process of sexual reproduction. Interestingly, the generation of polyploid/aneuploid nuclei is a strategy used by some fungi to generate genetic diversity as a response to cell stress, such as antifungal therapy.

FIGURE 42–1.

A yeast cell showing the cell wall and internal structures of the fungal eukaryotic cell plan. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th ed. New York, NY: McGraw Hill; 2008.)

Nucleus, mitochondria, and endoplasmic reticulum present

❋ Fungal cell wall distinguishes from mammalian cells

❋ Ergosterol, not cholesterol, makes up cell membrane


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