Tom Volk's Fungus of the Month for January 1999

This month's fungus is Candida albicans, cause of most "yeast infections" in humans.

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scanning EM of Candida budding I can hear you thinking, "Well this is certainly a change from the usual fungus of the month." I'm teaching Medical Mycology again this semester, so I thought I'd include the most commonly discussed of the human pathogenic fungi. Judging by the frequency of commercials on television, yeast infections are a very common cause of disease and discomfort, especially among women. Most cases of yeast infection in the United States are caused by Candida albicans. "Yeast" is the term for a fungus that exists as a single-celled organism rather than as hyphae. There are about 500 species of yeasts in 60 genera, or about 1000 species of yeasts or yeast-like organisms. Some fungi are called yeast-like because they exist as a yeast for part of their life cycle, but can be hyphal for a significant portion of it.

The terms yeast and yeastlike are vernacular terms for unicellular fungal organisms that reproduce by budding. This is an inadequate definition, mainly because

The term "yeast" is of no taxonomic significance. It is useful only to describe a morphological form of a fungus. Most yeasts have affinities to Ascomycota, but a small percentage have affinities to Basidiomycota.

cultures of Candida albicansIn the strictest sense of the word there are no inherently pathogenic yeasts-- those associated with human or animal disease are incapable of producing infection in the normal healthy individual. Some alteration of the host's cellular defenses, physiology, or normal flora must take place before colonization, infection, and disease production can take place. Pathogenicity among yeasts is extremely variable-- the most virulent is Candida albicans. There are also other pathogenic Candida species, as well as pathogenic species of Cryptococcus (especially C. neoformans), Torulopsis, Trichosporon, and Rhodotorula. Most of these have airborne spores or conidia and can be isolated as contaminants from skin, sputum, feces or other clinical specimens. This can lead to confusion about which organism is actually the pathogen. Only a few species in a few genera have regularly been associated with production of disease in humans or animals. In compromised host there are many others that can be opportunists.

Candida albicans between the toesUnlike mycelial fungi, many yeasts are normal flora inhabitants of humans. Many Blastomycetes (yeasts and yeastlike fungi) constitute a resident population regularly part of the skin, surfaces, buccal mucosa, intestinal tract and vagina mucosa. When the normal flora of these areas is disrupted, often Candida albicans or other yeasts can become a problem as they take advantage of reduced competition.

Clinical manifestations of candidiasis are extremely varied, ranging from acute, subacute, chronic and episodic. Involvement may be localized to the mouth, throat, skin, scalp, vagina, fingers, toes shown to the left), nails, bronchi, lungs or gastrointestinal tract. It may also be systemic as in septicemia (circulating in the blood and causing damage to blood vessels and sometimes blood cells), endocarditis and meningitis. Pathologic processes evoked are diverse and vary from irritation and inflammation to chronic and acute suppuration or granulomatous response. There are many common names for the diseases because the fungus is so commonly associated with infections. historically this has been a well-known pathogen. Hippocrates described thrush --oral candidiasis-- in the 4th century B.C. The word thrush is derived from ancient Scandinavian or Anglo-Saxon words for the disease. The French word for the condition is le Muguet, which means "lily-of-the-valley." Factors predisposing people to candidiasis include AIDS, burn patients, young individual, pregnancy, oral birth control, high fruit diets, steroids, antibiotic therapy, immunosuppressants, cancer treatments, heart surgery, genetic deficiency, endocrine deficiency diabetes, use of catheters, and use of dirty needles

Candida albicans chlamydosporesMethods of isolation characterization and identification of yeasts are very different from those of the mycelial fungi. Morphology is much less important, although C. albicans will form chlamydospores, such as you see to the left, on certain media. Most yeasts, however, look pretty much the same. Physiology is far more important. Techniques in zymology (the study of yeasts) are similar to those of bacterial identification. Physiological tests include:

  1. carbohydrate assimilation and fermentation
  2. nitrogen utilization
  3. production of extracellular substances, such as capsules
  4. production of enzymes
There are a number of commercial kits now available to identify yeasts. Most are based on variations on these physiological tests.

There are many other kinds of yeasts that cause human disease. These include: In addition the true human-pathogenic fungi have a yeast phase inside the human body, thus getting around the human immune system-- although the mycelial form is the cause of infection. These include: The other true human-pathogenic fungus is Coccidioides immitis, cause of coccidioidomycosis, also known as Valley fever. It is restricted mostly to the desert southwest USA and the San Joachin Valley in California. It is probably the most virulent of the human pathogenic fungi. It is dimorphic, producing a mycelial form in nature and a spherule form in the host organism.

Many thanks to Dr. John Rippon, Dr. Al Rogers, and Dr. Tex Beneke for providing this month's pictures.

Please do not email me asking me about diagnosing your infections or how to get rid of your yeast or other fungal infections. I am not a medical doctor or even a medical mycologist-- I just play one on the internet.

If you have anything to add, or if you have corrections or comments, please write to me at

This page and other pages are © Copyright 1999 by Thomas J. Volk, University of Wisconsin-La Crosse.

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