Fungi are eukaryotic organisms (each containing a membrane-bound nucleus) that develop from reproductive bodies called spores. Fungi may be the cause of any number of diseases in humans, animals, and plants; fungal infections are called mycoses (singular, mycosis).
Description
Mycology is the branch of science that studies organisms of the kingdom Fungi. Scientists estimate that over 200,000 species of fungus exist in nature. These species include yeasts, moulds, mildews, mushrooms, lichens, and smuts.
There are a number of characteristics that fungi share: they are eukaryotic (containing a nucleus that is bound by a nuclear membrane); they develop from reproductive bodies called spores; their cell walls are composed mostly of chitin, a nitrogen-containing carbohydrate; and they are heterotrophic (they cannot synthesize their own food and therefore absorb food from an external source through their cell walls).
Most fungi obtain their nutrients from dead organic matter and are called saphrophytes. Saphrophytes play an important ecological role in the decomposition of dead plants, animals, and other organic matter: they release large amounts of carbon dioxide into the atmosphere and recycle nitrogen and other important nutrients for use by plants and other organisms. Other fungi are parasites (obtaining their nutrients from a living host organism in a relationship that usually harms the host) or mutualists (involved in a mutually beneficial relationship with another organism).
Another important characteristic of fungi is that they do not contain chlorophyll. Chlorophyll is a green pigment that enables plants and such other photosynthetic organisms as algae and cyanobacteria to absorb energy from sunlight and use it to synthesize carbohydrates (photosynthesis). Because fungi are not reliant on sunlight as an energy source, they can grow in dark or low-light environments and in directions not normally observed in plants.
Morphology
Most fungi may be classified according to two major growth forms: yeasts or molds. Yeasts are round, unicellular (single-celled) organisms that form a vegetative body called a thallus. The thallus may consist of cells in groups or in branched chains called pseudo-hyphae. Examples of yeasts include Saccharomyces cerevisiae, used in making bread and alcoholic beverages; and Candida albicans, the causative agent of yeast infections.
Molds, on the other hand, are composed of long filaments called hyphae (singular, hypha). Hyphae may be further classified as septate (containing cross walls) or aseptate. A mass of hyphae is called a mycelium.
Whereas yeast cells each contain a single nucleus, cells in septate hyphae may be uninucleate (containing one nucleus), binucleate (containing two nuclei), or multinucleate (containing many nuclei). An example of a mold is Penicillium roqueforti, used to make blue cheese.
Some fungi are dimorphic: they may exist in either yeast or mold form. What form a fungus assumes depends on such environmental factors as the temperature or nutrients present. Some examples of dimorphic fungi include Histoplasma capsulatum and Coccidioides immitis.
Reproduction
All fungi can reproduce asexually by the production of single-celled structures called spores. The number of chromosomes (structures in the nucleus containing genetic material) remains unchanged when cells duplicate their genetic material and then divide. This is not the ideal state for a fungus and is thus called the imperfect state. (It is often observed in the laboratory when fungi that are normally pathogenic to humans are allowed to reproduce.)
Sexual reproduction can also occur in most fungi and is called the perfect state. In this process, one cell divides to become two haploid cells (each containing a single set of unpaired chromosomes). Two cells can then fuse together to become a diploid cell (containing a full set of chromosomes); that cell can then divide.
Role in human health
Some fungi have been found to be directly or indirectly beneficial to humans, while others are pathogenic (disease-causing). Still others are pathogenic to plants and animals important in the food chain.
Food manufacturing
Different yeasts in the genus Saccharomyces are employed by bakers, brewers, and vintners to make their bread, beer, or wine. For instance, S. cerevisiae is commonly used as baker's yeast and in the production of ales. Candida milleri is a yeast used in conjunction with an acid-producing bacteria to yield sourdough bread.
Various species of mushrooms are cultivated specifically for human consumption. These include Agaricus bisporus (accounting for 38% of the world's cultivated mushroom supply), Lentinus edodes (shiitake mushrooms), Volvariella volvacae (the paddy straw mushroom), and the Pleurotus family (oyster mushrooms). Other edible fungi include truffles (fungi of the family Tuber that grow in a special subterranean (mycorrhizal) association with certain trees), morels (of the Morchella family), and the blue-green mold of the Penicillium family that is essential in the production of certain cheeses.
Medicinal and recreational drugs
Discovered in 1929, a metabolite of the fungus Penicillium notatum (later to be called penicillin) became the first antibiotic (a substance produced by a microorganism that can selectively treat an infectious disease). Other fungi that are the source of clinically important antibiotics include those in the family Streptomyces: S. nodosus (amphotericin B), S. erythreus (erythromycin), S. fradiae (neomycin), S. griseus (streptomycin), S. orientalis (vancomycin), and S. rimosus (tetracycline).
Some species of fungi are known as hallucinogens (substances inducing false sensations in the absence of true stimuli) and have been used by many cultures during religious ceremonies (for example, Amanita muscaria). Claviceps purpurea (the ergot fungus) and fungi of the Psilocybe family are also known for their hallucinogenic effects.
Biomedical research
Phycomyces blakesleeanus is a fungus that grows on animal feces in nature. The sporangiophores (the stalks on which spores are produced) of Phycomyces have been shown to respond to a variety of stimuli, including light, gravity, wind, and nearby objects. One important finding was that the light sensitivity of the sporangiophore is about the same as the eyes of humans. Furthermore, like humans, the sporangiophore can adapt to a one-billion-fold change in ambient light intensity.
Biologists have recently shown that Neurospora crassa, also known as red bread mold, can produce spores at approximately 24-hour intervals (known as a circadian rhythm) when in a constant environment. The fungus is therefore being used as a model organism for investigating circadian rhythms, which occur in many different organisms including humans.
Common disease and disorders
Human mycoses can be classified as superficial, cutaneous, subcutaneous, systemic, or opportunistic.
Superficial and cutaneous mycoses
These fungal infections do not invade underlying muscle or bone and are mostly restricted to the outer layers of the skin, nails, and/or hair. Superficial mycoses involve only the outermost layers of skin and result in a change in hair or skin pigment. For example, tinea nigra, caused by Exophiala werneckii, results in black lesions on the skin. Piedraia hortai, the causative agent of black piedra, creates hard dark-colored nodules on scalp hair, eyebrows, and/or eyelashes.
Cutaneous mycoses are generally caused by infection with a dermatophyte (a skin-infecting fungus). Common families of dermatophytes are Epidermophyton, Microsporum, and Trichophyton. Some of the more commonly seen cutaneous infections include:
In the case of subcutaneous fungal infection, muscle, bone, connective tissue, and/or overlying skin may be involved. Subcutaneous mycoses may begin at the site of a laceration or even a seemingly innocuous scratch or puncture wound; fungi are introduced from soil or plant material. These mycoses, however, typically remain localized rather than spread from the site of infection. Sporotrichosis (caused by Sporothrix schenckii) and mycetoma (caused by Madurella grisea, among others) are two noted exceptions; sporotrichosis may spread along the lymphatic system, and mycetoma to deeper muscle and bone.
KEY TERMS
Cyanobacteria—Photosynthetic bacteria, commonly known as blue-green algae.
Dermatophyte—A fungus that can cause a skin infection.
Hypha—Cellular unit of the fungi; typically a branched and tubular filament.
Lichen—A fungus that grows a symbiotic relationship with algae.
Meningitis—Inflammation of the meninges, the membranes surrounding the brain and spinal cord.
Metabolite—A substance produced by way of a metabolic process.
Morphology—The study of the shape and structure of an organism.
Mutualism—Close relationship of two or more organisms, which typically involves exchange of food or other resources.
Mycorrhiza—Subterranean symbiotic relationship between a fungus and a plant root.
Septum—Wall that separates the cells of a fungal hypha into segments.
Systemic mycoses
Systemic fungal infections usually involve more than one type of body tissue. The lungs are often a site of primary infection when airborne spores are inhaled. Often the primary infection is asymptomatic (shows no signs of infection) or resolves quickly. If the fungus spreads to the bloodstream, however, it may disseminate to other organs or systems. The following are the most commonly seen systemic mycoses:
Coccidioidomycosis, caused by Coccidioides immitis; begins as a pulmonary infection (although 60% of infections are asymptomatic) but may disseminate to the central nervous system, bone, and/or skin.
Cryptococcosis, caused by Cryptococcus neoformans; begins as a pulmonary infection but may disseminate to the central nervous system to cause meningitis.
Paracoccidioidomycosis, caused by Paracoccidioides immitis; begins as a pulmonary infection but may disseminate to the mucous membranes, lymph nodes, and/or skin.
Opportunistic mycoses
Opportunistic fungi do not normally cause disease in healthy humans, but can cause infection in individuals who are immunocompromised, such as those with acquired immune deficiency syndrome [AIDS] or those who have undergone organ transplantation. Some important opportunistic mycoses include:
Aspergillosis, a mycosis caused by members of the Aspergillus family. Common mechanisms of infection include hypersensitivity (an allergic reaction); local pulmonary infection; opportunistic infection (leading to pneumonia and the development of a characteristic "fungal ball"); and systemic infection (leading to abscesses in the brain, liver, kidneys, skin, or bone.
Candidiasis. Candida albicans is a yeast that causes oropharyngeal candidiasis, also known as thrush. Thrush is an often-seen opportunistic infection in patients with acquired immune deficiency syndrome (AIDS). Candida albicans is also the cause of the majority of cases of vulvovaginitis (yeast infection).
BOOKS
Fisher, Frances W. Fundamentals of Diagnostic Mycology. Philadelphia: W. B. Saunders Company, 1998.
Spicer, W. John. Clinical Bacteriology, Mycology, and Parasitology. London, UK: Churchill Livingstone, 2000.
