|MicrobiologyBytes: Infection & Immunity: Pathogenic fungi||Updated: April 8, 2009||Search|
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General CharacteristicsFungi include moulds, yeasts and higher fungi. All fungi are eukaryotic and have sterols but not peptidoglycan in their cell membrane. They are chemoheterotrophs (requiring organic nutrition) and most are aerobic. Many fungi are also saprophytes (living off dead organic matter) in soil and water and acquire their food by absorption. Characteristically they also produce sexual and asexual spores. There are over 100,000 species recognised, with 100 infectious agents of man. Moulds are composed of numerous, microscopic, branching hyphae known collectively as a mycelium.
Growth occurs from the apical tip, the apical vesicles contain materials and enzymes for the formation of new hyphal wall. Older hyphae are less biochemically active and contain many storage vacuoles. In most moulds these hyphae have septa (crosswalled divisions), but in some there are none and the hyphae are coenocytic (aseptate). Septa, (plural of septum) are cross-wall formations which divide fungal hyphae into cells. They may add strength to the hyphae or serve to isolate adjacent parts to allow differentiation, such as during production of the reproductive structures.
The structure of the septa is of taxonomic importance, and may be simple or complex, septate or aseptate (coenocytic). Growth of a septum starts at the hyphal wall and proceeds towards the centre of the hyphae, septa may consist of a solid plate, have a single pore at the centre, or have multiple pores creating a sieve-like appearance.
Click here to see a still image of a septum in the septate fungus Sordaria fimicola.
Part of the mycelium is involved in gaining nutrients, the vegetative mycelium, and part in growth and reproduction, the aerial or reproductive mycelium.
From the reproductive mycelium, spores are formed. Asexual spores are produced by the aerial mycelium of a single organism, whereas sexual spores are formed by the fusion of cells and nuclei from opposite mating strains.
There are many types of spore with names such as chlamydospore and sporangiospore. Examples of moulds include Penicillium, the source of penicillin that was the first antibiotic.
Yeasts are unicellular organisms, normally ovoid or spherical in shape. Typically, they replicate by budding rather than binary fission. Here, the cytoplasm and dividing nucleus from the parent is initially a continuum with the bud, or daughter yeast, before a new cell wall is deposited to separate the two. In some cells, these buds fail to detach and may form a short chain of cells called a pseudohypha. Although yeasts are single cells and produce smooth bacterial-like colonies on laboratory agar media, unlike bacteria they have a distinct nucleus and are thus eukaryotes. An example of a yeast is Saccharamyces cerevisiae that produces alcohol under anaerobic fermentation.
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Human fungal infections in the United Kingdom are uncommon in normally healthy persons, being confined to conditions such as candidiasis (thrush) and dermatophyte skin infections such as athlete's foot. However, in the immunocompromised host, a variety of normally mild or nonpathogenic fungi can cause potentially fatal infections. Furthermore, the relative ease with which people can now visit "exotic" countries provides the means for unusual fungal infections to be imported into this country.
Candida albicans is a yeast causing candidiasis or "thrush" in humans. As a superficial mycoses, candidiasis typically infects the mouth or vagina. C. albicans is part of the normal flora of the vagina and gastrointestinal tract and is termed a "commensal". However, during times of ill health or impaired immunity the balance can alter and the organism multiplies to cause disease. Antibiotic treatment can also alter the normal bacterial flora allowing C. albicans to flourish.
Histoplasmosis. This is caused by Histoplasma capsulatum. The organism is dimorphic (being a mould that can convert to a yeast form). H. capsulatum is endemic in many parts of the world including North and South America. It is found in the soil and growth is enhanced by the presence of bird and bat excreta. Environments containing such material are often implicated as sources of human infection. The lungs are the main site of infection but dissemination to the liver, heart and central nervous system can occur. Pulmonary infection can resemble symptoms seen in tuberculosis.
Aspergillosis. This is the name given to a number of different diseases caused by the mould Aspergillus. It produces large numbers of spores and occurs world-wide. In the United Kingdom, A. fumigatus is the most common species causing disease. The organism can infect the lungs, inner ear, sinuses and, rarely, the eye of previously healthy persons. In the immunosuppressed host, Aspergillus can disseminate throughout the body.
Candidosis. In severely immunocompromised patients (e.g. those receiving chemotherapy) C. albicans, that is part of the normal human flora (see above), can proliferate and disseminate throughout the body.
Cryptococcosis. This is a systemic infection caused by the yeast Cryptococcus neoformans. The commonest manifestation is a subacute or chronic form of meningitis resulting from the inhalation of the organism. Pulmonary infection can also occur. The disease affects both healthy and immunosuppressed individuals and occurs world-wide. C. neoformans can be isolated in large numbers from pigeon droppings in the environment, although such birds do not appear to harbour the yeast.
Certain fungi, such as mushrooms, can produce poisonous toxins that may prove fatal if ingested (e.g. Amanita phalloides: "death cap"). Others (Psilocybe) affect the central nervous system inducing hallucinogenic responses.
Many moulds produce secondary metabolites (mycotoxins) that are highly toxic to humans. Ergotism is caused by eating bread prepared from rye infected with the fungus Claviceps purpurea. Historically, several large scale outbreaks of madness in local populations have been attributed to ergotism.
Pneumocystis. This is an infection of the lung caused by Pneumocystis jirovecii (formerly Pneumocystis carinii). The organism is a common cause of fatal pneumonia in AIDS patients. An intracellular parasite, with a life cycle of trophozoite and cyst, it was formerly considered to be a protozoan. The cysts contain 8 nuclei which can be seen in smears of pulmonary aspirates.
© AJC 2007.