Coccidioides immitis
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Coccidioides immitis
Coccidioides immitis is a dimorphic fungus that exists in two distinct forms, saprophytic and parasitic. In the soil, the organism exists as a mold with septated hyphae resembling a shape of a barrel. When entering a host, the arthroconidia (spores) break off from the hyphae and evolve into round structures called spherules. Then when inside the host, spherules grow and undergo internal division, forming smaller structures called endospores. Large spherules can rupture to release packets of endospores, resulting in new foci of infection within the host. The main habitat ofCoccidioides immitis is in soil.
C. immitis colonies grow rapidly. At 25 or 37°C and on Sabouraud dextrose agar, the colonies are moist, glabrous, membranous, and grayish initially, later producing white and cottony aerial mycelium. With age, colonies become tan to brown in color.
Formal description of Coccidioides immitis was performed by Rixford and Gilchrist from a case observed in California in 1986. However, the parasite was then still thought to be a protozoan. The correct taxonomic status of C. immitis as an ascomycete fungus was demonstrated by Ophuls and Moffit in 1900 by culture on artificial of the fungal mycelia. It is important to have its genome sequenced because Coccidioides immitis, along with its relative Coccidioides posadasii, can cause a disease called Coccidioidomycosis (Valley Fever), and it is a rare cause of meningitis, mostly in immunocompromised persons, and the disease can be fatal. Persons afflicted with HIV/AIDS are highly susceptible to Coccidioidomycosis and suffer a high mortality rate from the disease.
| Kingdom | Fungi |
|---|---|
| Phylum | Ascomycota |
| Class | Euascomycetes |
| Order | Onygenales |
| Family | Alcaligenaceae |
| Genus | Coccidioides |
| Species | C. immitis |
| Binomial | Coccidioides immitis |
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[edit] Surface Characteristics
Like most pathogenic fungi, the cell wall of Coccidioides immitis is rich in chitin and chitin metabolism is a reasonable target for the design of antifungal agents. The antigenic composition of an alkali-soluble, water-soluble cell wall extract of Coccidioides immitis is heterogeneous in composition, containing two distinct antigenic components. One is present as a polymer that is antigenically identical to a polymeric antigen in coccidioidin, designated antigen 2. The other component presented an unusual precipitin pattern in that a cathodal leg was demonstrable in the absence of an anodal leg. The fungal cell wall also contains mannoproteins and glucans. Any disruption in its integrity should affect growth. The cell wall provides a unique therapeutic opportunity for antifungal agents by targeting a structure not found in mammalian cells. The echinocandins are cyclic hexapeptides, members of a new class of antifungal agents. They appear to inhibit the synthesis of 1, 3-β-d-glucan, a major cell wall component which provides structural integrity and osmotic stability in most pathogenic fungi.
Metabolism: Of various carbohydrates and amino acids tested, glucose, mannose, fructose, and glutamate were the most efficient substrates metabolized by the endospores and spherules of Coccidioides immitis. Strain M-11 of Coccidioides immitis was found to utilize mannitol for growth in the mycelial form but not in the spherule form. Cell-free extracts of both forms, grown on glucose, were capable of reducing nicotinamide adenine dinucleotide with mannitol-1-PO but not with mannitol. The extracts accomplished a rapid oxidation of reduced nicotinamide adenine dinucleotide by fructose-6-PO, the expected product of mannitol-1-PO oxidation.
[edit] Pathogenic Activity
The distribution of Coccidioides immitis has been shown to be restricted to America through ecological and epidemiological studies. Coccidioides immitis has been known for its saprophytic nature, tolerance for high soil salinity in climates of high temperature and low rainfall. Epidemiological studies and case reports have extended the apparent range of this organism; however, almost all soil isolates in Arizona and California have been from areas with the physical and biotic parameters of this zone. The repeated isolation of C. immitis from the soil has been from small, well defined areas. C. immitis has been observed to grow in the soil if naturally infected soil is simply enriched with sterile water. The ease with which it can be grown is paradoxical to its naturally restricted distribution to soils in endemic areas (15). Animals, although not the major natural reservoir, cannot be ignored as possible factors in the ecology of C. immitis. Currently, Coccidioides immitis is an endemic in the United States.
[edit] Virulence
Coccidioides immitis Coccidioides immitis possess branching septate hyphae that are 2 to 4m in diameter and allow them to develop infectious arthroconidia along its length. Such structure is the most major virulent factor for Coccidioides immitis. As the arthroconidia mature, they become spores that are easily airborne and can be carried long distances by the wind. It takes only a few inhaled arthroconidia to produce infection in the host. When inhaled, the arthroconidia is small enough (< 10m) to escape upper respiratory defenses to lodge in the lungs where it transforms into a multinucleated spherule rather than a yeast. Using internal division, the spherule produces hundreds of uninucleated endospores. The spherule ruptures on maturity releasing the endospores, which in turn matures to from new spherules.
The initial host immune response is a transient polymorphonuclear leukocyte response directed at arthroconidia. As spherules develop, the inflammatory response becomes mononuclear and persists throughout the course of the infection. The primary lung infection is asymptomatic in 60% of individuals. The other 40% develop a mild to moderate influenza-like syndrome 1 to 3 weeks after exposure that includes cough, fever, night sweats, pleuritic chest pains, arthralgias, myalgias,
