D. FUNGAL PATHOGENECITY
The overall purpose of this Learning Object is to introduce factors that promote fungal pathogenicity by permitting fungi to colonize and harm a host.
LEARNING OBJECTIVES FOR THIS SECTION
Mycology (def) is the study of fungi. Fungi include yeasts, molds, and fleshy fungi. They:
1. are eukaryotic;
2. have a rigid cell wall;
3. are chemoheterotrophs (organisms that require organic compounds for both carbon and energy sources);
4. obtain their nutrients by absorption;
5. obtain nutrients as saprophytes, organisms that live off of decaying matter, or as parasites, organisms that live off of living matter.
We will be concerned mainly with the yeasts and molds, especially those causing mycoses (fungal infections).
In this section we will look at fungal pathogenicity.
As with the bacteria, fungal virulence factors can be divided into two categories: virulence factors that promote fungal colonization of the host; and virulence factors that damage the host.
A. Virulence Factors that Promote Fungal Colonization
Virulence factors that promote fungal colonization of the host include the ability to:
1. contact host cells;
2. adhere to host cells and resist physical removal;
3. invade host cells;
4. compete for nutrients;
5. resist innate immune defenses such as phagocytosis and complement; and
6. evade adaptive immune defenses.
Examples of virulence factors that promote fungal colonization include:
1. A compromized immune system is the primary predisposing factor for serious fungal infections. A person highly immunosuppressed, such as a person taking immunosuppressive drugs to suppress transplant rejection, or a person with advancing HIV infection, or a person with other immunosuppressive disorders, becomes very susceptible to infections by fungi generally considered not very harmful to a healthy person with normal defenses.
2. As with bacteria, the ability to adhere to host cells with cell wall adhesins seems to play a role in fungal virulence.
3. Some fungi produce capsules allowing them to resist phagocytic engulfment, such as the yeast Cryptococcus neoformans and the yeast form of Histoplasma capsulatum.
4. Candida albicans stimulates the production of a cytokine called GM-CSF and this cytokine can suppress the production of complement by monocytes and macrophages. This may decrease the production of the opsonin C3b as well as the complement proteins that enhance chemotaxis of phagocytes.
5. C. albicans also appears to be able to acquire iron from red blood cells.
6. C. albicans produces acid proteases and phospholipases that aid in the penetration and damage of host cell membranes.
7. Some fungi are more resistant to phagocytic destruction, e.g., Candida albicans, Histoplasma capsulatum, and Coccidioides immitis.
8. There is evidence that when the yeast form of Candida enters the blood it activates genes allowing it to switch from its budding form to its hyphal form. In addition, when engulfed by macrophages, it starts producing the tubular germ tubes which penetrate the membrane of the macrophage thus causing its death. For a movie of Candida killing a macrophage from within, see the Theriot Lab Website at Stanford University Medical School. Click on "Candida albicans killing macrophages from inside out."
9.The yeast Cryptococcus neoformans inhibits the productions of the cytokines TNF-alpha and interleukin-12 (IL-12) while stimulating the production of Interleukin-10 (IL-10). TNF and IL-12 activate macrophages while IL-10 suppresses their activation. As a result, macrophages are not activated. Macrophage activation is needed in order to kill the organisma that have been engulfed.
10. Factors such as body temperature, osmotic stress, oxidative stress, and certain human hormones activate a dimorphism-regulating histidine kinase enzyme in dimorphic molds, such as Histoplasma capsulatum, Blastomyces dermatitidis, and Coccidioides immitis, causing them to switch from their avirulent mold form to their virulent yeast form. It also triggers the yeast Candida albicans to switch from its yeast form to its more virulent hyphal form.
B. Virulence Factors that Damage the Host
1. As fungi grow in the body, they can secrete enzymes to digest cells. These include proteases, phospholipases, and elastases. In response to both the fungus and to cell injury, cytokines are released. As seen earlier under Bacterial Pathogenesis, this leads to an inflammatory response and extracellular killing by phagocytes that causes further destruction of host tissues.
2. Many molds secrete mycotoxins (def), especially when growing on grains, nuts and beans. These toxins may cause a variety of effects in humans and animals if ingested including loss of muscle coordination, weight loss, and tremors. Some mycotoxins are mutagenic and carcinogenic.
Aflatoxins, produced by certain Aspergillus species, are especially carcinogenic. A mold called Stachybotrys chartarum is a mycotoxin producer that has been implicated as a potential serious problem in homes and buildings as one of the causes of "sick building syndrome." Mycotoxin symptoms in humans include dermatitis, inflammation of mucous membranes, , cough, fever, headache, and fatigue.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
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Updated: Oct., 2007
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