Cryptococcus neoformans
Andrew Schott

Kingdom: Fungi
Phylum: Basidimycota
Subphylum: Basidimycotina
Order: Sporidiales
Family: Sporidiobolaceae
Genus: Filobasidiella (Cryptococcus)
Species: Filobasidiella neoformans
              (Cryptococcus neoformans)

Figure 1: Cryptococcus neoformans in the lung of a patient with AIDS. The inner capsule of the organism stains red here in this photomicrograph.

Figure 2: India Ink preparation

Among the 37 species contained in the genus Cryptococcus, Cryptococcus neoformans is the only species that is pathogenic.  This species, also called by its teleomorph name, Filobasidiella neoformans, is a member of the large class of organisms called "club fungi" or Division Basidiomycota, which is one the five major types of fungi.  C. neoformans can live in and cause disease in both plants and animals.  Cryptococcus neoformans is a yeast-like fungus enclosed by a capsule.  It was first identified in nature from samples of peach juice, but has proven to be found more abundantly in soil contaminated with pigeon droppings, eucalyptus trees, decaying wood from living trees, and in goats with pulmonary disease.  The places this fungus likes to live can be extremely beneficial, as I will tell about near the end.  Cryptococcus neoformans grows normally as unicellular yeast, and even reproduces by budding.  However, it is enclosed by a polysaccharide capsule, unlike yeast.

Cryptococcus neoformans is the causative agent of cryptococcosis.  The most common clinical form of cryptococcosis is meningoencephalitis, and the most common development motivator of cryptococcosis is AIDS.  Meningitis is caused by C. neoformans, especially as a secondary infection for AIDS patients and is normally sub acute or chronic.  Cryptococcosis can also involve the skin, the lungs, eyes, bones, joints, prostate gland, urinary tract, and myocardium.  ES Jacobson, ND Jenkins, and JM Todd studied Superoxide dismutase (SOD) production by C. neoformans to determine whether SOD levels increase at 37oC to account for possible decreases in the production of melanin at this same temperature. (C. neoformans was found to produce sufficient levels of melanin to successfully protect itself from the oxidative compounds produced by macrophages, as discovered by Jacobson and Tinnell.  This is what led Jacobson, Jenkins, and Todd to study SOD production by C. neoformans.) These investigators observed an increase in SOD levels at 37oC, which suggests that SOD may participate in free radical scavenging at this higher temperature in vivo.

Figure 3: Pathway of melanin synthesis by C. neoformans.

Why would a yeast-like organism have a capsule around it?  Certain properties of the capsule that surrounds C. neoformans seems to enable the host to more effectively to rid C. neoformans from the host tissues.  On the other hand, other properties of this capsule protect C. neoformans from host defenses (see figure 1, above).  Complement is fixed by cryptococcal capsules by the alternate pathway, and this process makes chemotactic peptides like C5a. Chemotaxis of leukocytes formed by either of these mechanisms would actually be beneficial to the host.

The picture to the right is a proposed means of infection of Cryptococcus neoformansC. neoformans enters the host through the respiratory system as dehydrated haploid yeast or as basidiospores.  At this stage they are very small, so that they fit into the alveolar spaces inside the lungs.  In the lungs, C. neoformans becomes rehydrated and gets back its thick polysaccharide capsule.  After a while of sitting in the lungs, the organism spreads to extrapulmonary tissues.  C. neoformans has a preference for the brain, so this is the reason that people infected with it get meningoencephalitis.   If untreated, cryptococcal meningoencephalitis is 100% fatal.   If the host does not have enough T-cell­ dependent immune function, the most effective antifungal drugs are no match for cryptococcal infections.

There are two different varieties of Cryptococcus- var. neoformans and var. gattii.  The two varieties are basically identical except for the fact that basidiospores of var. neoformans species are round, while the basidiospores of var. gattii are elliptical.

So why is it beneficial for C. neoformans to live in places such as soil contaminated with bird excrement?  Recent studies suggest that colonies of Cryptococcus neoformans and certain related fungi growing on the ruins of the melted down reactor of the Chernobyl Nuclear Power Plant might be able to use the energy of radiation (mostly beta radiation) for "radiotrophic" growth.  This type of growth is where the organism, in this case C. neoformans, uses the pigment melanin to convert gamma radiation into chemical energy that can be used for growth. This proposed mechanism can be compared to anabolic pathways for the synthesis of reduced organic carbon like carbohydrates in phototrophic organisms, which collect photons from visible light with pigments like chlorophyll, where the energy is used in photolysis of water to generate usable chemical energy in photophosphorylation of photosynthesis as ATP.  It is not known, however, whether or not melanin-containing fungi use a multi-step pathway like photosynthesis, or a chemosynthesis pathway.  More studies will surely be done in the future on this topic, especially as toxic waste continues to pile up (almost) exponentially.


*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.


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