Sudden fungal outbreaks have long been routine among plants, and more recently, animals. A recent outbreak among humans in the Pacific Northwest raises the disturbing prospect that we are not immune either. The mystery of this outbreak’s origins is detailed in “Strange Fungi Now Stalk Healthy People” in the December issue of Scientific American.
The outbreak is ongoing but, in spite of appearances, Cryptococcus gattii doesn't exist to plague us. The fungus prefers to live in soil and on trees, where it subsists quite happily on decaying matter. So how can an organism that seems to enjoy a full and rich life on plants and dirt possibly find itself suited to living inside humans? The answer, it turns out, may be an accident of evolution.
Life in the wild is not all sunshine and rotting roses for C. gattii. “Microorganisms are in a constant fight for territory, for food sources, for their place in that microbial community,” says Karen Bartlett of the University of British Columbia, an expert in the behavior of biological aerosols. Yeasts have many predators, and formidable among them are amoebas. These protists ooze their way through the soil and water of the world, engulfing and digesting tiny prey. To prevent amoebic annihilation, Cryptococcus species have evolved mechanisms to elude their would-be predators, such as a drying- and digestion-resistant coat, UV-protective pigments and the ability to survive being swallowed by predators.
Those same mechanisms allow yeast to evade a type of human immune cell that looks and acts just like an amoeba (similar cells are also found in other animals). We call them macrophages.
Written By: Jennifer Frazer
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