Accident of Evolution Allows Fungi to Thrive in Our Bodies


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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  1. Many years ago I contracted Gastroenteritis which recurred yearly about three times before developing into Acute Eneteritis.

    The periods of outbreak were short – perhaps four days – but excruciatingly painful.

    My Doctor issued me with a sample container and wooden spoon to enable me to send samples of my faeces for analysis.

    Anyway, my point in mentioning this unpleasant experience is that my GP told me that this kind of pathogen can eventually effect the central nervous system if carried for too long, a comment which I was reminded of in this article: ‘If a yeast cell finds its way from the lung (stomach) to the brain via a phage or other routes “that’s very bad news,” … “because once it gets into the central nervous system it’s in heaven.”

    I wonder if I had a lucky escape?!

    Although my wife would say that it wouldn’t have made a lot of difference because I’m not quite all there anyway.

    • In reply to #1 by Stafford Gordon:

      I wonder if I had a lucky escape?!

      Well, Cryptococcus isn’t like to be a problem unless you have a T-cell disorder like active HIV or are on steroids. C. gattii shouldn’t be a problem unless you’re sniffing gum trees in Papua New Guinea/North Oz/Vancouver Island. Sneaky bug though – TB does a similar thing, hiding in macrophages.

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