Killer Yeast from South America(?)

by Jennifer Frazer on May 23, 2010

The thick yeasty capsids (coats) of Cryptococcus gattii relative C. neoformans. C. gattii also has a thick capsid. CDC/Dr. Leanor Haley

It’s not every day you read about suggestions for potential health-related travel advisories to U.S. states. But such is the case for the Pacific Northwest, where an emerging fungal infection has — very unusually for fungi — begun felling otherwise healthy people. A growing number of cases of the potentially fatal disease cryptococcosis have been diagnosed in people who merely traveled to Washington, Oregon, or British Columbia.

There are several very scary things about this fungus, but foremost among them is that it a) was introduced somehow from the tropics, most likely South America/Brazil or also possibly Africa and is somehow not finding it a problem to survive in Canada; b) is actively spreading up and down the Pacific Northwest coast; c)hangs out in the environment, where it can get along apparently fine without us (and seems to particularly like the bark of and soil around trees like Eucalyptus); d) is acquired from the environment, that is, you don’t have to have contacted with an infected person or animal; e) doesn’t manifest itself symptomatically for 2-12 months after one picks it up and e) can kill otherwise healthy people.

So let’s review. This means that you can just be hiking along in the woods, minding your own business, and a few months later you’re dead. Now I’m not saying this is *common*. Only 200 people have been seriously affected in 10 years. And let’s face it, there are a lot of other things that can kill you while hiking along minding your own business, much, much quicker and more commonly than these fungi: bears, mountain lions, lightning, exposure, rockfalls, your own stupidity, deranged axe murderers, errant militias, etc. But the very fact that, like anthrax, you can pick it up for no apparent reason from the environment, and much more easily, is disturbing. (Histoplasmosis is also picked up by inhalation but it is well known you must spend time around bird or bat droppings. Aside: can you think of any other diseases that are picked up just by inhalation from the wild?)

This particular organism is Cryptococcus gattii, a relative of the Cryptoccoccus neoformans that gained some fame in infecting AIDS patients when the immune disease emerged in the ’80s. Cryptococcosis begins as a lung infection, but can progress to meningitis. C. gattii, a cousin of C. neoformans, first emerged on Vancouver Island in 1999, but in the last few years, two new strains have emerged that seem to be killing otherwise healthy people in numbers much higher than earlier strains.

There are anti-fungal drugs that can help show C. gattii the door, but the wikipedia article for C. gattii contains this ominous sentence:

Antifungals alone are often insufficient to cure C. gattii infections, and surgery to resect infected lung (lobectomy) or brain is often required.

Resectioning *brain*??! Oh hell no.

Fortunately, infection with this organism remains relatively rare, but that’s no guarantee of safety. Why? This fungus is having sex. And that is one possible origin of the two newly virulent strains that have emerged in the Pacific Northwest — and possibly the source of future, more dangerous strains. It is also possible that the two new strains were newly introduced from their tropical diversity centers. But the scientists *know* there’s a lot of C. gattii hanky-panky going on. They looked at a couple dozen genetic markers in this recent PLoS study, and compared which isolates had which markers and how they were organized in the genome. They also tried to reconstruct the evolutionary and geographic relationships between various North American isolates using these data. They found huge genetic diversity among their isolates, and that at least one north American strain came from Australia. They also that the fungus is bumping up its numbers by quick and easy asexual reproduction.

But the initial creation of the new hyper-virulent types, they concluded, was likely due to sex. Only sexual reproduction provides the opportunity for lots of large-scale genome reorganization; that’s one of sex’s main advantages. When two entire genomes get together, a process called “crossing over” takes place that allows all the various chromosomes to swap corresponding bits. The result is sometimes good, sometimes bad, but whatever the outcome, generation of diversity and evolution often happen a heck of a lot faster per generation with sex than without.

Lest you think that’s the only worrying bit, it turns out that this fungus has a rather kinky sex life. Not only is it having sex with all the other mating types (fungi often have many “genders”, which for convenience scientists refer to as “mating types”), it can also, somewhat unusually for fungi, have sex (and I mean produce viable offspring) with its own type, a process called unisexual mating. Sheesh.

So that’s what was chiefly covered in the news reports. But it turns out there’s more to this story. Because this fungus is actually a yeast — an infectious yeast. Now I know what you may be thinking: yeast are the source of everything good and worthy on Earth: fresh, crusty, heavenly leavened bread, wine, beer, soy sauce, etc. And you’re right. But that doesn’t mean there aren’t evil yeast. Nasty yeast. Parasitic yeast. Because “yeast” is a lifestyle, not a taxonomic group or single species.

The yeast you’re probably most familiar with, Saccharomyces cerevisiae, commonly called baker’s yeast, is but one fungus that has adopted this way of life. And that way of life is: forsaking multicellularity for swingin’ single cellhood. In a way, yeast have devolved (although that’s a bit misleading; it doesn’t mean they’re less adapted or successful than they were before; just that being unicellular is more like their long-ago ancestors). The standard fungal body plan today is a multicellular filament, or tube. The tube branches, and the collection of branched tubes makes up the fungal body, or cottony “mycelium” we are familiar with from the surface of refrigerator contents gone rogue. Yeast are fungi that have given up this form, and have gone back to single celled living, a bit like a protist. It would be as if all your spleen cells decided that they’d rather just go off and live on their own in pond water — they’re sick and tired of cleaning up your bloodstream, thank you very much! — and they want out of the cooperative.

But it gets more complicated than that. Some fungi can go back and forth between their normal, filamentous lifestyle and the yeast lifestyle. Histoplasmosis, mentioned earlier, can do this. In the wild, it is a brownish mycelium that lives in bird droppings and bat guano. Once inhaled and installed in your lungs, it becomes a yeast. Cryptococcus neoformans and Cryptococcus gattii are among this group too. Sometimes they are filamentous. Sometimes not.

And here’s where it gets really weird. Because yeast is a lifestyle, not a taxonomic group (like a genus or family), yeast come from all over the fungal family tree. Baker’s yeast is an ascomycete like morels and truffles. But C. gattii is . . . a yeast evolved from the jelly fungi! Jelly fungi are super-awesome amazing organisms in the basidiomyctes (a major division in the fungi, like ascomycetes, that make their sexual cells on special pronged cells called basidia) that usually decay wood and produce fantastically fun-to-touch-and-play-with fruiting bodies that are gelatinous and sometimes quite colorful.For reference, here is what one looks like:

Ascocoryne sarcoides, Photo by Daryl Thomas, Mushroom Observer. Creative Commons Attribution-Unported 3.0 license

The jelly fungus you might be familiar with, if you are familiar with one at all, is the wood ear mushroom*, Auricularia auricula, commonly used in Asian cuisines — in particular, in hot and sour soup. Again, these fungi are perfectly innocuous. Some fungi produce very different forms when they live asexually or sexually. Because they’re so different, mycologists often give the sexual form a different name from the asexual form (an explanation of whether this is a good idea and why mycologists continue the practice could take up a book). The sexual jelly fungus form of C. gattii is Filobasidiella bacillispora, although it’s not necessarily the case that the jelly form would produce a structure big enough for you to see with your naked eye. I know of no jelly fungi that can hurt you either by touch or consumption. But the yeast form of these particular fungi apparently can. Weird!

A larger question might be: what are these fungi doing in us? Are they only accidental parasites? What are they doing out in the wild when they aren’t infecting us? As they say . . . further studies are needed.

But in the meantime, the weirdness parade goes on. C. gattii has *also* been isolated from a regular menagerie of other mammals, including koalas, llamas and. . . are you ready for this? . . . dolphins. How dolphins managed to spend time frolicking in the woods (or how the fungus managed to spend time frolicking with dolphins) is a subject for another blogger.


*According to the Wikipedia article, the Japanese name for these jelly fungi is “tree jellyfish”(!) Sometimes literal foreign phrases smack you upside the head with a perfectly obvious insight you never thought of before.
Byrnes, E., Li, W., Lewit, Y., Ma, H., Voelz, K., Ren, P., Carter, D., Chaturvedi, V., Bildfell, R., May, R., & Heitman, J. (2010). Emergence and Pathogenicity of Highly Virulent Cryptococcus gattii Genotypes in the Northwest United States PLoS Pathogens, 6 (4) DOI: 10.1371/journal.ppat.1000850

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May 24, 2010 at 2:42 am

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cgauthier May 24, 2010 at 12:21 am

This is terrifying. As you say, infection is still rare, but, I’ve read elsewhere that a general increase in fungal fecundity is expected in wetter areas as the globe warms. The past few years, here, in Oklahoma, have been wetter and warmer and, sure enough, I see way more mushrooms and molds than I used to and in stranger and stranger places. This isn’t a big problem yet, but I have (hopefully) another half-century or so in this life. I would just as soon die tomorrow in a car wreck than live to see civilization choked to death by jelly mold.

Jennifer Frazer May 24, 2010 at 8:28 am

Although it is a bit alarming, I wouldn’t get too worried about a jelly fungus taking over the planet . . . yet. : ) Numerically, cancer, heart disease, influenza, AIDS, syphillis, etc. are still far bigger killers. But it is creepy knowing that anyone who spends time in the woods in the Pacific Northwest is potentially susceptible (although the odds still seem pretty low — only 200 people have been seriously affected and 24 killed in 10 years), and there’s absolutely no known way to prevent infection.

And certainly, if parts of the country continue to get warmer and wetter, molds and fungi are going to have it easier, and not just in our homes and in the forest — on our crops and livestock too. Anyhow, if I had to pick an invertebrate that I thought was actually going to take over the world, it’d be jellyfish and comb jellies. Together, they seem to be taking over the vast ocean voids we’ve left behind by removing all the fish. Oh, and slime molds. Slime molds have long been plotting to take over (see Spinal Tap Director’s Cut and upper right corner of this page).

kati May 24, 2010 at 10:30 pm

yes, slime molds! :)

as a native pacnw-er, i’d guess we’ve always had more fungi, etc, than OK and we are still surviving! except for those unlucky 200 or so but still… ;)

Warren May 25, 2010 at 9:28 am

Wow. This was super informative.

But the tree-jellyfish thing looks like a “tree jelly-bean”. Or rather, a platter of jelly beans that was left out in the sun for too long.

Yummy information. Thanks.


Jennifer Frazer May 25, 2010 at 9:34 am

Well, some of them look more like jellyfish than others. : ) I think it’s more the texture than anything — jelly fungi are wonderful to pet, and they have this fantastic slippery rubbery feel that is very similar (I imagine) to jellyfish texture. But if you check out this photo of a wood ear, you might see a passing resemblance to a tree jellyfish, sans tentacles.

Jo May 25, 2010 at 4:53 pm

Wow! Thanks for such an informative post about such an unusual topic. I have to go off now and process this information. The world seems like a different place all of the sudden, infectious yeast, tree jellyfish, sexual types, fungus turning into yeast, and brain resectioning! Yikes! That dolphins and hikers in the Pacific Northwest are both falling victim to this is crazy and disturbing in a way that brings home how little we know about the world around us. Do you know what kind of dolphins they are or if it is more than one kind, or where the infected dolphins lived?

Jennifer Frazer May 25, 2010 at 8:15 pm

The study said “a male Atlantic Bottlenose dolphin in San Diego”, and linked to this paper.This Washington Post article talks about the previous, less virulent form of the disease, and has lots of great details. It says in 2007 over two dozen local porpoises had died around Vancouver Island and suggests they got it by breathing in spores carried over the water. Actually, if you’ve made it this far, I highly recommend reading the WaPo article — quite revealing regarding the human face of the disease and the climate change angles.

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