Yeast STD's (S. cerevisiae L-A Virus

Sorry, so I'm a bit retarded and had to revise this post since I posted it before I was finished writing

Anyway, I was doing some reading on dsRNA viruses and one of the papers I was reading cited this article: Double-stranded RNA viruses of Saccharomyces cerevisiae. Seeing how S. cerevisiae and I have grown to be very good friends lately, I checked it out and was pretty amazed. Yeasts get STDs. Unlike human STD's, these only seem to make you sick if you're not infected.

To summarize the relevant bits of that article... the majority of S. cerevisiae strains harbor dsRNA viruses that are transmitted during mating and asexual reproduction. There is no pathogenic effect on the host cell, but some of these viruses encode toxins in their genomes. The toxins kill any S. cerevisiae cell that is not infected with the specific strain of L-A virus that encodes the toxin.

My question is... does anyone know if common brewers' yeasts harbor toxin-producing viruses? I can see why companies like wyeast might engineer or select strains that lacked toxin-encoding dsRNA viruses since people might be pitching blends of S. cerevisiae strains, but who knows... And how do people who brew with wild yeasts (S. cerevisiae, not Brett) get by if they pitch strain X and then add strain Y to the secondary/bottling bucket?

Just thinking out loud here, after a couple of brewskis (you lost me after the first couples of phrases...) but could those strains be called killer strains, such as in wine yeasts (Lalvin EC-1118 and K1V-1116 are good examples)?

mrfocus said:

Just thinking out loud here, after a couple of brewskis (you lost me after the first couples of phrases...) but could those strains be called killer strains, such as in wine yeasts (Lalvin EC-1118 and K1V-1116 are good examples)?

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I think you're right. I googled killer yeast strains and there was a wikipedia article on them. Should have checked the googles first, I guess. It doesn't really answer my original question explicitly, but provides a bit of perspective. And it makes it sound like most brewing strains do not harbor viruses that encode toxins. But who knows. Those wine strains are either toxin producers or they just have a reputation for producing killer brews. My money is on the former, though.

Disclaimer... I'm not much of a yeast guy. I spend my time with mammalian cells and some prokaryotes. But this sort of host-virus interaction strikes me as really cool.

This would be an interesting question to pose to Chris White (White Labs) -- cwhite at whitelabs dot com

I'll shoot him an email. My curiosity has been piqued.

When considering the prevalence of latent viruses within the human genome (somewhere on the order or 5-10% of our genome) and how little we actually understand about the non-coding regions of DNA my guess is that it is an absolute certainty that all brewers yeast contains integrated viral genomes. At the same time this is true for all genomes of every living cell, from bacteria to yeast to humans, as all life owes its combinations of genes to virus mediated genetic transfer.

As for brewers yeast harboring the toxin producing strains it is entirely possible. I would think that it would be difficult to detect such viruses for a pair of reasons. 1) The virus employs a toxin-antidote strategy, similar to many bacterial plasmids. The nature of this system ensures that the yeast will always be healthy due to the antidote being produced to counter the toxin. It is in the best interests of the virus to keep its host alive, and therefor itself. 2) Because the yeast are healthy there would be no need for the lab to do the genetic research looking for that viral genes. Although Wyeast and White Labs are no doubt large modern facilities, I am quite certain they do little more then aseptically maintain and culture new yeasts strains from existing breweries, and may not have the capacitiy to do such intensive genetic research on all their strains.

But I could be, and likely am, entirely wrong. This is an interesting review article. I have also recently discovered that pubmed can be used to find primary research articles dealing with yeast and beer fermentation. Haha, so although my supervisor thinks that I am working hard in the lab, pouring over journal articles, I am actually reading about yeast stress during lagering or wort aeration.

jim_reaper1066 said:

As for brewers yeast harboring the toxin producing strains it is entirely possible. I would think that it would be difficult to detect such viruses for a pair of reasons. 1) The virus employs a toxin-antidote strategy, similar to many bacterial plasmids. The nature of this system ensures that the yeast will always be healthy due to the antidote being produced to counter the toxin. It is in the best interests of the virus to keep its host alive, and therefor itself. 2) Because the yeast are healthy there would be no need for the lab to do the genetic research looking for that viral genes.

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I could definitely see the commercial yeast producers just maintaining and expanding cultures... but then again I have this image of an eccentric beer-guzzling microbiologist tinkering with the strains to optimize flavor profiles, shelf life, growth parameters, etc...

I just read a page about culturing yeast from bottles (I want some Ommegang yeast!), and it sounded like some commercial brewers use different yeasts at different stages of brewing. Ommegang doesn't, but others apparently do. Anyway, if the strain used in the primary is a toxin producer, the one you add to the secondary would have to produce the same sort of toxin or it would die and not do anything.

You could detect it pretty easy if you had access to reference strains producing all of the different types of toxin. Grow up your reference strains, spin 'em down, take the supernatant (with the toxin) and add some of it to a culture of the yeast to be tested. If it fails to kill your yeast, you have an idea what toxin gene your yeast is expressing. Then switch it around and see what the supernatant of your test yeast kills. Whether or not a people would bother with this is beyond me, though.

jim_reaper1066 said:

I have also recently discovered that pubmed can be used to find primary research articles dealing with yeast and beer fermentation. Haha, so although my supervisor thinks that I am working hard in the lab, pouring over journal articles, I am actually reading about yeast stress during lagering or wort aeration.

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What?!?!?! Those things exist? Sweet. Aw hell. There goes my productivity this summer...

My guess is that the would-be healthy yeast around the infected yeast will die off leading to low cell counts in that culture(laboratory wise). Its likely that culture wouldn't be passaged onto another round of testing. Is scientists would likely select against these strains by just picking healthy cultures early on so that they could develop into their lab strains they then sell. Once you pick your initial cells to culture they can perpetually produce more healthy, naive yeast. This is also because there is no likely issue with contamination from outside sources where the virus may be brought into the mix.

Secondly I don't there are cures for these viruses other than a possible defense mechanism the yeast may have to it. Viruses are difficult to target due to their obligate intercellular life style. We have very few anti-viral medications available for human use because of the expenses and research required to produce a working product.

There is always the chance the yeast could dilute the virus out over time if the virus does not confer any selective advantage to the yeast by poisoning other nearby cells. Then the virus would likely be diluted out of the cell over time. This would mean the virus would constantly be selected for by either producing toxins to let the hosts cell get a leg up on competition to stay or not producing toxins so that the cell replicates slower over time(because it has to take care of the viral genome as well as its own) and it would be out-paced by healthy cells that would snuff out the infection.

There are tons of variables to consider on this topic, but all in all I don't think you have to be worried all that much when using your laboratory-grade yeast cells.

Lastly I think the thing you should be most scared of are the two prion diseases that the papers mentions yeast being able to harbor. Prion diseases can affect cross-species and is COMPLETELY without treatment. Oh wait you can gamma radiate your body to stop it... but then you're dead. See the diseases of Kuru and Bovine Spongiform Epithelium(Mad cow disease).


PS: Google scholar is a great alternative to searching papers.

On a related tangent, a blood test looking for anti-saccharomyces cerevisiae antibodies is used to confirm Chrohn's Disease...

onemanlan said:

I don't there are cures for these viruses other than a possible defense mechanism the yeast may have to it.

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Oh, yeah... these viruses are here to stay. And they evade innate antiviral defenses by replicating their genomes and transcribing mRNA inside of the nucleocapsid. They never actually "uncoat". It's pretty phenomenal, really.

onemanlan said:

There is always the chance the yeast could dilute the virus out over time if the virus does not confer any selective advantage to the yeast by poisoning other nearby cells. Then the virus would likely be diluted out of the cell over time. This would mean the virus would constantly be selected for by either producing toxins to let the hosts cell get a leg up on competition to stay or not producing toxins so that the cell replicates slower over time(because it has to take care of the viral genome as well as its own) and it would be out-paced by healthy cells that would snuff out the infection.

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I'm a little confused by what you mean about "dilution". Dilution of the toxin in the environment probably has something to do with the reason that there is so much diversity among the toxin-encoding L-A virus satellites, but since the virus is replicating intracellularly it is almost certainly going to be passed on to daughter cells. Unless it wasn't actively replicating (even at a slow rate), I don't see how it could be diluted in the cytosol.

As far as the fitness impact of these things goes, producing a toxin could raise fitness by eliminating eliminate competitors like you said, but from the sounds of the article there is no decline in growth rate associated with harboring the virus.

onemanlan said:

There are tons of variables to consider on this topic, but all in all I don't think you have to be worried all that much when using your laboratory-grade yeast cells.

Lastly I think the thing you should be most scared of are the two prion diseases that the papers mentions yeast being able to harbor.

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I'm not worried about using the yeast outside of having the strains "fighting" in the fermenter if I end up pitching multiple strains for whatever reason. I'm primarily just curious. Yeast prions are interesting. Every now and then, you hear about a new fungal prion or prion-like protein being discovered. Probably not a cause for concern, though, since any homologous proteins in humans are probably different enough that the yeast prion couldn't serve as a seed for conversion/aggregation. And people have been drinking beer for thousands of years with no ill effects beyond being more likely to hook up with ugly people

Hoppocrates said:

On a related tangent, a blood test looking for anti-saccharomyces cerevisiae antibodies is used to confirm Chrohn's Disease...

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Wow... all I know about Chrohn's disease is that the immunomodulatory properties of certain worms can reduce the severity of symptoms for some people.