New & Noteworthy

You Can Thank Fruit Flies for Those Yummy Beers

October 14, 2014

A wonderful side effect of the close relationship between yeast and fruit flies is great tasting beer. Image from Wikimedia Commons

It is as simple as this, beer tastes good.  And if a new study in Cell Reports by Christiaens and coworkers pans out, you can thank fruit flies for some of those delicious flavors.

No, fruit flies aren’t in your beer. Instead, they have forced the evolution of our favorite beast, Saccharomyces cerevisiae, down a path towards making the aromatic compounds that make beer so darned tasty.

See, yeast can’t get around on their own and so they often rely on insects to move to new pastures. In order to have this happen, they need to attract insects. Plants have worked this out by evolving colorful flowers and sweet nectar. And one way that yeast may do this is by generating aromas that fruit flies find irresistible.

The researchers in this study first stumbled onto this possibility around fifteen years ago. Back then the P.I. was a graduate student who left his yeast flasks out on the bench over the weekend.  Over that same weekend fruit flies escaped from a neighboring Drosophila lab and invaded the yeast lab.

In a “you got peanut butter on my chocolate” moment, the yeast researchers found the fruit flies swarming around one set of flasks and ignoring some of the others. A quick look at the flasks showed that fruit flies were ignoring the yeast strains in which the ATF1 gene was knocked out.

The ATF1 gene encodes the alcohol acetyltransferase responsible for making most of a yeast’s fruity acetate esters.  So it makes perfect sense that fruit flies ignored strains deleted for ATF1 because they didn’t smell as good anymore. To confirm this hypothesis, the authors did a fun, controlled experiment.

In this experiment, the authors set up a chamber where they could use cameras to track fruit fly movement.  One corner of the chamber had the smells from a wild type yeast strain and another corner had smells from that same strain deleted for ATF1.  As you can see in the video here, the fruit flies cluster in the corner with the wild type strains.  Fruit flies definitely prefer yeast that can make flowery sorts of acetate esters.

Christiaens and coworkers took this one step further by actually looking at the effect these chemicals had on Drosophila neurons.  They used a strain of fruit fly containing a marker for neuronal response, so that the researchers could “see” how the flies were reacting to wild type and atf1 mutant yeast smells.  As expected from the previous experiments, the olfactory sensory neurons responded differently to each smell.

To confirm that the esters were responsible for this difference, the authors observed the effect of adding esters back to media in which the atf1 mutant yeast were growing. They found that as more esters were added, the activity pattern of the Drosophila neurons shifted towards that seen with the wild type yeast.

OK, so fruit flies like good smelling yeast.  The next question the researchers asked was whether this had any effect on the dispersal of the yeast – and it definitely did.

To test this, they labeled wild type and atf1 mutant yeast with two different fluorescent markers, so the strains could be distinguished from each other. They then spotted each strain opposite from one another on a specially designed yeast plate and let a fruit fly roam the plate. They then removed the fly and the original spots of the yeast cells.

After letting the plate incubate for 48 hours, so that any yeast cells that had been moved around on the plate could grow up into colonies, they washed the plate to remove the cells that had been dispersed by the fly and used flow cytometry to determine the amount of each strain. They found that wild type yeast were transported about four times more often than the aft1 mutant yeast.

These results show that fruit flies are more likely to disperse yeast if the yeast are producing fruity smells.  Given the close relationship between fruit flies and yeast, and the fact that insect vectors are very important for yeast out in the wild, it is reasonable to think that yeast may smell good in order to attract fruit flies to carry them to new places.

This research also again points to the importance of expanding studies to include more than one organism (see our last blog here). By increasing the diversity of organisms in an experiment, we can learn much more about how things work in the real world. And maybe even learn why yeast evolved to give us such delicious beer.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: beer , Saccharomyces cerevisiae , Drosophila

Finally, Microbes Get the Recognition They Deserve

April 15, 2013

Anyone reading the SGD blog knows that the yeast Saccharomyces cerevisiae is an amazing little organism. Not only does it give us bread, wine and beer, but it also is an invaluable tool in understanding human biology. It has helped us better understand cancer, Alzheimer’s, and lots of other diseases, not to mention basic biological processes like gene regulation and cell cycle control. This little one celled beast is the rock star of biology!

And now, finally, government is starting to take notice. In a 58-0 vote, the Oregon House recently decided that yeast should be the official state microbe. If the Senate and the governor agree, then yeast will be getting the recognition it deserves. Take that, C. elegans, Drosophila, and all of you other model organisms!

beer bubbles

Yeast is getting recognition for beer, but it is so much more!

Unfortunately, this recognition is not for yeast’s scientific value. Craft beer making is huge in Oregon, and designating yeast as the official state microbe is a way of celebrating this important state industry. Given all of yeast’s other important contributions to the well-being of us all, this feels a bit like celebrating Hugh Jackman for his role as Wolverine in X-Men while ignoring his roles on Broadway or his role as Jean Valjean in Les Miserables. Yes, he was great in X-Men, but that is an incomplete picture of him as an actor. Same thing with yeast.

Yeast should be celebrated for wine and bread, for medicines like anti-malarials and antifungals, for the deep biological understanding it has given us, and even for its possible future as a source for biofuels. Still, this honor is way better than nothing, and at least yeast will be the first microbe officially recognized by a state. Well, it will be if Oregon hurries.

Hawaii is voting on an official state microbe too, Flavobacterium akiainvivens. This bacterium was discovered by a high school student during a science fair project and is only found in the state of Hawaii. The Oregon senate should vote soon, or yeast will be the second officially recognized microbe.

Of course, the bill could die in the Senate. This is what happened in Wisconsin back in 2009 when their House passed a bill making Lactococcus lactis the official state microbe. This bacterium is important for making Wisconsin’s famous cheese but it wasn’t important enough for the Senate to approve it as Wisconsin’s official state microbe. Hopefully Oregon won’t make the same mistake with yeast.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: beer , Saccharomyces cerevisiae

I See Dead Yeast (in My Beer Foam)

November 19, 2012

A study by Blasco and coworkers confirms that beer foam is littered with corpses of dead yeast. Or at least with bits of their cell walls.

glass of beer

This one has a lot of Cfg1 protein.

This has been known for awhile. But what these researchers did was to identify one of the key proteins in the cell wall important for maintaining a good head on beer.

The authors knew from previous studies that certain mannose binding proteins play an important role in beer foam. So they used primers that lined up with the 5’ and 3’ ends of one of the known foam-related genes from S. cerevisiae, AWA1, to look for similar genes in the brewing yeast S. pastorianus. This allowed them to PCR out the CFG1 gene.

To show that this protein was involved in the foaminess of beer, they next knocked the gene out of S. pastorianus and used this deletion strain to do some brewing. What they found was that while beer made with this strain had about the same amount of foam, it didn’t last as long. This strongly suggested that CFG1 was involved in maintaining a good head on a mug of beer, earning the gene its name: Carlsbergensis Foaming Gene.

As a final experiment, they added the gene back to a strain of S. cerevisiae, M12B, that makes beer without foam. When this strain expressed CFG1 and was used to brew up some beer, that beer was foamless no more. This suggests that CFG1 may be important for foam formation as well as stabilization.

What is probably happening is that during fermentation, yeast cells are autolysing, releasing their cell wall proteins into the beer. Since Cfg1p is hydrophilic on one end and hydrophobic on the other, it forms very stable bubbles. And foam is simply a bunch of stable bubbles.

Hopefully scientists can use this information to tweak the amount of foam a given beer yields. Then a drinker can choose lots or little foam, long lasting or short lived foam, or any combination he or she wants.

Root beer foams for a different reason

NPR’s take on this study

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: Saccharomyces pastorianus , beer , brewing , Saccharomyces cerevisiae

Better Beer Through Beards

October 16, 2012

Small time craft brewers are always looking for ways to push the envelope of beer taste.  They are trying to find variations in beer’s fundamental ingredients — hops, barley, and yeast — that will make their beer distinctive.  Of these three, the most important is probably yeast (of course, we’re biased here at SGD!).

Think of the tasty beers that could come out of those beards.

Something like 40-70% of beer taste comes from the yeast used to make it alcoholic.  This is why brewers search high and low for new strains of yeast that will give their beer that special something which will make it stand out.  They have looked on Delaware peaches, ancient twigs trapped in amber, Egyptian date palms, and in lots and lots of other places. 

But brewers don’t always have to go far away because sometimes the best yeast is right under their noses.  Literally.

A brewery in Oregon found the yeast they were looking for in one of their master brewers’ beards.  They are now using this yeast to brew a new beer!  This seems uniquely revolting but the beer supposedly is quite tasty.  Perhaps if they don’t advertise the source of their yeast, this beer could become popular.

They aren’t sure where the yeast in his beard came from, but they think it may have come from some dessert he ate in the last 25 years or so (he hasn’t shaved his beard since 1978).  What would be fun is if his beard wasn’t just an incubator, but a breeding ground for new yeast.  Maybe yeast from a dessert from 1982 hooked up with a beer yeast blown into his beard while he was working at the brewery.  The end result is a new improved hybrid yeast! 

Of course we won’t have any real idea about this yeast until we get some sequence data from it.  And all kidding aside, the more yeast that are found that are good for making beer, the better the chances that scientists can home in on what attributes make them beer worthy.  So this beard borne yeast may help many beers in the years to come despite its troubling beginning.

Perhaps brewers also need to start searching through more beards to look for likely beer yeast candidates.  Beard microbiome project anyone?

More information

About beard yeast

What’s in a beer, anyway?

Original lager yeast found in Patagonia

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: beer , Saccharomyces cerevisiae , fermentation

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