PLoS Biology: Starving yeast poisons other organisms

Yeast is not a simple single-celled microbe as we once thought, but a competitive killer
.
When glucose is lacking, yeast releases a toxin that poisons other microbes that enter its surrounding habitat, or even its own clones
.
This toxic phenomenon, previously unknown, helps us understand the behavior of single-celled microbes, the evolution of single-celled to multicellular organisms, and potentially useful applications for
the food industry.

Bread baking has become a popular new hobby during the pandemic, so now you may find small packets of dry yeast
hidden in many kitchen cupboards.
This tiny live fungus has been a staple of our diets for thousands of years, allowing us to enjoy fluffy bread, sweet wine, and sparkling beer
.
Until recently, yeast was thought to be a simple single-celled microbe, but researchers at the University of Tokyo have now discovered that it has a deadly survival strategy
.

Tetsuhiro Hatakeyama, assistant professor at the Graduate School of Arts and Sciences, explains, "In the critical survival scenario of glucose starvation, yeast releases toxins into their habitat, killing other microorganisms while the yeast itself acquires
resistance.
" "We call this phenomenon a latecomer killing
.
We were even more surprised to learn that yeast produces toxins that can also kill clones they don't adapt to, so they run the risk of killing not only invading microbes, but also their own offspring
.
This seemingly dangerous, near-suicidal behavior has never been found in single-celled organisms before, or even thought to exist
.
”

Although cooperative behavior forms are well known in many bacteria and fungi, this study is the first time competition
has been found in clonal cells of single-celled organisms.
This has important implications
for understanding the ecology of microbes and why some specific microbes grow during fermentation while others do not.
To achieve this discovery, the team cultured clonal cells (i.
e.
, from the same parental cell)
under glucose-restricted and glucose-rich conditions, respectively.
When these cells bonded, their growth patterns suggested that yeast cells that had adapted to glucose starvation were able to poison latecomers and reserve food resources
for themselves.

"Our study reveals a surprisingly selfish side of yeast behavior," Hatakeyama said
.
"The phenomenon we found is similar to a thought experiment proposed by the ancient Greek philosopher Carneades of Cyrene, known as Carnedis's plank: If a sailor grabbed a plank that could only support one person, escaped from a shipwreck, and pushed away another sailor close behind, would he be charged with murder?" The researchers believe that this strategy may help yeast avoid mass starvation in populations, while also helping to select toxin-producing offspring that are more likely to continue their lineage
.
The strategy has been observed in several different types of yeast – originally extracted from beer, bread and wine – which may mean that the phenomenon may be occurring
more widely in this different species.

This discovery could be used to develop useful growth control mechanisms for economically important yeast species, such as those
used in the food industry.
Although not included in the study, it may also pave the way
for better control of yeast species that have negative effects on human and animal health.
The team's next step will be to explore the implications
of this discovery on cell evolution.
Hatakeyama explains: "For the development of multicellular organisms, it is necessary not only to activate cell growth with each other, but also to mutually inhibit cell growth or programmed cell death
in clonal cells.
" "We know that fungi are more prone to evolutionary transitions between single cells and multiple cells than other organisms, so we wanted to unravel the relationship between
post-attack killing and multicellular biological evolution.
" We hope that this research will make a significant contribution
to our understanding of ecosystem development and evolutionary transitions.
" ”