Appointment of the microbe of the year 2022

The bak­er’s yeast Sac­cha­romyces cere­visi­ae pro­duces cakes and bread in addi­tion to wine — the basis of sparkling wine — and beer. Yeasts are tiny sin­gle-celled organ­isms and are clas­si­fied as microbes, even though — unlike bac­te­ria — they have a cell nucle­us (eukary­otes). This rela­tion­ship with humans makes them an ide­al research object. As small “cell fac­to­ries”, they pro­duce med­i­cines and raw mate­ri­als on an indus­tri­al scale. The Asso­ci­a­tion for Gen­er­al and Applied Micro­bi­ol­o­gy (VAAM) select­ed this microor­gan­ism, which is impor­tant for our enjoy­ment and sus­tain­able pro­duc­tion, as Microbe of the Year 2022.

“Sug­ar fun­gus of beer” means the Latin name Sac­cha­romyces cere­visi­ae. The Microbe of the Year 2022 is a great brew­mas­ter, although it is so tiny that ten of its cells stacked togeth­er bare­ly reach the thick­ness of paper. The brew­ing yeast did not become vis­i­ble until the inven­tion of the light micro­scope (1680) in the form of many tiny par­ti­cles that make beer cloudy. It took near­ly 200 more years for Louis Pas­teur to iden­ti­fy liv­ing yeast cells as the cause of alco­holic fermentation.

Nat­u­ral­ly, yeast cells feed on sug­ar com­pounds from leaves and fruits. They break down glu­cose or fruc­tose into car­bon diox­ide (CO₂) bub­bles and the alco­hol ethanol. The alco­hol gives the yeast an advan­tage: It kills com­pet­ing microor­gan­isms. Once the yeast has eat­en the sug­ar, it can con­tin­ue to break down the ethanol it has pro­duced itself.

Peo­ple have been using yeast fer­men­ta­tion for thou­sands of years: Even the ancient Egyp­tians made a type of beer. In ear­li­er cen­turies, this was a drink even for chil­dren, because it was much low­er in germs than the fre­quent­ly pol­lut­ed water. Wine and sake are also based on the fer­men­ta­tion activ­i­ty of yeast. For the for­ma­tion of foam in sparkling wine, a yeast vari­ant (Sac­cha­romyces bayanus) is used in the sec­ond fer­men­ta­tion, which goes back to three dif­fer­ent yeasts, includ­ing bak­er’s yeast.

Baking yeast

The uni­cel­lu­lar yeast fun­gi also pro­duce car­bon diox­ide bub­bles in cake dough: Flour con­sists of linked sug­ars (car­bo­hy­drates), which Sac­cha­roym­ces cere­visi­ae con­verts to CO₂. Vig­or­ous knead­ing dis­trib­utes the yeast cells in the dough; slight heat stim­u­lates their metab­o­lism and mul­ti­pli­ca­tion. The result­ing bub­bles cause the yeast dough to become loose — it rises.
Bak­eries, brew­eries, and wine and sparkling wine cel­lars use a vari­ety of dif­fer­ent yeast strains and species. In the sour­dough used for bread, lac­tic acid bac­te­ria sup­port the yeast. The exact com­po­si­tion and their con­di­tions of use are often well-kept trade secrets.

Biotech­no­log­i­cal mod­el organ­ism for med­i­cines and sus­tain­able raw materials
Sac­cha­romyces cere­visi­ae was the first eukary­ot­ic organ­ism with a ful­ly sequenced genome. Today, there are strain col­lec­tions in which every sin­gle one of the approx­i­mate­ly 6,300 yeast genes can be mod­i­fied. Using bak­er’s yeast as a mod­el organ­ism, it is com­par­a­tive­ly easy to study the basic struc­ture and func­tion of eukary­ot­ic cells, because yeast cells have a sim­i­lar struc­ture to human cells.

Yeast cells also serve as a cel­lu­lar fac­to­ry. Dia­bet­ics, for exam­ple, have ben­e­fit­ed from this for decades: The human insulin gene has been “implant­ed” into the yeast genome, so this tiny organ­ism pro­duces much of the human hor­mone for dia­betes ther­a­py. Research teams also used genes from fun­gi and bac­te­ria to enable yeast to con­vert nat­ur­al sug­ars from wood (xylose) into ethanol. This means that plant waste can now serve as a raw mate­r­i­al and ener­gy source. Mod­i­fied yeast cells can also pro­duce suc­cinic acid, a build­ing block for the indus­tri­al pro­duc­tion of poly­ester. The anti­malar­i­al drug artemisinin (award­ed the Nobel Prize in 2015) is pro­duced by a sophis­ti­cat­ed “redi­rec­tion” of yeast metab­o­lism. This process also served as the start­ing point for the pro­duc­tion of the chem­i­cal­ly relat­ed replace­ment avi­a­tion fuel farnesene.

An impor­tant role for biotech­nol­o­gy is played by a prop­er­ty that char­ac­ter­izes yeasts, like all eukary­otes: They pos­sess mem­brane-enclosed organelles that allow spa­tial sep­a­ra­tion of var­i­ous bio­chem­i­cal process­es. This makes it pos­si­ble, for exam­ple, to sep­a­rate tox­ic inter­me­di­ates with­in the cell. Researchers recent­ly suc­ceed­ed in “pack­ag­ing” enzymes for the pre­cur­sor of nylon in vesi­cle-like vesi­cles. This is an exam­ple of how the divi­sion of labor in the cell can be opti­mized by new reac­tion spaces. Sac­cha­romyces cere­visi­ae will play an impor­tant role in soci­ety’s trans­for­ma­tion to more sus­tain­able forms of economy.