Bakers add yeast to bread dough. As the dough bakes, the yeast produces carbon dioxide. This causes the bread to rise. The carbon dioxide also produces hundreds of tiny bubbles in the bread, which you can see as holes in a slice of bread. Yeasts are unicellular, but they can clump together to form chains of cells.
Yeasts reproduce by forming spores or by budding.
During budding, a portion of the yeast cell pushes out of the cell wall and forms a tiny bud. In time, the bud forms a new yeast. In 1928, Scottish scientist, Sir Alexander Fleming discovered that a substance produced by the blue-green mold Penicillium could kill certain bacteria that caused infections. Fleming named the substance penicillin. Since that time, the anibiotic penicillin has saved many lives.