Yeast - The helpful and harmful organism

Yeast is a big part of our lives and human culture in general. It is involved in everything from alcohol production and bread making, to scientific research and probiotics. However, some yeasts can also be pathogenic and cause human infections.

Yeasts are unicellular fungi that mainly metabolism sugars for energy. They are very common in the wild where they can be found on the skins of fruit and other sugar-rich places like cereal grain crops. Even though yeast plays a huge role in many aspects of our everyday lives, most species are only about 4 microns in diameter, which is about half the size of our red blood cells. in comparison, a single human hair is roughly 80 microns thick.

When we talk about yeast in general, we are referring to the species Saccharomyces cerevisiae which is used in the production of bread and alcohol. In bread production, yeast is added to convert some of the sugars in the dough to carbon dioxide which makes the bread rise. Another product of the metabolism of sugars besides carbon dioxide is alcohol which has been used for beer and wine production for over 7000 years. In an airtight container, brewers yeast will produce ethanol which is the desired alcohol for consumption. However, if the yeast and sugar batch is exposed to air (specifically oxygen) during fermentation, methanol will be produced, which is very toxic to ingest in large quantities.

Yeast cells reproduce by ‘budding’ the majority of the time. During this form of asexual reproduction, the yeast cell copies its DNA in a process called mitosis and divides into two individual cells. This means that the new daughter cell is a genetic clone of the parent cell. Because yeast is easily accessible, mostly harmless, easy to culture and simple cells, they have been used as a scientific model organisms for human cells and other eukaryotic cells for many years. This has helped describe both DNA replication, cell division and many other cellular functions and properties in detail.

Yeast is also a part of the human normal flora on both the skin and in the gut. Several species of yeast lives here, and in healthy individuals normally does not cause any problems. in fact some yeast species are used in probiotics because of their beneficial properties in the gut flora. But if the immune system gets compromised, like during chemotherapy, as a baby or in older individuals, some yeasts are able to cause infections. One of the most common is Candida albicans which can cause yeast infections in the mouth, vagina, nails and skin called candidiasis or thrush.