What are yeasts?
Yeasts are eukaryotes, meaning that they share quite some cellular features with mammalian and human cells. Bacteria, on the contrary, are prokaryotes, sharing far less characteristics with fungi or plants and animals. Yeasts belong to the kingdom of Fungi (Eukaryotes). In particular, yeasts are unicellular fungi, they thus only consist of one cell. An important remark here is that C. albicans is a yeast, but can also exist multicellularly (the filaments or hyphae actually consist of multiple cells). Candida is therefore called a polymorphic yeast, as it can exist as multiple forms.

As yeasts are unicellular, their cells contain all necessary machinery for the organism to survive. Within the group of yeasts, there is not so much cellular diversity. Genetically, these yeasts look very alike. However, an important variable characteristic is pathogenicity. While some yeasts obtained the trait to grow in and infect the human body, such as some Candida species, others did not, such as baker’s or brewer’s yeast. This indicates that they evolved separately, both in different niches.

Do you want to know about how brewer’s yeast can produce alcohol? Watch this TED-Ed video: A brief history of alcohol - Rod Phillips.

Good yeasts - probiotics
We consist of millions of microbes, which reside in our gut, on our skin and in/on many other body parts. Most of these microbes are beneficial, helping us with digestion and acquiring vitamins from our diet. If you want to know more about these microbes, watch this TED-Ed video: You are your microbes - Jessica Green and Karen Guillemin

In some cases, the good microbes are out of balance, causing disease. One of the conditions caused by this type of imbalance is antibiotic-associated diarrhea. Antibiotics do not only kill the pathogenic bacteria, but they often also kill the beneficial bacteria in our gut or vagina. In these cases, probiotics can be an effective treatment. Probiotics are living micro-organisms that, when administered in adequate amounts, have a positive effect on health.

Want to catch up on probiotics? Take a look at these web pages:
Probiotics: What You Need to Know - NCCIH
The International Scientific Association for Probiotics and Prebiotics - Nature.com

Mostly, bacteria are commercialized as probiotics, however, yeasts can also have a positive effect on human or animal health. They have the advantage over bacteria that they are more resistant to stomach acid and thus survive the gastro-intestinal tract more effectively. Furthermore, in the case of antibiotic therapy, yeasts can be combined with the treatment, reducing the risk of antibiotic-associated diarrhea. Yeasts are eukaryotes and are thus not sensitive to the antibiotic.

Bad yeasts - Candida albicans
It is important to understand that C. albicans lives inside most people’s bodies as a harmless commensal. Normal colonization occurs mostly in the vagina, the gut and the oropharyngeal cavity. The fact that Candida is present in our bodies, however, and can thus start an infection any time we are weakened, poses a serious disadvantage. Especially people who are extremely weakened, such as recipients of immune suppressants, or the elderly, are at risk for a systemic infection with Candida. In these cases, Candida present in any of the previously mentioned body sites (especially the gut) will cross the epithelial cells, immune cells (which are present in reduced amounts) and all other barriers normally limiting access to the blood stream. It will do so by forming filaments or hyphae. The filaments penetrate the cells and can do this via different methods. Once the bloodstream is reached, the filaments can also revert to normal yeast-shaped cells.

Alternatively, Candida can also reach the bloodstream through contamination during surgery or by implanting contaminated catheters or other indwelling material. Most often, a biofilm is formed on these devices, consisting of Candida filaments and yeast-shaped cells, and most often other microbes and extracellular matrix. These biofilms are extremely hard to eradicate as the matrix does not allow easy passage of drugs, the cells are more resistant to drugs and the biofilm also sheds persister cells, which can cause a new infection at a distinct location. In any case, when Candida occurs in the bloodstream, it will, by forming filaments once again, reach vital organs, such as the kidneys and liver. People with a systemic Candida infection do not survive in about 50% of the cases, however, it is often hard to depict what the actual cause was of death, as these patients often suffer from multiple complications.

Want to know more about Candida infections? Check out this website: Candidiasis - CDC

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