# Is yeast, as in baking yeast or brewers yeast, a bacteria or fungus?

1. Dec 4, 2003

### Royce

Is yeast, as in baking yeast or brewers yeast, a bacteria or fungus? My daughter, the nurse, and I are arguing about this. I say it is bacteria and she says it is a fungus. Whose right or are there more than one kind?

2. Dec 4, 2003

3. Dec 4, 2003

### Monique

Staff Emeritus
Sorry to say Royce, but your daughter is right

I am not exactly 100% sure on the nomenclature and whether all yeasts are fungi, but I can say the following:

Fungi that are responsible for life-threatening infections (in immuno-comprised individuals) are Candida, Aspergillus (mainly Aspergillus fumigatus) and Cryptococcus neoformans.

The thing I am confused about is that the state the organism is in might classify it as a yeast or a fungi. I am talking about Candida albicans of which I know it is a dimorphic yeast, it can undergo a morphogenetic switch from a yeast-form to a hyphal form. The yeast form are buds, while in the hyphal form the cell starts growing in a single direction and becomes very long (causing the pathogenicity). I guess that spherical cells are called yeast and hyphal cells fungus. (I am not sure if the other two fungi I mentioned are also yeast, but at least C. albicans is both for a fact).

You are definately wrong by saying yeast is a bacteria, that distinction is very clearly made :) Bacteria are protozoa, very primitive cells and they don't have any internal compartments. Yeast to the contrary is a very simple eukaryote (we are eukaryote too) and has subcellular compartmentalizations (organelles).

It is interesting to understand why the yeast cells are not pathogenic, while the hyphal cells are. The reason for this is that yeasts have a cell wall that covers the cell membrane. We humans only have a cell membrane. A cell wall is very rigid and very strong, when the yeast cell undergoes the change to hyphal growth, it will actually grow right through our tissues and organs, there are no barriers since our cells are so weak.

Also, our immune cells are able to engulf and eat foreign cells, that is exactly what will happen to such a hyphal cell, it gets eaten by a human cell. But the hyphal cell doesn't die directly, and is able by shear stress to burst the cell it engulphed by the linear growth.

Interesting to note is that the change from a yeast to a hyphal growth occurs with blood as a trigger, this is very very dangerous and survival rates are very slim when such a diagnosis is made, maybe 10% live?

Another interesting thing is that there is very little medicine can do against such a systemic (inside the body) fungal infection (that is why so few survive). Since the organism is eukaryotic, and we are too, a medicine against the pathogen will have severe side effects for the patient (unlike with a bacterial infection, where we have lots of medications).

I know of a single fungicidal drug (will actually kill the mycosa), which is Amphotericin B (Adrenaline might know of it :P). The way it works is that it relies on an interaction with ergosterol. Ergosterol is the equivalent molecule of cholesterol that we humans have in our cell walls, but it is slightly different. This interaction takes place in the cell membrane, where a large pore will be formed through which ions will leak, causing the cell to die. The problem is that it also has an interaction with human cholesterol (although less strong) and the side effects are thus severe (nephrotoxic).

Besides the fungicidal drug, there are several fungistatic drugs (will inhibit the growth of the mycosa, but it won't kill it). Those are fluconazole, itraconazole, ketoconazole. These all inhibit the biosynthesis of ergosterol, causing an intermediate to accumulate. Side effects are mild endocrine effects, but the medicine is not effective since the mycosa are not killed! When the drug treatment is stopped, the infection might come back, this also induces the development of resistant cells for the treatment.

So the need for other drugs is great and many pharmaceutical and academic labs are very busy in trying to identify new targets for medicine development. New drugs might come to the market soon, based on echinocandins (non-competative inhibitors of $$\inline\beta$$-glucan synthesis) of which clinical trials are currently being conducted.

Whoo! Did you read all that? you might impress your daughter with this newly acquired knowledge

4. Dec 4, 2003

### Monique

Staff Emeritus
Re: yeast

Just wondering, are you talking about baking yeast (Saccharomyces cerevisiae) in specific?

Again, it is not a bacteria for sure (100%) but I am not sure whether it is classified as a fungus per se.. at least some yeasts are fungi :)

5. Dec 5, 2003

Fungi ...

6. Dec 5, 2003

### Royce

Thanks all. I appreciate even if I was wrong. I usually am with her. Monique, I now know more than I ever wanted to about yeast, thanks again.

7. Dec 5, 2003

### Monique

Staff Emeritus
well, people underestimate the information that is available in biology. People think you get a yeast infection and that is a disease so you die. But on a microscopic or molecular level all processes are going on that are fully understood or subject of much research.

For some reason people don't really care for this information, I wonder why. I find it amazing that a cell gets caught by another cell and is eaten, after which it initiates a survival mechanism and breaks free, killing the cell by which it was engulfed.

There is probably a whole molecular/biochemical process that I could tell you of how a cell knows it is being eaten, I am sure there are a handfull of research groups figuring out how it works, it would be a perfect target for therapy.

As for all the information I gave in the post, last monday I attended a lecture of a academic clinical researcher who discussed the whole topic of fungal infections and current research and apparently it stuck

8. Dec 6, 2003

### Another God

Staff Emeritus
I will be starting honours in 2005 in a lab which works on Sacharomyces Cerevisiae, concentrating on Oxidative Damage and Gene Expression in general (Using gene array chips etc). I can't wait. I will be working in Oxidative Damage and its affect on life span

9. Dec 6, 2003

### Monique

Staff Emeritus
Cool! So do you happen to know how many genes are orthologous between the yeast and the humans? Ofcourse you are going to look at metabolic pathways so you'd expect those to be very conserved right?

10. Dec 6, 2003

### Another God

Staff Emeritus
i guess so. I haven't started getting into it yet, but by the end of 2 years you know I'm going to be overloaded with Yeast knowledge.

11. Dec 6, 2003

### Monique

Staff Emeritus
Next week I am going to do a rotation in a yeast lab so I'll be sure to ask :)