Eukaryotic Microbes

00:01 Hello and welcome to Fungi. We're going to delve into this topic in a little more detail and I want to start with some general principles. And after this lecture, I hope that you'll be able to distinguish the two general types of fungi. You should know the different types of human fungal diseases and their origins, and you should be familiar with the drugs that are used to treat fungal infections.

00:29 There are two kinds of fungi morphologically. Most of the fungi that we know of, are called filamentous fungi, and these are diagrammed here. They can be made up of very extensive networks of filaments. These are composed of hyphae and mycelia. So mycelium is the name for the entire filamentous growth and it's made up of smaller segments called hyphae.

01:00 So these constitute most of the fungi that we know of in nature. These reproduce by dividing.

01:05 They also produce spores, you can see a structure on this mycelium that produces spores and of course the spores disperse in nature on animals and in the wind and they seed new growth, so the fungi elsewhere. So these are filamentous fungi. Mushrooms are fungi. They are produced by filamentous fungi. The filamentous part of the mycelium of course grows on or in the ground, you don't see it very easily. It’s microscopic. But periodically the mycelia are simply rearranged and grouped together to form a mushroom. The mushroom comes out of the ground and its sole purpose is to produce spores, which drop out of the cap of the mushroom, they drop on the ground, they are dispersed and they initiate the growth of a new fungus.

01:51 Now I say the sole purpose is to make spores, but of course humans like to eat mushrooms, some of them are very good to eat, some and of course are poisonous, but not everyone likes to eat them and those are the mushroom kickers as I said before, and the rest are mushroom pickers. So you have a large mycelial growth, periodically mushrooms coming up to make new spores. The other kind of fungus are the yeasts, and these are the unicellular fungi. A famous one is Saccharomyces cerevisiae or Baker's yeast if you like bread or beer, you have the yeasts to thank for it, because they carry out the fermentation that make the gases that make those products possible. So yeasts are unicellular, you can see a diagram of yeasts on the left part of the cell and an actual photograph of a living yeast on the right. They divide by budding, you have a single yeast cell and a small new one starts to form as a bud from the side. It starts out smaller and smaller, it gets bigger and bigger and then it pinches off and becomes a new yeast cell, and you can learn more about this process in our basic discussion of fungi.

03:06 So those are the two major types, we have filamentous and yeasts among the fungi. All of these organisms, all these fungi are what we call heterotrophs. They can't make their own precursors, the molecules they need to build themselves. There are other organisms, other microorganisms that are called autotrophs, they can take minimal organic compounds and use them to build the precursors that they need, fungi cannot do that, they have to absorb what they need premade from the environment, so we call that heterotrophy and typically fungi grow outside in the soil or on rotting wood, many fungi secrete enzymes that then digest the material around them and then the fungi absorb it. So plants as you know, a major component of plants is cellulose and fungi can secrete enzymes that will digest the cellulose and then they can take the components from the digestion, absorb it and use it to make more molecules. And so that's how fungi grow. The mycelium can get bigger and bigger by absorbing these molecules; they are used to build precursors, like amino acids and lipids, which then get made into larger structures as well. So fungi are what we call heterotrophs.

04:24 Another important property of fungi is that almost all of them are strict aerobes, that is they require air and more specifically oxygen in which to grow. If you take the oxygen away, most fungi will not grow. Now in contrast, there are many other microbes that can grow in the absence of oxygen, we call those anaerobes, but this is not the case for fungi, most of them are strict aerobes, of course there are exceptions to this and that's why we say almost all fungi are strict aerobes.

04:59 The cell wall of a fungus is very different from that of plants or bacteria or even other eukaryotic organisms. And the cell wall is diagrammed on the right. There is a cell membrane surrounding the cytosol of the fungus and that's a phospholipid bilayer, very much like the cell membrane of other eukaryotic organisms, it has proteins embedded in it. But where a eukaryotic membrane would stop there, the fungal cell wall has other layers on top of it. And just above the lipid bilayer is a layer of chitin, and chitin is a polymer of the sugar N-acetylglucosamine and that is shown on the left-hand part of the slide, in pink is the glucosamine six carbon sugar molecule and the N-acetyl group is shown in green. So long chain polymers of this sugar is chitin, that's right on top of the cell membrane and then above that is a structure made of beta glucans, this is another carbohydrate molecule, the carbohydrates are polymerized in a specific way. And then on top of the beta glucans we have mannoproteins, these are glycoproteins, which means sugars conjugated to proteins and they are called mannoproteins because the largest sugar here is mannose, or I should say the most prevalent sugar in the mannoprotein is mannose.

06:33 Now fungi can grow in many places where other microbes cannot, in particular they can grow without water as long as they're in a damp environment. So for example, your bathroom wall may have fungi growing on it, there may be little dark spots between the tiles or even on the tiles and there isn't necessarily water on them, but it gets damp when you take a shower and the fungi can grow there, they can require very few nutrients in order to grow, but they will grow without overt water. So it can grow in damp environments and of course the soil in the forest is also damp as well. And as I mentioned earlier, many fungi produce spores for dispersal. This is predominantly the filamentous fungi, where they can make structures that aid in spore dispersal. We talked about the different ways spores are made in our previous discussion of fungi in an ascus or a basidium, depending on how they are presented, and of course the mushroom is a structure designed simply to produce spores for dispersal. Now spores have a role for the fungus to produce more fungal growth.

07:46 For humans, it can cause allergies, as we inhale spores on a regular basis. There have been some interesting studies done of air in various locations in cities or in the country, and spores are a large component of the air. And even now as I speak with you, it's likely that there are some spores in the air and I'm inhaling them. So we inhale them on a regular basis and for some of us they have no consequence whatsoever. I know that I've been in very moldy places and inhaled lots of spores and it had no allergic reactions, no long-term consequences, but for other individuals who are allergic, it can cause problems like sneezing and other lung problems. So spores are good for the fungi but not good for all humans.