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Fungi

by Vincent Racaniello, PhD
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    00:01 Hello and welcome to Fungi. After listening to this lecture, I hope you'll be able to define the two different types of fungi. I hope you'll understand how yeast reproduce and what we can use them for. I want you to be familiar with the structure of filamentous fungi and I want you to know some diseases that are associated with fungi.

    00:25 There are two types of fungi in general. Most of the fungi that we know are called filamentous.

    00:35 They form large networks of filaments that are called mycelia, and that is shown in this slide, and the mycelia in turn are made of smaller segments called hyphae. And this, you should be familiar with, if you've ever had moldy food in your house, bread or fruit that's covered with a fuzzy layer, that is a filamentous fungus. Mushrooms are also fungi, even though they are not microscopic, they are part of a very large mycelium. Remember that mycelium is the network of filaments that makes up the filamentous fungi, this typically grows underground. The mushrooms are actually aggregates of the mycelium that come up above the ground and they drop spores onto the ground to make more fungi. So the mushroom is big, it is not a microbe because you can see it, but it's obviously part of a microorganism, the fungus.

    01:34 The other kind of fungus are called yeasts, and this is because they are single cell and there is a diagram of single cell yeast here and a photograph of real yeast cells which are dividing. This particular yeast happens to be Saccharomyces cerevisiae. So we have filamentous fungi and yeast type fungi. Now an important property of all the fungi, is that they are heterotrophs, they acquire food by absorbing small molecules, they can't make any of their own molecules, they can't do it as bacteria do, they are not autotroughs.

    02:13 So they are heterotrophs, and often what the fungi will do is to secrete enzymes into their environment that digest precursors and then they absorb the digestion products. And here we have a mycelial growing on a rotting wood for example, they often grow outside in soil around wood. They absorb the nutrients that they need and that way the mycelium gets larger and larger. Typically the mycelia are beneath the surface of the soil, the mushrooms come out above. So whenever you see a mushroom, just think now underneath it, is a very big mycelium.

    02:53 The cell walls of fungi are different from the cell walls of any other organism, including eukaryotic cells and bacteria. They do have a cell membrane surrounding the fungal cell, which is a phospholipid bilayer and you can see that in this slide, it may also contain membrane proteins, but then just above the cell membrane is a layer called chitin.

    03:19 Chitin is a sugar polymer, it consists of multiple residues of N-acetylglucosamine, the glucosamine is the sugar shown in pink, the acetyl group is shown in green. So chitin is a very long polymer of this single sugar that lies just above the cell membrane. Above the chitin is a layer called beta-glucans, these are also a sugar polymers that are assembled as a protective layer and then on top of that is another layer called manno-protein, which is a glycoprotein sugar and protein and in this case most of the sugars are mannoses.

    03:57 So above our membrane, we have other layers of sugars and sugar polymers to protect the fungus and this makes it very different from all other kinds of cell membranes.

    04:08 Fungi can grow in many places, they often don't need water, they can grow in damp environments and that's an example here; the mold on your bathroom wall, the wall is not really wet, but it's damp because of the showering and if you've ever seen a black growth, especially in between the tiles on your bathroom wall, those are filamentous fungi that are growing.

    04:35 They get enough nutrients from the wall and they can absorb enough moisture to live.

    04:41 Many fungi produce spores in order to reproduce, so the fungal cells can divide, but spores are often produced to make additional fungi. So a mushroom for example, is a spore forming body. It comes up out of the mycelium, it's produced by bundling mycelia together and then once above the earth, the spores drop out of the mushroom cap onto the ground and then they disperse in the wind and they initiate new fungal growth everywhere. So spores are made for dispersal. For humans, spores are not so good because we can inhale them quite often. When you're walking around in the woods for example, you're often inhaling the spores as you disturb them by walking and many people have allergies to the spores. In fact, even in your house, there are lots of spores floating around because of their molds growing in various places, you're inhaling the spores.

    05:36 Okay, so that's filamentous fungi. There is one other general type and those are called yeasts and these are typically single cell organisms. There are about 1500 species of yeast, that's only 1% of all the fungi that we know of, so you can see the yeasts are in the minority. Yeasts are not only beneficial to us, we use them to make bread and wine and so forth, but they're also highly studied in the laboratory, because they're great models for eukaryotic cells but they're relatively easy to grow and easy to manipulate genetically.

    06:17 We have a lot of uses for yeasts in our world, saccharomyces cerevisiae is also called baker's yeast or brewer's yeast. So from that name you can guess we often use it for bread production, we use it to make wine and we use it to make beer. So once again the next time you see one of your favorite food or beverages, you should thank the yeast for making them for you. We also use yeast to produce ethanol fuel from sugars like sugarcane and field corn, so the next time you put gasoline into your car, it may be 10% ethanol, that ethanol came from a sugar and yeast helped to produce that ethanol. This lifestyle of a yeast is quite different from that of a filamentous fungus. The yeasts reproduce by budding, they divide asymmetrically, so if you think of a single spherical cell and we have some illustrations of that here on the slide, it can divide to make two cells of equal size or it can divide asymmetrically, which means one of the new cells is initially smaller than the other.

    07:33 You can see in this illustration, the process of budding. We have a spherical yeast and then a smaller yeast is coming out of that, that is asymmetrical budding. So yeasts divide by budding asymmetrically, but they also have a sexual stage, because there are haploid and diploid stages. There are stages where there is only one copy of each chromosome, that's the haploid stage, and then there are stages where there are two copies, that is the diploid stage. There also two mating types, A and alpha and they're shown here on this diagram, on the top you see alpha yeast, on the bottom you see A yeast. This is sort of like male and female in animals and these two come together to form diploid organisms.

    08:22 Each mating type, the alpha or the A, produces a pheromone, it's a chemical that attracts the other mating type. So an A and an alpha find each other, they get together and they fuse, they then form a diploid organism and that diploid cell can replicate or bud as a diploid or it can bud and replicate as a haploid organism. So this is a very unusual lifestyle compared to the filamentous fungi.

    08:52 Now many fungi produce spores in order to replicate. Here's an example of spores that are produced in what we call an ascus. The ascus is simply the sack that contains the spores, you can see here the spores are shown in green and there are many of them within the ascus, and the spores are called ascospores. So this is a way of reproducing, you produce a spore, the spore is released at some point, it goes into the environment and then can grow into a new fungus. We call fungi that reproduce in this way, by making spores in an ascus, we call them ascomyceytes. And ascomyceytes include yeasts, they include molds, they include many human pathogens and also some mushroom producing fungi that we like to eat, like truffles and morels, shown on the right of this slide. So they're all ascomyceytes, they make spores within a sac. Some examples of yeasts that are pathogenic for humans.

    09:57 Now these are yeasts, the single cell organisms. Cryptococcus neoformans is a single cell growing organism in human, it causes a disease called Cryptococcosis. This yeast is present in the soil, it's present in decaying wood, in tree hollows, it is also present in the droppings of birds and the way you acquire this particular yeast infection, you inhale the organism.

    10:27 It goes into your lung where it can replicate. Now typically Cryptococcosis is not a disease that we find in healthy people. It usually occurs in immunocompromised people, for example people with AIDS often have Cryptococcosis, or if you have an organ transplant and your immune system is being immunosuppressed by drugs so that you don't reject the organ, if you inhale the cryptococcus neoformans, you can develop Cryptococcosis. And when the yeast does enter a human, it's inhaled as I said, it can get into the lungs, it can replicate in the lungs and cause a pneumonia-like illness, it can also travel to the brain and cause disease there and we call that meningitis. This is one disease that is not contagious from person-to-person, even though you might think that a fungal infection might be once it gets into a human, that's the end, it can't transfer to another human through the air.

    11:29 Now Cryptococcosis, in particular Cryptococcal Meningitis is a serious illness worldwide.

    11:35 You can see from this graph that there are cases in many different regions of the world, there are about a million new cases every year with 625,000 deaths and the majority of them as you can see, are in sub-Saharan Africa and that's probably because it's relatively dry and the spores of the organism travel more readily and they can be inhaled.

    12:00 Another pathogenic yeast is related to the cryptococcus we just talked about, it's called Cryptococcus gattii. This is again a yeast forming fungus, it's present in the soil and it's usually present in tropical and subtropical regions, it's become noteworthy lately because there have been a lot of cases in the US Pacific Northwest and in British Columbia shown in green on this map. Again you acquire the infection by inhaling the spores, they go into your lungs and there they cause disease. And again this fungus causes disease in immunosuppressed person such as people with AIDS and organ transplants, but they can also cause diseases in healthy people, and also those with lung disorders that don't happen to be immunosuppressed.

    12:56 If you smoke, you put yourself at risk for infection with this yeast. Again the yeast may cause a pneumonia-like illness by growing in the lungs, or it also can spread to the brain and cause meningitis. Just like cryptococcus neoformans, cryptococcus gattii does not travel from person-to-person.

    13:24 Another pathogenic yeast, again a yeast cell, the single cell type fungi, is Candida albicans, and this can cause a disease called candidiasis. On the left is a picture of Candida growing on an agar plate in the laboratory, and the picture on the right, emphasizes the point that this yeast is a normal inhabitant of humans. It grows on our skin and also in our mouth. It's only when we have some kind of imbalance in our body, does Candida overgrow and cause a disease, and that's often associated with various medical procedures. When you have an outgrowth of candida in the mouth, it's known as thrush or oropharyngeal candidiasis, and this is associated with redness and soreness, difficulty swallowing and you can often see a white coating on the tongue or on the mucosal membranes of the mouth. It can also cause infections in the genital, vaginal areas, this is called Genital/vaginal candidiasis, again these are normal inhabitants of these regions and when there is some imbalance, for some reason the candida overgrows and causes a problem.

    14:50 In addition to Candida causing disease in the mouth and the genital areas, it can also become invasive, it can get into the bloodstream and spread to different organs, it can spread through blood to the heart, brain, eyes and these are very serious infections. They don't typically occur unless the patient has been in a hospital. So for example, these kind of invasive candidiasis are associated with a stay in an intensive care unit, you are very sick, they're associated with being there, they're associated with individuals who have a central venous catheter inserted into the large vein in your chest to deliver for example antibiotics or other materials, long-term presence of that is associated with invasive candidiasis. They are also at risk in people with weak immune systems, again from immunosuppression, or infections that cause immunosuppression, or the use of broad-spectrum antibiotics, say to treat a bacterial infection, this will often disturb your normal microbial flora and allow the candida to overgrow.

    16:06 We talked previously about how some fungi make spores within a sac, it's called an ascus, the other type of fungi produce spores from a structure called the basidium. And on the left, we see a typical yeast or fungal mycelium. And the very top, there are spores coming off the mycelium and that is a basidium. The spores come off and generate new fungi. And on the right is a close-up diagram of the basidium, and at the very tip the fungi produce spores, those are then released and fall into the environment to generate new organisms.

    16:44 Fungi typically live in soil or on dead matter, you often can't see them. If you just go for a walk in the woods, the soil is teeming with fungi, but you don't even see them. I think even in a city if you walk around there is plenty of fungi growing in damp places and they're just so small that you don't notice them, but they're there and you're inhaling them quite frequently. I'm sure you have had some rotting fruit in your refrigerator or on the table in your kitchen, that green growth or white fuzzy growth on the surface of the fruit, that is a mold, it's a filamentous fungus and these are growing and they're using nutrients from the fruit and they're producing spores. So if you can see an orange that looks like this one, you're breathing in the spores.

    17:39 Now remember the mycelia that fungi produce, these are large meshworks or networks of filaments that spread and spread and spread and these typically grow in the soil and periodically they produce mushrooms which pop above the soil to release their spores. These mycelia can be small or they can be very big. So the mycelium on the orange that we just looked at is relatively small, but some of these mycelia are miles in diameter. For example there is a site in Oregon in the US, it's a 2,400 acre site which is about 9.7 km², it has a single contiguous growth of mycelium. It's one organism that started from a spore and just kept growing and growing and growing. You can't see this, it's going in and under the soil, but periodically the mushrooms come up from the mycelium and they drop of course their spores. So we think this might be the world's largest organism, miles in diameter.

    18:49 Just to review the two different ways that fungi produce spores. They produce them from a basidium and these are called basidiospores and this is typical of the fungi called basidiomycetes, or the fungi can produce spores within a sac called an ascus, the spores are called ascospores and these fungi are called ascomycetes.

    19:19 Now in your home, there are lots and lots of different molds, and these are some of the common ones that you could have and they're called Cladosporium, Alternaria, Aspergillus, that may be a name you recognize, and Penicillium, another name you may recognize because that was the original source of the antibiotic, penicillin. So these are all and many more growing in your home, even if you think you have the cleanest home in the world, you will still have molds growing in them, you'll be inhaling the spores on a daily basis and most of you will probably never have a problem with this. However many people are sensitive to molds and the spores that they produce and these can cause stuffiness of your nose, they can irritate your eyes, they can even cause a skin condition and collectively we call this sick building syndrome. It is well known that since we live in a built environment, we live in our homes, we go to work in offices, they all have their own collection of fungi and some people are simply sensitive to it and always associate a certain building with a certain kind of illness. So sick building syndrome refers to the fact that our buildings are filled with fungi and many of us are sensitive to that. So fungi are found not just in your homes, but in many commercial buildings as well and some people for example are fine at home and they go to work and there is a particular fungus there and they have allergies during the day, they're sniffling and sneezing, they go home and they're fine, that is sick building syndrome.

    21:03 One of the diseases caused by breathing in spores that are found in your home is called aspergillosis and this is of course a consequence of breathing in spores produced by Aspergillus species. As I said many people breathe spores on a daily basis without ever getting sick.

    21:22 But there are people with weak immune systems or maybe lung disease, or if you smoke, this can destroy your immune system in your lung. These individuals are at higher risk for developing illness after inhaling these spores. You can have allergic reactions to the spores, which is a simple matter to take care of, or you can have more serious problems as well, you can get lung infections and the fungus can then spread from the lung to go to other tissues, such as the liver and the kidney and cause infections there as well. So what starts off as a rather innocuous inhalation of spores can become a very serious illness. In addition, a certain species of aspergillus, Aspergillus flavus, produces a toxin, it's called aflatoxin and this is often a contaminant of nuts. If you buy nuts and they happen to be a bit moldy, you probably should not eat them, because if that mold is aspergillus, they can be highly toxic as a consequence of the production of this toxin. Another type of fungal illness is called histoplasmosis and this is called by the species called histoplasma. This is a fungus that typically lives in the soil, especially soil that is rich in bird or bat droppings. This disease, histoplasmosis, occurs worldwide, can be found in the US, in the central and eastern states, parts of Central America and South America, Africa, Asia and Australia, so it's globally distributed. You acquire this disease by inhaling fungal spores.

    23:10 And again, most people inhale these spores on a daily basis and don't become ill, but if you have an immunosuppressive disease such as HIV-AIDS, or if you've had an organ transplant and are undergoing immunosuppressive therapy, or if you're infant or a very old person, these extremes of human age don't have very good immune systems; babies have not yet developed their immune system, people over 55 years old, their immune systems are slowly deteriorating, so they are at risk for infections with these otherwise innocuous spores as well. These fungi can cause infection of the lung in these at risk individuals, and just like some of the other diseases we talked about, those infections can become systemic, they can spread to the blood and go elsewhere and in particular histoplasmosis is known to go to the brain as well as the spinal cord. Just like many of the other fungal diseases of humans, this one is not contagious among humans.

    24:14 Fungi are major pathogens of plants, insects and amphibians. A couple of examples here, there is a disease called chytridiomycosis, it's caused by a fungus that I will not attempt to pronounce, which is causing global decline in frog populations. You may have heard about this, it's in the news quite often, I think we've lost 30% of the world’s amphibians as a consequence of this fungal infection. Another example is a disease called white nose syndrome. Here, what has been noticed is that there have been extensive deaths of bats due to a fungus, again, growing in the bat, which colonizes the skin of the animal and makes it appear white, that's why it's called white nose syndrome. And again, bats are very important to the ecology of the world and losing them is simply not acceptable. Fusarium is a fungal disease of cereal crops, cereal crops are very important for feeding most of the world, fusarium will destroy them very rapidly, so it's an economically important fungus and we have to figure out how to control it. And finally the last example is a fungal disease which infects ants. So the fungus enters the ant and alters its behavior. Normally ants like to move about higher in a plant, when they're infected with this fungus the ants go down to the forest floor, and there the temperature is lower and fungi like to reproduce at lower temperatures and the ant will find the lowest leaf on a plant and clamp onto one of the major veins and just remain there. So the ant will be taking nutrients from the plant, it will not move anymore because of the fungus that has gone to its brain and then eventually what comes out of the skull of the ants, well ants don't have skulls, what comes out of the head of the ant, is a fungal spore producing structure. It just pops right out of the ant's head, spores drop out of it and that releases new fungi into the environment, of course at that point the ant is then dead. So the fungus takes over the ant behavior, you could almost say it makes the ant into a zombie.

    26:45 We think that the increased fungal diseases in human is a consequence of global warming.

    26:53 As I've just told you fungi are major pathogens of plants and insects and amphibians, but there are not a lot of fungal diseases of human. However this is changing, at the end of the 20th century the number of human fungal diseases is going up. This may be partly a consequence of medical interventions, immunosuppressant for example, we're doing many, many more organ transplants than we ever did before, and to do that, to make those successful, we have to use immunosuppressive drugs. So, one reason for this increase is probably the increase in transplantation. Another reason is probably the onset of the AIDS pandemics in the 1980s, so this is immunosuppressing people and causing fungal infections where they wouldn't have had an infection before. But another change probably has to do with the climate.

    27:46 So humans of course are warm-blooded, endothermy refers to that fact. Fungi don't like to grow at the temperatures of a human body and we think that's part of the reason why human fungal infections are so infrequent. So most of our resistance comes from our immune system, and the fact that our temperature is too high for fungi. But as you probably know we're in a period of global climate change at the moment. Here is a graph showing the temperature of the earth, the mean temperature of the earth for many, many, many years, up to the present. So the main graph covers thousands of years and then the inset shows you 0 to 2000 A.D. So you can see that around 12 or 1300, there was an increase in temperature of a few degrees, it then went down again and then in the 1990s the temperature started rising again and that's the current status of our climate change. So the temperature has gone up from about twelve and a half degree average to over 14°. We think this characteristic is increasing human fungal diseases, selecting for fungi that can grow at higher temperatures.

    29:02 So just think, the fungi grow in most parts of the world, they're usually at the bottom of the forest where it's cooler, but overall the temperature is rising, so only the fungi that can grow at the high temperatures will survive, so we're selected for mutant fungi that can grow at higher temperatures and that is a fungus that will then invade humans.

    29:22 So this is the theory at the moment that somehow climate changes resulting in more fungal infections of humans.

    29:29 So I hope that by listening to this lecture on fungi, you've learned about the two general types of fungi, the yeast and the filamentous fungi. I hope you understand how yeasts reproduce and what we use them for, all the wonderful things we can make with yeasts. I hope you're familiar with the structure of filamentous fungi and what is a mycelium and a mushroom for example. And finally you should know some of the diseases associated with fungi.

    30:00 So you know they say in the world, there are two kinds of people, there are mushroom pickers and there are mushroom kickers, so I hope after this you're a mushroom picker.

    30:09 Thank you, see you next time.


    About the Lecture

    The lecture Fungi by Vincent Racaniello, PhD is from the course Microbiology: Introduction. It contains the following chapters:

    • Eukaryotic Microbes - Fungi
    • Yeasts
    • Pathogenic Yeasts
    • Filamentous Fungi (Molds)
    • Indoor Molds
    • Histoplasmosis
    • Global Warming and Fungal Diseases

    Included Quiz Questions

    1. Alternating alpha chains that create a semi-permeable mesh.
    2. A phospholipid bilayer that forms a cell membrane.
    3. A layer of chitin, a sugar polymer.
    4. A layer of beta-glucans.
    5. A layer of manno-proteins.
    1. A person who has recently received an organ transplant.
    2. An active cyclist.
    3. A kindergarten teacher.
    4. A member of the high school swim team.
    5. Someone taking a daily asprin.
    1. It will cause rapid dehydration from vomiting and diarrhea.
    2. Redness and soreness inside the mouth.
    3. Infections in the genital and vaginal areas.
    4. It can get into the bloodstream and spread to different organs, particularly the heart, brain, and eyes.
    5. A white coating on the tongue or on the mucosal membranes of the mouth.
    1. ...basidiospores and ascospores.
    2. ...sporoblasts and sporoclasts.
    3. ...candidaspores and neospores.
    4. ...basidiospores and neospores.
    5. ...sporoclasts and ascospores.
    1. Giardia.
    2. Fusarium.
    3. Aspergillosis.
    4. Histoplasmosis.
    5. Cryptococcal Meningitis.
    1. The network of filaments that makes up the filamentous fungi.
    2. A sugar polymer that consists of multiple residues of N-acetylglucosamine.
    3. A single cell growing organism in human that causes a disease called Cryptococcosis.
    4. A fungi associated with individuals who have a central venous catheter inserted into the large vein in their chest to deliver antibiotics.
    5. One of the diseases caused by breathing in spores that are found in the home.

    Author of lecture Fungi

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD


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