Welcome back. In this session, we're going to talk about Basic Microbiology. Zoonotic diseases are diseases of animals that can affect humans. So they are microbes that can spread from animals to people, and that includes viruses, bacteria, fungi, parasites, and prions. So what's the difference between a virus, a bacterium, a fungus, a parasite, and a prion? Let's talk a bit about viruses. Viruses have protein coats that surround the genetic material inside and viruses are technically not alive. They don't eat, they don't eliminate, they do not reproduce themselves and in fact rely on host cells to create new viruses and then move on to new hosts. So viruses come in many different shapes and sizes. They can vary from tiny polio viruses to massive Ebola viruses. So how does a virus work? A virus attaches itself to the outside of a cell and then gets incorporated into that cell. It uncoates itself allowing its genetic material to come out, incorporates its genetic material into the nucleus of the cell and turns the sell into a virus making machine, making many new copies of itself, and then eventually bursts out of the cell to infect other host cells. So how does an antiviral drug work? Now remember, viruses are technically not alive so you can't kill them. But what you can do is you can block their mechanism of action. So, you can block them from attaching themselves to the surface of a cell, you can block the virus from opening itself up and allowing its genetic material to become replicated, and it can also block the ability of the virus to bud out of the cell. So, those are different strategies where the different drug actions can work. Because viruses are not alive, they can't be killed, and antibiotics, which only work against bacteria do not work. So antivirals reduce symptoms but they won't completely stop them, it's up to your immune system to do that. So the best option with viruses is to vaccinate. Vaccines help to prime the immune system to fight off viral infections. And how do vaccines work? Well, you can have either a whole inactivated virus, you can have a virus that's been weakened, or you can have parts of viruses. And these are essentially target practice or boot camp for the immune system. So in order to stop the virus from spreading, you have to have enough people to get vaccinated to protect those who cannot be vaccinated for medical reasons, and this is called herd immunity. Herd immunity is when you have enough people vaccinated, those who are in the yellow to prevent the virus from propagating from person to person. This is a largely unvaccinated population, and it can spread quite widely throughout the entire group. Now, bacteria in contrast to viruses are alive. They eat, they eliminate, they reproduce and they are single celled organisms. They have a cell wall or membrane and they have genetic material. These are called prokaryotic organisms because they don't have specialized organelles the way the cells in your body and my body have. So the difference between prokaryotic cells versus eukaryotic cells bacteria are prokaryotic cells. They can have a membrane and a cell wall and they have genetic material. In contrast, eukaryotic cells which are animal cells, have more specialized organelles including a nucleus, golgi bodies, mitochondria and others, that provide specialized function within this cell. So when you look at the phylogenetic tree of life, bacteria are over here, archaea are in the middle, and eukarya which include animals, fungi, plants, and you and me, are over here. So we're not that closely related to bacteria. Different bacteria come in different shapes and sizes. You can have them in circular pairs, chains or clusters like a group of grapes. You can have them rod shaped. You can have them in curved forms and you can even have them in spiral forms. Now, in contrast to antiviral drugs, antibiotics actually will kill the bacteria and those are called bacteriasidal antibiotics. Antibiotics that don't kill the bacteria but slow their growth are called bacteriostatic. But both types of antibiotics rely on a functioning immune system to be able to rid the body of these harmful bacteria. So the antibiotics can either inhibit the bacteria from synthesising cell walls, nucleic acid synthesis or function or it can inhibit protein synthesis. So, fungi are more closely related to animals and people like you and me. They are indeed alive and they can include single-celled yeasts or multi-celled molds. They look similar to plants in that they have cell walls, but they're more closely related to animals. This is a picture of an absidia mold with a mature sporangial form at the end. And like animals, they must eat, and they can reproduce either asexually by fragmentation, budding, or they can produce spores. Fungi infect plants, and those kinds of infections are called blasts, rust, rots, scabs or smuts. And they can infect humans and other animals as well. So fungal cell structures are like other eukaryotic cells in that they have a nucleus and other specialized organelles. So, as I said, fungi can either come in yeast or molds, and you can compare that with bacteria. They are very small, but not quite as small as bacteria. So the classification of fungi are based on growth. So Candida albicans grows as a yeast, and you can have Candida species on your skin and overgrowth can lead to a condition called candidiasis. Histoplasma is another type of fungus, and it can exist as either a yeast or as a mold and that's called dimorphic. And histoplasma can live in the environment and particularly live in bird and bad droppings, and if you inhale dried bird or bat droppings, you can actually get sick from histoplasma if you don't have a competent immune system. Aspergillus is another type of mold. And that lives indoors and outdoors and people with weak immune systems are also susceptible to getting Aspergillus infections. So histoplasmosis, again, can live in the droppings of different animals such as birds and bats. If they become dried and aerosolized, you can inhale it and you can get sick. And certain areas are more endemic for histoplasmosis than other areas. So there are different classes of antifungal drugs, and they're listed here. Now, parasites are organisms that are similar to viruses in that they can exist inside or on host but unlike viruses, they are living. So they use the host bodies to be able to reproduce themselves. And there's many types of parasites. You can have the malarial parasite, which is spread by mosquitoes and can invade red blood cells, or you can have intestinal parasites like Giardia lamblia that can be transmitted by contaminated food or water. So diphyllobothrium latum is an adult worm, and you can get infected with this worm by inadvertently ingesting its eggs in contaminated food. These worms can grow to be more than 10 metres long or about 30 feet. Trichinella species are another type of parasite and you can also get them by ingesting contaminated food and they live inside muscle tissues as pictured here. So these organisms can be ingested and then they will invade your small intestine where they develop into adult worms. So the symptoms can be quite severe including diarrhea, abdominal pain, fever, muscle pains, weakness, and rashes. And this infection can be life threatening. So what's interesting is despite how bad worms can be in causing symptoms, for much of human existence, people have actually lived with these parasites in their guts. And there is some hypothesis that they can perhaps be helpful in dealing with inflammatory bowel diseases, because some people with inflammatory bowel disease will actually go into remission when they ingest these eggs. But in some people, the parasite seems to have a calming effect on their immune system and alleviates a lot of the symptoms of inflammatory bowel disease where the immune system attacks the small and large intestine in autoimmune diseases. Prions are misfolded proteins. And they are self-propagating, but they are technically like viruses not alive. So they can be infectious meaning that you can acquire them by eating contaminated food, mainly meat. They can be inherited or they can be spontaneous arising for unknown reasons. So you can think of a prion like a slinky. For those of you who've had slinkish kids, slinkies can be very smooth functioning toys. But if they get kinked or if they get twisted, they can never be returned back to this original form. In other words, it's a kinked slinky. So prion diseases are also called spongiform encephalopathies. And unfortunately, these diseases are always fatal. There's no vaccine, and there's no treatment. So pictured here is normal brain tissue. And you can see that brain tissue that's been contaminated with prions looks like it's full of holes or like a sponge. So the questions for this session then are, what are zoonotic diseases? What's the difference between a virus, a bacteria, a fungus, a parasite and a prion? Why don't antibiotics work against viruses? How do vaccines work? Why is herd immunity important? How do intestinal parasites get transmitted? Is there a vaccine or treatment for prion diseases? And with that, I'd like to thank you for watching this session.
