Understanding Infection and Pathogenicity

In Chapter 25, we're going to talk about infection and pathogenicity. So in this chapter, we're going to look at how organisms are categorized, the virulence factors, define some terms like pathogenicity, opportunistic pathogens. look at the process of infection. We will look at both viral and bacterial pathogens. And of course, how do they evade our host, human host defenses. So in an infection, the state occurring when an organism is growing and multiplying on or in a host. So the host is actually the larger organism or the organism that is supporting the survival of a smaller organism. And an infectious disease is a change from a state of health as a result of an infection. And again, With infections, that organism is growing in or on the host. When you think about the term pathogen, this is any parasitic organism that produces an infectious disease. So it's any organism that will cause disease. Pathogenicity is the ability of an organism to cause disease. So. Remember back to our biosafety levels 1, 2, 3, and 4. Depending on the pathogenicity of the organism, the virulence of the organism, then that would help to tell us where it would be classified and how we need to treat that organism. When you think of opportunistic pathogens, these are organisms that are normally Okay, so they're normally free living or part of our host normal microbiota, but they can adopt a pathogenic role if they are in certain areas of the host body or even if they transfer to a weaker host. When you think of virulence, this is the degree or intensity of the pathogenicity of an organism. So it's really determined by three different characteristics. When you think of virulence factors, the individual characteristics really determine what The organism structure helps to determine the virulence. For example, capsules on Klebsiella pneumoniae increases their virulence. Pili on some organisms, some bacteria, toxins that the LPS will secrete or produce. When you think of the invasiveness of the organism, this is the ability of that organism to spread. to neighboring tissue, adjacent tissue. And the infectivity is the ability of that organism to establish a focal point of the infection. So a pathogenic potential is the degree to which the pathogen can cause morbid symptoms, like the toxogenicity. And when we look at virulence, we measure it experimentally. by the lethal dose 50 or the infectious dose 50. So how much of the medicine is needed to kill 50% of the population? That's your minimum lethal dose that we talked about before. Well, with the lethal dose of organism, What about 50% of the amount of organism that will kill the population of host? So again, lethal is killing. Infectious would be to infect that population of host. When you think about extracellular and intracellular pathogens, again, this is where you're looking at extracellular. They're growing outside of the host tissue or fluids and intracellular are within. We've talked about facultative anaerobes. or facultative aneromes. These are organisms that will grow better in oxygen, but they can grow without. So facultative intracellular pathogens are going to be able to be inside the cell of the host or in the environment, but they can also grow in a pure culture without support of host. Now, an obligate intracellular pathogen is one that has to grow inside a host organism. So that would be like a virus. They have to grow within a host. When you think about the course of an infection, this is figure 25.1 that you see here. And this is a figure you wanna be comfortable with. Again, this is so similar to our growth curve that we talk about in microbial growth. And then, of course, our immune response that we talked about back in Chapter 23. So, of course, you've got to be exposed to the pathogen, right? And so once you're exposed to the pathogen and that comes into the host, then you've got an incubation period. So this is kind of like that lag phase. This is the time where the pathogen is coming in. And this is really before signs and symptoms are shown. So there's a very low level of intensity of symptoms because we really don't have. have it up and running so to speak. Then we have the prodromal stage. The prodromal stage right here is shown by the signs and symptoms have become expressed. So fever, maybe diarrhea, nausea, whatever the case may be. But in the prodromal stage, it's really not clear enough to get a definitive diagnosis of what it is. Then we have the illness period. So this is all of this section. In this period, this is when the disease is actively going. This is when the patient is really at the most severe point of that disease, whatever it is. And at this time, you'll see at this upper portion here that either the illness will plateau patient will begin to feel better and get over the infection in the convalescent period or the severity will continue until the patient has died. Again, in that convalescent period, this is where signs and symptoms really begin to disappear and you're going to have recovery of the host organism. So I'm asking you guys to define signs and symptoms. So what is the difference in the two? When you think about signs, these are objective changes in the body. So these are things that you can see and measure. So fever, rash, so you can see the rash on the body. You can measure the fever, that type of thing. Whereas symptoms are subjective. So is your opinion of the... of the condition. So if a person has nausea or loss of appetite or pain, then it's subjective. You know, some people have a higher pain threshold than others, that type of thing. And then the disease syndrome is the set of characteristic signs and symptoms of that disease. There are some diseases that have specific signs and symptoms, and those are the things that you are looking for in your patient. So when you think about the resources, think about that you have the person infected has the pathogen. And, um... So when an individual has been infected with a particular bacteria, then that individual has the pathogen. They have the protection, the nutrients, and the energy to use. So all of this is being provided to the pathogen. Also... Infectious agents are going to be able to develop ways to access and exploit the resources. So this is a survival mechanism. We talked about that back with spore formation, biofilms that can be formed, also viruses injecting their genome into host genome. using host enzymes and proteins. So there are resources that the host will provide for the pathogen. When you think about an infection, think about the events that are taking place during that infection. So in this area of the text, we're looking at what happens when you're moving the pathogen from one host to another or even to a reservoir. So the virulence of that organism is important. How many invading organisms are present? The adhesion and invasion factors of that organism. So you know, if the virulence factor is high, if there are a lot of the organisms available to invade, then they're going to be, that organism will more than likely be successful. So organisms actually compete against a person's, an individual's normal microbiota. And because of the, again, the size. the size of the population, the virulence factor, all of those things can promote or help these organisms to overtake the normal microbiota. And then, of course, at that point, that's where disease would occur. The organism is going to produce molecules and chemicals that will directly destroy. the host cells, also the potential of destroying or damaging the immune cells and other molecules and tissues. So looking at transmission and entry of a microorganism into a host. So again, when you're thinking about the source of the pathogen, you can be exposed to pathogens from animate objects. So another human, another animal, from an inanimate object, food, water, a reservoir, a natural environmental location that you would be exposed to, or a vector. like a mosquito. We talked about Chagas disease earlier. That is pass through the kissing bug. So when pathogens are transported or transmitted, this can happen directly or indirectly. So if it is direct contact, then you could have airborne contact, that's physical contact. a vehicle, a vector. Pregnant women can also pass pathogens onto their unborn fetus. So this figure is in your textbook, and it really breaks down the transmission of infectious diseases. So when you're thinking about direct and indirect, Direct exposure would be, again, horizontal contact, kissing, sex, something of that matter. Airborne droplets, coughing or sneezing on each other. A vector, again, would be a bug, a mosquito, something like that. And vertical contact is from the mother to the fetus. When you think of indirect contact, and you all swabbed several things in exercise six of our lab, and so doorknobs, gas pumps, water faucet handles, those types of things, those are fomites. So contact with the fomite. There's also the fecal oral contamination. So people that may not wash their hands well after using the bathroom, maybe produce has not been cleaned properly, that type of thing, contaminated food and water. So it's very important to make sure that you wash your fruits and vegetables before consumption. You wash your hands after using the bathroom or handling raw meat, cooking, wash your hands before and after. So there's ways to ingest organisms that can cause infectious disease. And then airborne indirectly again. So droplets that you may be spreading out. And then, you know, somebody, you know, because we talked about, the CDC talked about how far droplets can be released from a sneeze or a cough. And it's a good distance. And so, you know, other people that are passing through that area could potentially inhale the infected droplet or the droplet that contains the. pathogen. And then of course dust exhaust from vehicles and things like that that can also kick up dust materials that could pass organisms, maybe spores, things of that sort. So when you're thinking about transmission and virulence, We want to look at the opportunity for the tropism. So the transmission alone is not going to cause that infection to occur, but the pathogen has to make contact with that host organism. So be able to be taken in or on by that host. Airborne transmission, again, this is from coughing, sneezing. You can have droplets, direct transmission. So, again, talking with each other, maybe sneezing, things of that sort in close contact. It can travel one meter, so up to two millimeters in diameter of these droplets, and they can travel a meter. So just think of a meter stick that we use. So it can travel quite a bit. A droplet nuclei, this is indirect transmission. So these are much smaller. Look at this, one to five micrometers in diameter. Think about when you... looked and did your micrometer measurement exercise for. So this is going to result from the evaporation of larger droplets. And it can remain airborne for hours or days and can travel a long distance. Dust particles, indirect transmission, so microorganisms that are adhered, they're attached. to dust particles, they can survive long periods and even outside the host. So contact transmission, again, thinking about this is where we can have a person-to-person contact. We can have an indirect contact either with an inanimate object like a fomite or food and water. just like we've talked about before. Vehicles, again, that would be an inanimate, the food, water, biological materials. And then, again, the vehicle transmission is a single vehicle, a single item that would spread the pathogen to multiple hosts. So, you know, going back to the figure. the indirect vehicles. You know, this could be shared to many people because, you know, if, you know, 20 people are going to be eating out of this dish, out of this food item, then you can spread that to several people. So, a vector-borne transmission vectors include living organisms, that transmit a pathogen. So these are insects. And they can direct living transmitter of a pathogen. So again, ticks, fleas would be your insects, also vertebrates, dogs, cats, bats, skunks, all of these organisms that can pass. a pathogen. When a pathogen is transmitted by an insect, these are typically very virulent. They have a high virulence factor at that point. So they're going to cause disease, again, like Chagas disease, malaria, West Nile virus, some of those. And important things. It's important to remember that pathogens do not harm their vectors. So that's why the vector is such a good transmitter. They're not damaged. Vertical transmission, again, from the mother to the fetus. This is horizontal. That can occur gonorrhea, syphilis, herpes, German measles, toxoplasmosis. some of these some of the examples with these. Earlier I mentioned infectious dose and lethal dose. So when you're thinking about the number of the population with the infection you've got the percent animals with illness. So it could be a mouse model. We'll just use that as an example here and then the number of organisms one times 10 to the third. So as you're plotting the number per, it's the dose per unit of time, so it's the number of cells per unit of time against how many animals are infected. So the lethal dose of 50 or strain A is 3,000. So it takes 3,000 organisms to be, I'm sorry, not lethal, infectious dose. It takes 3,000 organisms. of this strain A organism to infect 50% of the animal population. With strain B, it takes 5,000 units of dose. One times 10 to the third is 1,000, right? So it takes 3,000 of strain A for 50%. It takes 5,000 for strain B. to infect 50%. So it varies with the organism. Some organisms are more pathogenic than others. So this one is more pathogenic, more virulent than B, okay? When you're thinking of adhering and colonization, so think about how that organism is going to survive. in or on that host. So it's got to be exposed to the host. So the first step is whatever this organism is, it has to either enter, well, it has to enter and attach to the host. So... Many of these, if it's a bacterium, again, some can survive on the tissue and some would be inside, go deeper into the host. So the portal of entry for many of these are going to be the skin. Your skin is your number one barrier of innate immunity. Respiratory is another. big entry point, also the GI tract, the urogenital tract, and your conjunctiva of your eye. So many of these places will be where you can get exposure to the pathogen. Adherence. This is where there's some adhesin proteins that are going to help to adhere that pathogen to the host. And then colonization is where the microbe has found its site of infection and it's going to replicate on or within that host. It doesn't necessarily result in tissue invasion or damage when we talk about the colonization. So this figure here is showing you a couple of examples of how microbes can adhere to the host. So this is figure 25.4, and we're looking at the Fembrae of Bacteria here, and you can see that along the sides and how they are attaching to the host cell. Opembrite will do that. Capsules. Here are the capsules that are surrounding the cell body, and they adhere to the host cell really well. And then viral proteins or spikes. So the spikes. that will bind to these pitted, these coated pits that are on the host cell. And they'll adhere to that and then be brought in to that organism. So when you think about invasion of these pathogens, again, we talked about infectivity. a while ago and how that infection, how that organism can establish a focal point of the infection and the invasiveness. We mentioned that earlier with the virulence. So how able is that organism to spread to adjacent tissues? Now we want to look at penetration. It can either be active or passive. So in this point, if it is an active occurrence, Then lytic enzymes can alter the host tissue by either attacking the extracellular matrix or the basement membrane of integument and the epithelial tissue and the intestinal lining. These lytic enzymes can also break down carbohydrate and protein complexes. either between the cells or on the cell surface. And these lytic enzymes can disrupt the host cell surface. The passive invasion, this is really not related to the pathogen itself. So if you have a lesion on your skin, if you have been bitten by a mosquito or a tick, if you have a wound, some sort of a damaged tissue. So that would be passive invasion. So looking at some of the specific examples here. So here with invasion, we're looking at specific examples of these. So once that organism has invaded the deeper tissues and has produced specific products or enzymes that help promote spreading, and again, that depends on its virulence factor, and it enters into the circulatory system. then you can get growth and multiplication of that pathogen. So, bacteri, bacteriemia is where the bacteria is in the bloodstream. Septicemia is when toxins of that bacteria has been released into the bloodstream. This all depends on what organism that you're talking about. So different organisms are going to act, of course, differently. So Clostridium tetani, which would lead to tetanus, this produces a number of virulence factors, but it's non-invasive. Bacillus anthracis, anthrax, and Yersinia pestis, which is a plague, has a lot of virulence factors and is very invasive. So again, you can have some variation with organisms that would be that you would be exposed to or a person could be exposed to. Here is talking about Listeria and Shigella and Rickettsine. So in your textbook, this is figure 25.5, and this is showing you an actin tail of Listeria. So you can take electron microscopes and... Add color to the picture, as you can see, because they're really not pink and green. But that way you can identify them under scope. The actin tail is really going to move that bacterial cell to the host cell surface, and then it's going to protrude. These protrusions are then engulfed by adjacent cells. cells and that bacteria is going to be able to enter it or them. And so you're continuing to pass that infection on, invade tissue. Now, we've talked about some invading ideas and concepts. Now we want to look at how do these organisms survive? Host defenses. So host defenses against the microbial invasion, there's primary defenses, secondary defenses, factors that influence the host defense. So the primary defense is where most of the strategies are going to prevent the pathogen from coming in. period. So that's physical and chemical barriers, but these are not always 100%. So that goes, that takes us back to the innate immunity that we talked about in chapter 22. Secondary defenses, again, these are our innate defenses like complement. activation, inflammation, fever, phagocytosis, those are going to come into play. Then if that's not handled, then we're going to move on to the adaptive immunity. So in innate immunity, you have your first line and your second line of defense. And then your third line of defense is over with adaptive immunity. And then there's factors that influence the host defense, like the age of your patient, the stress, nutritional deficiencies, genetic background. So successful pathogens can overcome the competition that they have and then really get past host defenses and some of the adaptive. immune system. So how are they doing this? This is just showing you a few. So producing a type 6 secretion system, finding shelter that they're not going to be recognized by defense cells. They can survive and replicate inside the host cell. They can get between host cells. They can make capsules to avoid phagocytosis. They can actually kind of get buried up in mucus, secrete exopolysaccharides to form a communal shelter that'll protect them in biofilms. They can produce enzymes that are going to inactivate innate immunity and then excrete. proteins that will kill host cells. So let's look at some of the strategies that they use. So evasion of host defenses by viruses and bacteria, looking at both. So antigenic drift. This is going with mutations that cause change in the antigenic site of the virion. So influenza, the GP120 protein on HIV, infection of your T cells like HIV, fusion of host cells. So this is allowing a... spread from cell to cell without really exposing the virus to an antibody-containing cell or even where antibody is in the fluid, in the plasma, in the serum with the plasma cells secreted. HIV, measles, the cytomegalovirus. Infection of neurons, having little or no MHC molecules like herpes viruses. Production and release of antigens that bind neutralizing antibodies. So this is going to tie up neutralization. So this is going to block neutralizing those antibodies. And really, you have insufficient antibodies to bind that. Viral particle, hepatitis B is an example of that. Also, bacteria can evade host defenses. by completely evading complement. So these capsules can be produced that are going to prevent complement activation. There's also a lengthened O chain in the LPS region of your outer membrane of E. coli or a gram-negative bacteria. Serum resistance. These are features on the surface of bacteria that really prevent membrane attack complex. So remember your MHCs that can form pores in cells. You can get resistance of phagocytosis, again, of the capsules. specialized proteins that are synthesized by bacteria, phagolysosome formation, leukocytins, enzymes that will destroy complement. And so there's so much that can protect these organisms from host immune defense. So how do you go about suppressing? How do these organisms suppress host response? Again, as I mentioned before, this O antigen on LPS, this is really going to eliminate the complement activation. And by doing that, you're not going to increase phagocytosis. You're not going to be able to remove that bacteria as well as otherwise. We've talked about biofilms. We've talked about biofilms as an important feature to organisms. Really, evading host defenses. And you've got a lot of different types of organisms that are in biofilms. So this figure is 25.6 in your book. And when you look at this here, and you can go back and look at biofilms in our earlier section, just to refresh your memory of what they look like. But biofilms are several layers of organisms, of bacteria. There's going to be sugars that the bacteria have released. There can be fungus in there. There can be spores in there. So what you're seeing with the legend here is the planktonic. bacterium are in the pink and you see those around the biofilm bacterium are in the blue and in the phagocyte enzymes so the enzymes that will help with the breakdown so biofilm bacteria are really protected from the loss of nutrition predators, environmental changes, host immune systems, antimicrobial agents, all of that. Some pathogenic bacteria in the biofilms will exchange plasmids and nutrients with each other. There's a quorum sensing molecule that really helps and they alter their behavior. So biofilms are less sensitive to antibiotics and host defenses. So biofilms, just like we mentioned before, are very, very difficult to remove and to treat if a person has a biofilm. What kind of damage to the host? would you expect from some of these organisms? So again, looking at the pathogenicity of a particular organism, when you think about these larger segments of bacterial chromosome, 10 to 200 kilobases that you can find, they also have... plasmid DNA that's going to encode virulence factors. So all of this is going to increase bacterial virulence. They're absent in non-pathogenic species, but they can spread between a non-pathogenic through horizontal transfer of the virulence factors, the plasmids from bacterium to bacterium. So again, we mentioned the toxogenicity a little bit ago. And again, this is the capacity of an organism to produce a toxin. And we looked at a toxin being a specific substance. Often it's a metabolic product. of the organism and it's going to damage the host in some specific manner. And intoxications are diseases that result from the entry of a specific toxin into the host. And then a term used, toxemia, are symptoms that are caused by toxins in the blood of a host. When you think of exotoxins, these are soluble heat labile proteins that are produced and released from a pathogen. So they're generally associated with gram-positive bacteria, and they may damage the host at some remote site. Toxins can be inactivated by neutralizing antibodies, so an antitoxin. or by chemical means that can create immunogenic toxoids. So these are things that we use, that are used for vaccines. So endotox, or sorry, exotoxins can be grouped into four different types based on their structure and their physiological activities. So AB toxins can be separated into two distinct portions. One. that is going to bind the host cell and one that is going to cause the toxicity. So here we have A is the subunit, the A subunit. This is responsible for the toxic effect. And B is the one that's going to bind to the specific cell. So here's A. Up here at the top is A and B. Diphtheria toxin is an example. It's going to bind to the host cell surface receptor by the B portion, and you can see that here, and is taken into the cell by the clathrin-coated vesicle. So these clathrin proteins here are going to help coat that vesicle. So now it's a coated vesicle. And those clathrin proteins will be released. And this pH is a neutral pH at 7. And then it's going to fuse with or form into an endosome and fuse with a lysosome so that you can have that little bit acidic pH. And by doing that... Now the toxin, the exotoxin, can be broken down. So the toxin is cleaved. It releases the A fragment, and you can see that here. Get my cursor to go. So here it is in the endosome. It releases the A fragment. Then A is going to enter the cytosol, which we see this here. to inhibit protein synthesis. So it's going to block protein synthesis of the cell and so the cell's going to die. and this is showing you this is figure 25.7 here and then our b portion of the figure here is showing you the pores that are formed with those toxins inserted. Our next type of exotoxin is a specific host site exotoxin. And so this is a neurotoxin that's going to damage nerve tissue, botulinum toxin, and tetanus toxin. Enterotoxins damage the small intestine like cholera toxin. And cytotoxins do a general tissue damage, the Shigella, so Shiga toxin. Some of your host site specific exotoxins are also AB toxins, like cholera will act as an AB toxin as well. Membrane disrupting exotoxins. So these are two subtypes, those that bind cholesterol in the host cell. and that bind cholesterol in the host cell membrane and form a pore like a leukocytin and hemolycins. These are phospholipases. So they're going to result in gangrene, so gas gangrene association. And so you can see that in this picture here. And then, of course, super antigens. These are pathogen proteins like staphylococcal, enterotoxins. These are going to provoke a massive cytokine release, and they will cause endothelial cell damage, circulatory shock, multi-organ failure. with those. And this is a figure of the super antigens. So they're really stimulating about 30% of the T cells in the immune system. This is figure 25.8 in your book. And you can see this is your antigen presenting cell, so your macrophage. And it has the MHC class 2 on it. And then here's your super antigen. It's really not attached to the MHC. And so because of that, it causes the T cells to overexpress and then release pro-inflammatory cytokines. And because of that, that can help damage a lot of the organs. host organs. So endotoxins, LPS of your gram-negative bacteria, these are released only when the microorganism is lysed or it divides. These are usually capable of producing fever, shock, blood coagulation, weakness, diarrhea, inflammation, intestinal hemorrhaging. Um, so many of these effects. are indirect and are mediated by host molecules and cells like macrophages and endogenous pyrogens, host cytokines as well. The toxic component is the lipid A portion. You can go back and look at the structure of LPS to see the A antigen, the O antigen, and then the core. So again, endotoxins, they're generally heat stable. They don't take a lot to cause toxicity, so nanogram amounts. They're weekly in eliciting an immune response. So they're really going to cause general system effects, as I mentioned earlier, with all these different symptoms. So septic shock cascade. This is in figure 25.9. And I ask you guys to look at this to get. familiar with it. So gram-negative bacteria are what cause endotoxin triggers, and there's several events that lead to the complications of shock, of the septic shock. So the endotoxin is released. You get coagulation, complement, the fibro-nollectic. You get the endogenous mediators. So all of these cytokines that are being released, platelet activating factor, endorphins, complement system. Then that moves to... the myocardium so depression of the heart tissue the heart muscle dilation um vasodilation vasoconstriction leukocyte aggregation organs the kidney liver lung brain dysfunction metabolic defect and then it goes into shock refractory hypotension multiple organ failure And then, of course, that would lead to death. With some instances, septic shock can, patients can recover and survive those. Mycotoxins, these are toxins that are produced by fungi. Aspergillus is an example. And these are in, you'd find these more in contaminated food crops, maybe water damaged buildings where the fungus has grown. Exposure to the aflatoxins causes both chronic and acute liver disease. It can also lead to liver cancer. Aflatoxins. are extremely carcinogenic, mutagenic, and immunosuppressive. So about 18 different types of aflatoxins exist. And with these, they're either classified according to their chemical structure, and they will... They're inhibitors of DNA, RNA, and protein synthesis. So they're going to induce inflammation, disrupt surfactant, phospholipids in the lungs, and can lead to pathological changes in tissue. So that takes care of Chapter 25. Let me know if you have any questions.

We will look at both viral and bacterial pathogens. And of course, how do they evade our host, human host defenses. So in an infection, the state occurring when an organism is growing and multiplying on or in a host. So the host is actually the larger organism or the organism that is supporting the survival of a smaller organism. And an infectious disease is a change from a state of health as a result of an infection.

And again, With infections, that organism is growing in or on the host. When you think about the term pathogen, this is any parasitic organism that produces an infectious disease. So it's any organism that will cause disease.

Pathogenicity is the ability of an organism to cause disease. So. Remember back to our biosafety levels 1, 2, 3, and 4. Depending on the pathogenicity of the organism, the virulence of the organism, then that would help to tell us where it would be classified and how we need to treat that organism.

When you think of opportunistic pathogens, these are organisms that are normally Okay, so they're normally free living or part of our host normal microbiota, but they can adopt a pathogenic role if they are in certain areas of the host body or even if they transfer to a weaker host. When you think of virulence, this is the degree or intensity of the pathogenicity of an organism. So it's really determined by three different characteristics.

When you think of virulence factors, the individual characteristics really determine what The organism structure helps to determine the virulence. For example, capsules on Klebsiella pneumoniae increases their virulence. Pili on some organisms, some bacteria, toxins that the LPS will secrete or produce. When you think of the invasiveness of the organism, this is the ability of that organism to spread.

to neighboring tissue, adjacent tissue. And the infectivity is the ability of that organism to establish a focal point of the infection. So a pathogenic potential is the degree to which the pathogen can cause morbid symptoms, like the toxogenicity. And when we look at virulence, we measure it experimentally.

by the lethal dose 50 or the infectious dose 50. So how much of the medicine is needed to kill 50% of the population? That's your minimum lethal dose that we talked about before. Well, with the lethal dose of organism, What about 50% of the amount of organism that will kill the population of host?

So again, lethal is killing. Infectious would be to infect that population of host. When you think about extracellular and intracellular pathogens, again, this is where you're looking at extracellular.

They're growing outside of the host tissue or fluids and intracellular are within. We've talked about facultative anaerobes. or facultative aneromes.

These are organisms that will grow better in oxygen, but they can grow without. So facultative intracellular pathogens are going to be able to be inside the cell of the host or in the environment, but they can also grow in a pure culture without support of host. Now, an obligate intracellular pathogen is one that has to grow inside a host organism.

So that would be like a virus. They have to grow within a host. When you think about the course of an infection, this is figure 25.1 that you see here. And this is a figure you wanna be comfortable with. Again, this is so similar to our growth curve that we talk about in microbial growth.

And then, of course, our immune response that we talked about back in Chapter 23. So, of course, you've got to be exposed to the pathogen, right? And so once you're exposed to the pathogen and that comes into the host, then you've got an incubation period. So this is kind of like that lag phase.

This is the time where the pathogen is coming in. And this is really before signs and symptoms are shown. So there's a very low level of intensity of symptoms because we really don't have.

have it up and running so to speak. Then we have the prodromal stage. The prodromal stage right here is shown by the signs and symptoms have become expressed. So fever, maybe diarrhea, nausea, whatever the case may be. But in the prodromal stage, it's really not clear enough to get a definitive diagnosis of what it is.

Then we have the illness period. So this is all of this section. In this period, this is when the disease is actively going.

This is when the patient is really at the most severe point of that disease, whatever it is. And at this time, you'll see at this upper portion here that either the illness will plateau patient will begin to feel better and get over the infection in the convalescent period or the severity will continue until the patient has died. Again, in that convalescent period, this is where signs and symptoms really begin to disappear and you're going to have recovery of the host organism. So I'm asking you guys to define signs and symptoms. So what is the difference in the two?

When you think about signs, these are objective changes in the body. So these are things that you can see and measure. So fever, rash, so you can see the rash on the body.

You can measure the fever, that type of thing. Whereas symptoms are subjective. So is your opinion of the... of the condition. So if a person has nausea or loss of appetite or pain, then it's subjective.

You know, some people have a higher pain threshold than others, that type of thing. And then the disease syndrome is the set of characteristic signs and symptoms of that disease. There are some diseases that have specific signs and symptoms, and those are the things that you are looking for in your patient. So when you think about the resources, think about that you have the person infected has the pathogen.

And, um... So when an individual has been infected with a particular bacteria, then that individual has the pathogen. They have the protection, the nutrients, and the energy to use.

So all of this is being provided to the pathogen. Also... Infectious agents are going to be able to develop ways to access and exploit the resources. So this is a survival mechanism.

We talked about that back with spore formation, biofilms that can be formed, also viruses injecting their genome into host genome. using host enzymes and proteins. So there are resources that the host will provide for the pathogen. When you think about an infection, think about the events that are taking place during that infection. So in this area of the text, we're looking at what happens when you're moving the pathogen from one host to another or even to a reservoir.

So the virulence of that organism is important. How many invading organisms are present? The adhesion and invasion factors of that organism. So you know, if the virulence factor is high, if there are a lot of the organisms available to invade, then they're going to be, that organism will more than likely be successful.

So organisms actually compete against a person's, an individual's normal microbiota. And because of the, again, the size. the size of the population, the virulence factor, all of those things can promote or help these organisms to overtake the normal microbiota.

And then, of course, at that point, that's where disease would occur. The organism is going to produce molecules and chemicals that will directly destroy. the host cells, also the potential of destroying or damaging the immune cells and other molecules and tissues. So looking at transmission and entry of a microorganism into a host. So again, when you're thinking about the source of the pathogen, you can be exposed to pathogens from animate objects.

So another human, another animal, from an inanimate object, food, water, a reservoir, a natural environmental location that you would be exposed to, or a vector. like a mosquito. We talked about Chagas disease earlier.

That is pass through the kissing bug. So when pathogens are transported or transmitted, this can happen directly or indirectly. So if it is direct contact, then you could have airborne contact, that's physical contact.

a vehicle, a vector. Pregnant women can also pass pathogens onto their unborn fetus. So this figure is in your textbook, and it really breaks down the transmission of infectious diseases.

So when you're thinking about direct and indirect, Direct exposure would be, again, horizontal contact, kissing, sex, something of that matter. Airborne droplets, coughing or sneezing on each other. A vector, again, would be a bug, a mosquito, something like that. And vertical contact is from the mother to the fetus.

When you think of indirect contact, and you all swabbed several things in exercise six of our lab, and so doorknobs, gas pumps, water faucet handles, those types of things, those are fomites. So contact with the fomite. There's also the fecal oral contamination.

So people that may not wash their hands well after using the bathroom, maybe produce has not been cleaned properly, that type of thing, contaminated food and water. So it's very important to make sure that you wash your fruits and vegetables before consumption. You wash your hands after using the bathroom or handling raw meat, cooking, wash your hands before and after. So there's ways to ingest organisms that can cause infectious disease.

And then airborne indirectly again. So droplets that you may be spreading out. And then, you know, somebody, you know, because we talked about, the CDC talked about how far droplets can be released from a sneeze or a cough.

And it's a good distance. And so, you know, other people that are passing through that area could potentially inhale the infected droplet or the droplet that contains the. pathogen. And then of course dust exhaust from vehicles and things like that that can also kick up dust materials that could pass organisms, maybe spores, things of that sort.

So when you're thinking about transmission and virulence, We want to look at the opportunity for the tropism. So the transmission alone is not going to cause that infection to occur, but the pathogen has to make contact with that host organism. So be able to be taken in or on by that host.

Airborne transmission, again, this is from coughing, sneezing. You can have droplets, direct transmission. So, again, talking with each other, maybe sneezing, things of that sort in close contact.

It can travel one meter, so up to two millimeters in diameter of these droplets, and they can travel a meter. So just think of a meter stick that we use. So it can travel quite a bit.

A droplet nuclei, this is indirect transmission. So these are much smaller. Look at this, one to five micrometers in diameter. Think about when you...

looked and did your micrometer measurement exercise for. So this is going to result from the evaporation of larger droplets. And it can remain airborne for hours or days and can travel a long distance. Dust particles, indirect transmission, so microorganisms that are adhered, they're attached. to dust particles, they can survive long periods and even outside the host.

So contact transmission, again, thinking about this is where we can have a person-to-person contact. We can have an indirect contact either with an inanimate object like a fomite or food and water. just like we've talked about before. Vehicles, again, that would be an inanimate, the food, water, biological materials.

And then, again, the vehicle transmission is a single vehicle, a single item that would spread the pathogen to multiple hosts. So, you know, going back to the figure. the indirect vehicles. You know, this could be shared to many people because, you know, if, you know, 20 people are going to be eating out of this dish, out of this food item, then you can spread that to several people. So, a vector-borne transmission vectors include living organisms, that transmit a pathogen.

So these are insects. And they can direct living transmitter of a pathogen. So again, ticks, fleas would be your insects, also vertebrates, dogs, cats, bats, skunks, all of these organisms that can pass. a pathogen. When a pathogen is transmitted by an insect, these are typically very virulent.

They have a high virulence factor at that point. So they're going to cause disease, again, like Chagas disease, malaria, West Nile virus, some of those. And important things. It's important to remember that pathogens do not harm their vectors.

So that's why the vector is such a good transmitter. They're not damaged. Vertical transmission, again, from the mother to the fetus. This is horizontal.

That can occur gonorrhea, syphilis, herpes, German measles, toxoplasmosis. some of these some of the examples with these. Earlier I mentioned infectious dose and lethal dose.

So when you're thinking about the number of the population with the infection you've got the percent animals with illness. So it could be a mouse model. We'll just use that as an example here and then the number of organisms one times 10 to the third. So as you're plotting the number per, it's the dose per unit of time, so it's the number of cells per unit of time against how many animals are infected.

So the lethal dose of 50 or strain A is 3,000. So it takes 3,000 organisms to be, I'm sorry, not lethal, infectious dose. It takes 3,000 organisms. of this strain A organism to infect 50% of the animal population. With strain B, it takes 5,000 units of dose.

One times 10 to the third is 1,000, right? So it takes 3,000 of strain A for 50%. It takes 5,000 for strain B.

to infect 50%. So it varies with the organism. Some organisms are more pathogenic than others.

So this one is more pathogenic, more virulent than B, okay? When you're thinking of adhering and colonization, so think about how that organism is going to survive. in or on that host.

So it's got to be exposed to the host. So the first step is whatever this organism is, it has to either enter, well, it has to enter and attach to the host. So... Many of these, if it's a bacterium, again, some can survive on the tissue and some would be inside, go deeper into the host. So the portal of entry for many of these are going to be the skin.

Your skin is your number one barrier of innate immunity. Respiratory is another. big entry point, also the GI tract, the urogenital tract, and your conjunctiva of your eye. So many of these places will be where you can get exposure to the pathogen.

Adherence. This is where there's some adhesin proteins that are going to help to adhere that pathogen to the host. And then colonization is where the microbe has found its site of infection and it's going to replicate on or within that host. It doesn't necessarily result in tissue invasion or damage when we talk about the colonization. So this figure here is showing you a couple of examples of how microbes can adhere to the host.

So this is figure 25.4, and we're looking at the Fembrae of Bacteria here, and you can see that along the sides and how they are attaching to the host cell. Opembrite will do that. Capsules. Here are the capsules that are surrounding the cell body, and they adhere to the host cell really well. And then viral proteins or spikes.

So the spikes. that will bind to these pitted, these coated pits that are on the host cell. And they'll adhere to that and then be brought in to that organism.

So when you think about invasion of these pathogens, again, we talked about infectivity. a while ago and how that infection, how that organism can establish a focal point of the infection and the invasiveness. We mentioned that earlier with the virulence.

So how able is that organism to spread to adjacent tissues? Now we want to look at penetration. It can either be active or passive.

So in this point, if it is an active occurrence, Then lytic enzymes can alter the host tissue by either attacking the extracellular matrix or the basement membrane of integument and the epithelial tissue and the intestinal lining. These lytic enzymes can also break down carbohydrate and protein complexes. either between the cells or on the cell surface. And these lytic enzymes can disrupt the host cell surface. The passive invasion, this is really not related to the pathogen itself.

So if you have a lesion on your skin, if you have been bitten by a mosquito or a tick, if you have a wound, some sort of a damaged tissue. So that would be passive invasion. So looking at some of the specific examples here.

So here with invasion, we're looking at specific examples of these. So once that organism has invaded the deeper tissues and has produced specific products or enzymes that help promote spreading, and again, that depends on its virulence factor, and it enters into the circulatory system. then you can get growth and multiplication of that pathogen. So, bacteri, bacteriemia is where the bacteria is in the bloodstream.

Septicemia is when toxins of that bacteria has been released into the bloodstream. This all depends on what organism that you're talking about. So different organisms are going to act, of course, differently. So Clostridium tetani, which would lead to tetanus, this produces a number of virulence factors, but it's non-invasive. Bacillus anthracis, anthrax, and Yersinia pestis, which is a plague, has a lot of virulence factors and is very invasive.

So again, you can have some variation with organisms that would be that you would be exposed to or a person could be exposed to. Here is talking about Listeria and Shigella and Rickettsine. So in your textbook, this is figure 25.5, and this is showing you an actin tail of Listeria.

So you can take electron microscopes and... Add color to the picture, as you can see, because they're really not pink and green. But that way you can identify them under scope. The actin tail is really going to move that bacterial cell to the host cell surface, and then it's going to protrude. These protrusions are then engulfed by adjacent cells.

cells and that bacteria is going to be able to enter it or them. And so you're continuing to pass that infection on, invade tissue. Now, we've talked about some invading ideas and concepts. Now we want to look at how do these organisms survive?

Host defenses. So host defenses against the microbial invasion, there's primary defenses, secondary defenses, factors that influence the host defense. So the primary defense is where most of the strategies are going to prevent the pathogen from coming in.

period. So that's physical and chemical barriers, but these are not always 100%. So that goes, that takes us back to the innate immunity that we talked about in chapter 22. Secondary defenses, again, these are our innate defenses like complement.

activation, inflammation, fever, phagocytosis, those are going to come into play. Then if that's not handled, then we're going to move on to the adaptive immunity. So in innate immunity, you have your first line and your second line of defense. And then your third line of defense is over with adaptive immunity.

And then there's factors that influence the host defense, like the age of your patient, the stress, nutritional deficiencies, genetic background. So successful pathogens can overcome the competition that they have and then really get past host defenses and some of the adaptive. immune system. So how are they doing this? This is just showing you a few.

So producing a type 6 secretion system, finding shelter that they're not going to be recognized by defense cells. They can survive and replicate inside the host cell. They can get between host cells.

They can make capsules to avoid phagocytosis. They can actually kind of get buried up in mucus, secrete exopolysaccharides to form a communal shelter that'll protect them in biofilms. They can produce enzymes that are going to inactivate innate immunity and then excrete. proteins that will kill host cells. So let's look at some of the strategies that they use.

So evasion of host defenses by viruses and bacteria, looking at both. So antigenic drift. This is going with mutations that cause change in the antigenic site of the virion.

So influenza, the GP120 protein on HIV, infection of your T cells like HIV, fusion of host cells. So this is allowing a... spread from cell to cell without really exposing the virus to an antibody-containing cell or even where antibody is in the fluid, in the plasma, in the serum with the plasma cells secreted.

HIV, measles, the cytomegalovirus. Infection of neurons, having little or no MHC molecules like herpes viruses. Production and release of antigens that bind neutralizing antibodies. So this is going to tie up neutralization. So this is going to block neutralizing those antibodies.

And really, you have insufficient antibodies to bind that. Viral particle, hepatitis B is an example of that. Also, bacteria can evade host defenses. by completely evading complement. So these capsules can be produced that are going to prevent complement activation.

There's also a lengthened O chain in the LPS region of your outer membrane of E. coli or a gram-negative bacteria. Serum resistance. These are features on the surface of bacteria that really prevent membrane attack complex.

So remember your MHCs that can form pores in cells. You can get resistance of phagocytosis, again, of the capsules. specialized proteins that are synthesized by bacteria, phagolysosome formation, leukocytins, enzymes that will destroy complement.

And so there's so much that can protect these organisms from host immune defense. So how do you go about suppressing? How do these organisms suppress host response? Again, as I mentioned before, this O antigen on LPS, this is really going to eliminate the complement activation. And by doing that, you're not going to increase phagocytosis.

You're not going to be able to remove that bacteria as well as otherwise. We've talked about biofilms. We've talked about biofilms as an important feature to organisms. Really, evading host defenses. And you've got a lot of different types of organisms that are in biofilms.

So this figure is 25.6 in your book. And when you look at this here, and you can go back and look at biofilms in our earlier section, just to refresh your memory of what they look like. But biofilms are several layers of organisms, of bacteria. There's going to be sugars that the bacteria have released. There can be fungus in there.

There can be spores in there. So what you're seeing with the legend here is the planktonic. bacterium are in the pink and you see those around the biofilm bacterium are in the blue and in the phagocyte enzymes so the enzymes that will help with the breakdown so biofilm bacteria are really protected from the loss of nutrition predators, environmental changes, host immune systems, antimicrobial agents, all of that.

Some pathogenic bacteria in the biofilms will exchange plasmids and nutrients with each other. There's a quorum sensing molecule that really helps and they alter their behavior. So biofilms are less sensitive to antibiotics and host defenses. So biofilms, just like we mentioned before, are very, very difficult to remove and to treat if a person has a biofilm.

What kind of damage to the host? would you expect from some of these organisms? So again, looking at the pathogenicity of a particular organism, when you think about these larger segments of bacterial chromosome, 10 to 200 kilobases that you can find, they also have...

plasmid DNA that's going to encode virulence factors. So all of this is going to increase bacterial virulence. They're absent in non-pathogenic species, but they can spread between a non-pathogenic through horizontal transfer of the virulence factors, the plasmids from bacterium to bacterium. So again, we mentioned the toxogenicity a little bit ago. And again, this is the capacity of an organism to produce a toxin.

And we looked at a toxin being a specific substance. Often it's a metabolic product. of the organism and it's going to damage the host in some specific manner. And intoxications are diseases that result from the entry of a specific toxin into the host.

And then a term used, toxemia, are symptoms that are caused by toxins in the blood of a host. When you think of exotoxins, these are soluble heat labile proteins that are produced and released from a pathogen. So they're generally associated with gram-positive bacteria, and they may damage the host at some remote site.

Toxins can be inactivated by neutralizing antibodies, so an antitoxin. or by chemical means that can create immunogenic toxoids. So these are things that we use, that are used for vaccines. So endotox, or sorry, exotoxins can be grouped into four different types based on their structure and their physiological activities.

So AB toxins can be separated into two distinct portions. One. that is going to bind the host cell and one that is going to cause the toxicity. So here we have A is the subunit, the A subunit.

This is responsible for the toxic effect. And B is the one that's going to bind to the specific cell. So here's A.

Up here at the top is A and B. Diphtheria toxin is an example. It's going to bind to the host cell surface receptor by the B portion, and you can see that here, and is taken into the cell by the clathrin-coated vesicle.

So these clathrin proteins here are going to help coat that vesicle. So now it's a coated vesicle. And those clathrin proteins will be released. And this pH is a neutral pH at 7. And then it's going to fuse with or form into an endosome and fuse with a lysosome so that you can have that little bit acidic pH. And by doing that...

Now the toxin, the exotoxin, can be broken down. So the toxin is cleaved. It releases the A fragment, and you can see that here. Get my cursor to go.

So here it is in the endosome. It releases the A fragment. Then A is going to enter the cytosol, which we see this here.

to inhibit protein synthesis. So it's going to block protein synthesis of the cell and so the cell's going to die. and this is showing you this is figure 25.7 here and then our b portion of the figure here is showing you the pores that are formed with those toxins inserted.

Our next type of exotoxin is a specific host site exotoxin. And so this is a neurotoxin that's going to damage nerve tissue, botulinum toxin, and tetanus toxin. Enterotoxins damage the small intestine like cholera toxin. And cytotoxins do a general tissue damage, the Shigella, so Shiga toxin. Some of your host site specific exotoxins are also AB toxins, like cholera will act as an AB toxin as well.

Membrane disrupting exotoxins. So these are two subtypes, those that bind cholesterol in the host cell. and that bind cholesterol in the host cell membrane and form a pore like a leukocytin and hemolycins. These are phospholipases. So they're going to result in gangrene, so gas gangrene association.

And so you can see that in this picture here. And then, of course, super antigens. These are pathogen proteins like staphylococcal, enterotoxins. These are going to provoke a massive cytokine release, and they will cause endothelial cell damage, circulatory shock, multi-organ failure.

with those. And this is a figure of the super antigens. So they're really stimulating about 30% of the T cells in the immune system.

This is figure 25.8 in your book. And you can see this is your antigen presenting cell, so your macrophage. And it has the MHC class 2 on it. And then here's your super antigen.

It's really not attached to the MHC. And so because of that, it causes the T cells to overexpress and then release pro-inflammatory cytokines. And because of that, that can help damage a lot of the organs.

host organs. So endotoxins, LPS of your gram-negative bacteria, these are released only when the microorganism is lysed or it divides. These are usually capable of producing fever, shock, blood coagulation, weakness, diarrhea, inflammation, intestinal hemorrhaging. Um, so many of these effects.

are indirect and are mediated by host molecules and cells like macrophages and endogenous pyrogens, host cytokines as well. The toxic component is the lipid A portion. You can go back and look at the structure of LPS to see the A antigen, the O antigen, and then the core. So again, endotoxins, they're generally heat stable. They don't take a lot to cause toxicity, so nanogram amounts.

They're weekly in eliciting an immune response. So they're really going to cause general system effects, as I mentioned earlier, with all these different symptoms. So septic shock cascade. This is in figure 25.9.

And I ask you guys to look at this to get. familiar with it. So gram-negative bacteria are what cause endotoxin triggers, and there's several events that lead to the complications of shock, of the septic shock.

So the endotoxin is released. You get coagulation, complement, the fibro-nollectic. You get the endogenous mediators.

So all of these cytokines that are being released, platelet activating factor, endorphins, complement system. Then that moves to... the myocardium so depression of the heart tissue the heart muscle dilation um vasodilation vasoconstriction leukocyte aggregation organs the kidney liver lung brain dysfunction metabolic defect and then it goes into shock refractory hypotension multiple organ failure And then, of course, that would lead to death. With some instances, septic shock can, patients can recover and survive those.

Mycotoxins, these are toxins that are produced by fungi. Aspergillus is an example. And these are in, you'd find these more in contaminated food crops, maybe water damaged buildings where the fungus has grown. Exposure to the aflatoxins causes both chronic and acute liver disease. It can also lead to liver cancer.

Aflatoxins. are extremely carcinogenic, mutagenic, and immunosuppressive. So about 18 different types of aflatoxins exist. And with these, they're either classified according to their chemical structure, and they will... They're inhibitors of DNA, RNA, and protein synthesis.

So they're going to induce inflammation, disrupt surfactant, phospholipids in the lungs, and can lead to pathological changes in tissue. So that takes care of Chapter 25. Let me know if you have any questions.

Transcript for:Understanding Infection and Pathogenicity

Transcript for:
Understanding Infection and Pathogenicity