this is chapter 15 microbial mechanisms of pathogenicity so in this chapter we'll look at some specific properties of microbes that contribute to their ability to cause disease so microbes aren't trying to hurt us or make us sick they're just living their life and getting food and defending themselves so sometimes just the mere presence of microbes can be enough to induce symptoms in the host most microbial properties that contribute to their pathogenicity aren't really well known but what we do know is that if the microbe is able to overpower the host defenses then disease results when we talk about pathogenicity we're just talking about the ability of a microorganism to cause disease so is it pathogenic or non-pathogenic virulence is the degree of pathogenicity so some microbes are more virulent or more pathogenic than others so most microbes have a preferred portal of entry or way that they're able to gain entrance inside of the host mucous membranes are one of the most common portals of entry so remember all of our open body cavities are lined with these mucous membranes so the easiest way to get into the body is pretty much right through the front door right so the respiratory tract is usually the easiest so just by simple inhalation to the respiratory system and then it has access to the rest of the body the digestive system is another portal of entry although most microbes will be killed by the hydrochloric acid in the stomach but there are some that can make it through and cause infection the genital urinary tract so for urinary tract infections or sexually transmitted infections as well as the conjunctiva or the membrane of the eye so the conjunctival membrane of the eye is also connected to the respiratory system so our tear our tear ducts drain out into the nasal cavity so something could potentially land on your eye right and get into your respiratory system through that portal of entry the skin is a pretty good impenetrable barrier as long as it's unbroken remember the keratin and the skin cells helps make them more durable and waterproof and also some skin secretions right help to prevent microbial attachment to the skin surfaces however some microbes can still enter through open hair follicles or sweat gland ducts some fungal cells can also just grow on keratin in the skin itself so things like ringworm when we talk about the parenteral route this is when the microbe or the infection has been deposited directly into the tissues when our those barriers have been penetrated or injured so things like punctures injections bites cuts surgery anything that damages the skin barrier where now those microbes can gain entry so even after microbes have entered the body they don't necessarily cause disease the pathogenicity of a microbe is going to depend on several factors one of which is which portal of entry is used so we said most pathogens have a preferred portal of entry where they're able to cause the most damage so their preferred portal of entry required for them to be able to cause disease so one example would be streptococci that are inhaled can cause pneumonia but if you were to swallow them right you're not going to get pneumonia so they're just going to be killed by the acid in the stomach so they have to enter through the respiratory system however some pathogens can cause disease from multiple portals of entry so for example anthrax so you can become infected with anthrax through ingestion or inhalation as well as cutaneous infection or contact infection generally if only a few microbes enter the body they'll probably be overcome by our host defenses however if a large number gains entry in a relatively short period of time they're going to be more likely to overpower our host defenses so with the covid 19 pandemic one concern was for healthcare professionals right that are getting higher viral loads or levels of exposure to the virus that's going to just overpower their defenses whereas with a normal infection you may only be exposed to a small amount of viral particles that you're able to kind of build a defense against right before they can increase their numbers however if you're just constantly bombarded with those viral particles you would just get overwhelmed your immune system would get overwhelmed trying to overcome those so we look at the id 50 or the infectious dose for 50 of a population so this helps us to measure the virulence of the microbe but again this can vary based on the portal of entry so the previous example we said anthrax right has multiple portals of entry but the id 50 right or the virulence of the anthrax microbe is going to vary based on this portal of entry how it entered the body so if the portal of entry is the skin it only takes a relatively small number of those endospores or infectious cells to cause infection in at least 50 percent whereas via ingestion you're going to have to ingest a large much larger amount of endospores right to become infected the ld50 is the lethal dose for 50 of a population so this is more so measuring the potency of a toxin so different toxins have different potency so botulinum toxin requires 0.03 nanograms to be lethal for 50 whereas a staphylococcal enterotoxin much higher amount so first off in order to infect a host cell the pathogen has to be able to adhere or stick to the host cell to gain entry so this is called adherence so almost all pathogens have to attach to our host tissues in this process called adherence or adhesion so think like an adhesive like tape greater glue it's just going to help the microbes stick to the host cell to cause this infection adherence is done by proteins called adhesins or ligands on the pathogen that bind to receptors on the host cells so these adhesins could be found on external structures like the glycocalyx or other surface structures like the fimbriae or pilot so this is showing some of these adhesion proteins on the outer surface of our pathogen cell that are able to bind or adhere and stick to receptors on the surface of our host cell so now this pathogen can gain entry and cause infection of our host cells along with adherent some microbes may form biofilms so basically these large aggregate communities of microbes that can share nutrients and increase the degree of adhesion and ultimately become resistant or more resistant to phagocytosis so this is showing some adherence of e coli bacteria with pili on the urinary tract in a urinate tract infection so they're able to stick to those urinary tract cells and adhere so they're not as easily washed out with the urine some pathogens can damage the surface of the host cell but most are going to have to penetrate the actual tissues to cause disease so looking at how some bacterial pathogens can penetrate or evade through our host defenses one mechanism is production of capsules so we talked about capsules before there are these kind of thick hardened glycocalyx cases sugar cases around the cell wall so these are to help inhibit phagocytosis it makes the cells more resistant to be being eaten so our macrophages our immune system cells aren't able to kind of bite through this hard shell this hard capsule so some common types of pathogens that produce these capsules are strep pneumoniae so pneumonia anthrax plague and meningitis causing bacteria let's remember from lab we did the capsule stain so the capsules appear as these clear little halos around our stained cells certain cell wall components may also help our bacterial pathogens penetrate through our host defenses so one example would be the m protein so these are kind of spike proteins on the surface of some cells that are going to help mediate attachment to the host cell while resisting phagocytosis so similar to the capsule it just makes it harder for our macrophages and immune cells to kind of bite down and eat these pathogens some cell walls may be composed of this waxy lipid called mycolic acid so our acid fast bacteria that we saw in lab right in the mycobacterium so this waxy lipid cell wall is just going to help resist digestion by immune cells so as we saw in lab acid fats bacteria mycobacteria have their own special type of stain that's able to penetrate through that thick waxy glycolic acid the bacteria can produce enzymes to help them penetrate through the host defenses so an example are coagulases and kinases so remember ace tells us as an enzyme and enzymes are usually named for what they do so a coagulase is going to cause coagulation right or clotting of the fibrinogen proteins and to form these protective clots so we kind of wall off the bacterial wall off itself to protect it against our immune system kinases can then digest and break down these clots and release the bacteria into the bloodstream hyaluronidase and collagenase are enzymes that work to digest and break down the tissues to help bacteria penetrate through the tissue layer so we're going to break down the polysaccharides that hold the cells together in those epithelial sheets right those tissue sheets and then collagenase enzymes break down the collagen holding the connective tissue under the skin so now the bacteria have made an opening and they're able to now invade those deeper tissues iga proteases are enzymes that destroy iga protein antibodies so iga antibodies are naturally produced in our body secretions to help prevent adhesion to the mucosa right so it's in our saliva and our mucus so some bacteria can produce enzymes that specifically target and break down these antibodies remember everything goes back to structure reflects function so if we destroy the structure of these antibodies they'll no longer be able to protect against infection many pathogens can use antigenic variation to alter their surface antigens to help them penetrate or evade our normal host offenses so antibodies are specific specifically shape to recognize and bind to specific shaped antigens right or proteins on the surface of these pathogens so a lot of pathogens viruses and bacteria are able to mutate and change the shape of their surface proteins and antigens so now our antibodies are basically rendered ineffective so this is one reason why we have to get a new flu shot every year so it mutates so the antibodies that you developed last year from last year's flu shot right we'll no longer recognize the new shape of this year's flu strain some pathogens can actually penetrate into the host cell cytoskeleton so they produce these surface proteins called invasives that allow them to invade into the host cell by disrupting the cytoskeleton and it causes what's called membrane ruffling so it kind of ruffles and fluffs out the membrane so the phytoskeleton kind of stitching and holding the membrane and the cell together forming these little openings and pockets where now the bacteria can enter the cell some pathogens are able to actually survive inside our phagocytes so they get into the cell and our immune cells our phagocytes are able to capture them normally we would fuse our captured pathogen with a lysosome containing digestive enzymes right break it down and then exocytosis the waste products but some pathogens are able to alter the ph inside the phagolysosome rendering those enzymes ineffective allowing it to survive some are able to escape from the phagosome before it fuses with the lysosome so it's never exposed to those lysozyme enzymes some pathogens can prevent fusion of the lysosome so again it's never in contact with those digestive enzymes so just because a pathogen has been captured by a phagocyte doesn't necessarily mean that that's the end of that particular pathogen so once pathogens have entered into the host cell and evaded our defenses so in order to cause infection they then have to go on to cause damage or harm to those host cells so one way that bacterial pathogens can damage the host cells is by using the host nutrients so if the pathogen uses up all of your nutrients they're not available for your cells so your metabolism and your cells aren't able to function properly so one way that bacteria can kind of steal our nutrients and resources is with sidereports so iron is important for most pathogenic bacteria to reproduce so they produce cydera four proteins that are just going to bind iron more tightly than our host cells right so the bacteria are able to get all of the iron for themselves right before the host cells have a chance to pathogens can also cause direct damage to the host cell by disrupting normal whole cell function using the whole cell nutrients producing harmful waste products some pathogens can also reproduce inside the host cells and cause them to rupture however most whole cell damage is going to be caused by toxins the toxins are poisonous substances that are produced by microorganisms so these toxins are usually the primary factor of an organism's pathogenic properties so toxins may produce symptoms like fever or cardiovascular problems digestive problems like diarrhea they can physically destroy cells and cause the body to go into shock toxigenicity is just the ability of a microbe to produce a toxin toxemia is presence of toxin in the host blood exoemia meaning blood toxins in the blood an intoxication is the presence of a toxin but without the microbial growth so the toxin is there but the actual microbe that produced that toxin is no longer there so this is showing two examples of two well-known toxins botulinum toxin and tetanus toxin so botulinum toxin causes a paralysis so it's going to stop muscle contraction right so normal mechanism we have the acetylcholine right binding to the muscle cell triggering that muscle contraction so botulinum toxin just blocks the release of that neurotransmitter so the muscle never gets the signal to contract and is essentially paralyzed so a potential bonus question on your next exam botulinum toxin is used in botox pretty common well-known cosmetic treatment for wrinkles you get some botox injections so what they're actually injecting into your face is botulinum toxin so bonus question what toxin is commonly used in cosmetic procedures botox or botulinum toxin tetanus toxin is the opposite so instead of causing paralysis of the muscle we're causing uncontrollable or excessive muscle contraction or a tetanus so the normal mechanism we have some inhibitory neurotransmitters to stop the muscle contraction so with tetanus toxin it's blocking those inhibitory neurotransmitters so we just get a constant influx of those stimulatory acetylcholine so the muscle cell keeps getting signals to contract and contract and contract and it never gets too relaxed exotoxins are proteins produced and secreted by bacteria so these are mostly gram-positive bacteria that are producing these exotoxins so exotoxins are generally soluble in body fluids and they can destroy host cells or inhibit metabolic functions so again exotoxin it's being secreted outside the cell right generally you're gram-positive bacteria antitoxins are antibodies against specific exotoxins so our immune system produces antibodies against certain antigens or surface proteins on cells to help protect us um they can also produce antibodies against specific toxins right so these are just called anti-toxins so but they work essentially the same way as antibodies they'll bind to the toxin and essentially render it ineffective so it's not able to go and contact any other cells toxoids are inactivated exotoxins that are used in vaccines like the tetanus vaccine a b toxins contain an enzyme component or an active component and a binary or b component so the a b exotoxin is released from your gram positive cell the b component binary component binds to a host cell receptor it gains entry into the host cell so now the a and b components will separate and the active enzyme component can now carry out its function generally some way of inhibiting protein synthesis of the host cell membrane disrupting toxins lyse the host cell by disrupting the plasma membrane or forming pores in the membrane so this is common in gas gangrene infections so leukocytins are these toxins that are going to kill phagocytic leukocytes right so some white blood cells hemolysins are going to lyse or kill erythrocytes or red blood cells by forming these channel proteins so essentially either too much can flow in and cause the cell to swell and burst or too much can flow out and cause the cell to shrink and die streptolysins are just specific type of hemolysins or blood lysing toxins produced by streptococci so we'll see in lab when we look at hemolysis on our blood augur plate right so we have the three different categories of hemolysis or the degree to which these bacteria are able to lyse and break down the blood cells so different species can contain different classes of hemolysin toxins right so each hemolysin or type of hemolysin enzyme is going to produce a different pattern right so we have our beta alpha and gamma hemolysis um so another potential bonus question for your next exam blood agar contains five percent sheep blood so what type of blood is used in blood auger sheet blood super antigens can cause an intense immune response due to your release of cytokines from the host cells or the t cells the immune system cells so this can lead to what's called a cytokine storm and have symptoms of fever nausea vomiting diarrhea shock and even death so essentially what happens is the super antigen causes kind of an over reaction of the immune system right so it's detecting that antigen in various locations of the body and has a massive release of these cytokines and causes these cytokine storms so this is one cause of concern in coven 19 infections is kind of the overzealous response of the immune system against this infection right to lead to these cytokine storms genotoxins are going to damage the genes or dna and cause mutations that can disrupt cell division or lead to cancer remember all cell structure and function is based on the genetic code right those instructions so if we change or mutate those instructions we could have abnormal cell function or uncontrolled abnormal cell growth or cancer endotoxins are produced within bacterial cells and make up part of that outer cell wall in our gram-negative bacteria we're talking about the lipopolysaccharide layer in gram-negative bacteria so they have that extra layer of cell wall so the lipid a in the lipo polysaccharide portion is that endotoxin so endotoxins are generally released either during replication or cell death so it's possible to have some endotoxins present even after you kill the cells so the endotoxins are more heat stable than the cells themselves so once the cell dies the cell wall breaks down these endotoxins are now going to be released so these endotoxins can cause the macrophages to release cytokines and lead to those cytokine storms and cause septic shock it can also cause disseminated intravascular coagulation or just capillary blood clots which lead to tissue death so the tissues aren't able to get oxygen because of all the blood clotting damage to blood vessel epithelia and can lead to these tissue deaths endotoxins can also stimulate the pyrogenic response or the fever response so genic means genesis or formation of pyro means fire right so pyrogenic response is just raising body temperature in a fever so macrophage ingest and gram negative bacteria kills it as the cell dies and breaks apart it's going to release those endotoxins that are going to cause the macrophage and the immune system cells to release cytokines so the cytokines travel through the blood where they are detected by the hypothalamus so the hypothalamus is like your body's thermostat so the detection of these cytokines these immune system alarm chemicals tell the brain there's an infection right so we need to crank up the thermostat and produce that fever so fever response can help the body to recover more quickly it can increase metabolism and can also inhibit some bacterial growth kind of slow them down if maybe we can sweat them out so summary exo versus endotoxins so your exotoxins are mostly from gram-positive endotoxins are only from gram-negative because they're part of that lipopolysaccharide exotoxins are usually protein-based right so like the a-b toxins endotoxins are lipid base so they're the lipid a of that lipopolysaccharide endotoxins are also more heat stable so they can withstand autoclaving whereas exotoxins are more easily destroyed and some examples of exotoxin diseases include gas gain green tetanus botulism right so the botox diphtheria and scarlet fever some examples of endotoxin diseases things like typhoid fever urinary tract infections and meningitis the limitless amoeba site lysate sa or the lal essay is used to test for endotoxins so this test used the blood of horseshoe crabs that contain special cells called amoebocytes these amoebosites will lyse um or die and clump and clot in the presence of the endotoxin and produce that clomping or those clots so similar to like when blood typing right so if it has a reaction the endotoxin is there the cells will start to clot up and we'll get that positive test result so horseshoe crab blood is blue because it contains a pigment a blue pigment called hemocyanin so our blood pigment is hemoglobin right heme there's the red pigment they just have a different color pigment in their blood to carry their oxygen but another potential fun fact bonus question for your next test um this horseshoe crab blood runs for about sixty thousand dollars a gallon so you ever need a backup job plan you can always go into uh bleeding out horseshoe crabs right for this limitless amoeba site lysate solution so bonus question how much does a gallon of horseshoe crab blood cost sixty thousand dollars so we've talked in earlier chapters about plasmids lysogeny and how that can contribute to pathogenicity so remember the plasmids carry the bonus gene so they're not part of the main chromosome they're kind of an extra chromosome that may carry genes for things like toxin production antibiotics antibiotic resistance and certain enzymes so through the process of lysogeny we had that lysogenic conversion we can incorporate another microbes dna of phage dna right into the host cell and have that prophage so now this prophage this recombined kind of hybrid cell can produce some of those new traits so the cell may now start to produce a capsule or some toxins so we can change the characteristics of the microbe due to an incorporation of this prophage or this recombined chromosome so the viral mechanism for evading host defenses is a bit different than bacteria because they essentially hide and grow inside the host cell right so this is one reason why antibiotics are not effective against viral infections because they hijack your host cell metabolic machinery so they're kind of hiding in your own cells so viruses are sneaky and that their attachment sites can mimic some of our regular neurotransmitters and substances like acetylcholine you can kind of trick ourselves into attaching to the viruses and letting them in the one way the hiv virus avoids immune detection is by attacking and killing the immune cells directly so if there's no immune system to fight off the infection then it can just spread freely but one mechanism for defense against these viral infections is production of interferons so interferons basically interfere with viral replication so interferons are produced by virally infected cells so one cell gets infected by the virus but kind of in its last dying breath it's going to release these interferon signals to neighboring cells to help protect them from infection to hopefully prevent the spread of this virus so interferons can signal neighboring uninfected cells to destroy rna and reduce protein synthesis so essentially lock your doors right before the virus gets there um it can signal to neighboring infected cells to undergo apoptosis or basically cell suicide before the virus has a chance to replicate more and spread more just go ahead and kill these infected cells early and interferons can also activate other immune cells to help counteract this infection many viruses display what are called cytopathic effects so these are just physically visible effects of a viral infection on a cell so viruses can affect cells by stopping cell synthesis they can cause lysosomes to release enzymes those digestive enzymes and they kind of just digest themselves they can create inclusion bodies in the cell cytoplasm so this picture is showing some infected cells that have dark inclusions within the nuclei that are representing the sites of viral assembly so these cells are infected with viruses and you can see these darker granules or inclusions where the viruses are being assembled and multiplying some viruses can cause cells to fuse together and create a sensation so synthetium is kind of just a single coordinated unit so we have a bunch of cells kind of fused together and coordinated or moving and functioning kind of a coordinated unit viruses can also change host cell function including chromosomal changes so they can cause mutations and lead to cancers or abnormal cell growth they can also induce antigenic changes on the cell surface and a loss of contact inhibition which can lead to cancer so contact inhibition is just a cell overgrowth so normally cells can detect kind of where other cells are so they don't overgrow too much so with this loss of contact inhibition the cells just kind of over grow on top of each other kind of in an uncontrolled manner so there are other microbes that can be pathogenic and cause disease and infection besides bacteria and viruses so look at some pathogenic properties of fungi protozoa helmets and algae so fungi mainly produce infection or disease by their toxic metabolic products and they inhibit protein synthesis normal cell function a lot of them can induce an allergic reaction or modify the host cell membranes some fungi can have capsules to prevent phagocytosis just how some bacteria can have capsules to prevent phagocytosis protozoa infections can be caused by either the presence of the protozoan and or their waste products right that can cause symptoms um so malaria is caused by a plasmodium right protozoan infection a lot of protists can avoid host defenses by either digesting the cells and tissue fluids they can grow within the phagocytes and they can also display antigenic variation so our immune system is not able to recognize them helmets are the parasitic worms that must use host tissues for growth so most common well-known example is elephantiasis so it's a helmet infection that blocks the lymphatic circulation so it kind of takes up residence in the lymphatic system and the lip vessels and blocks that fluid circulation so it builds up into these large masses right so helmets can cause extensive cellular damage they also can produce waste products that can cause symptoms as well algae can produce a neurotoxin called saxotoxin during red tide right so they say don't eat fish or shellfish from red tide because it can cause paralytic shellfish poisoning so these algae hatch in the ocean and then they're going to be filtered out by the shellfish so this shellfish are going to kind of secondarily carry this toxin when eaten by humans the portals of exit are generally the same as portals of entry so infections are going to be spread or leave the body right generally the same way organ system that they came in so respiratory tract illnesses you get through inhalation and then they're spread and exit the body through coughing and sneezing right also through the respiratory tract gi infections are spread through feces and saliva genito urinary tract can be passed through the urine or secretions from the genitals during things like stis skin infections can be spread through contact right so things like herpes ringworm hpv warts wound drainage or it can all be portals of exit to spread those skin infections um and bloodborne infections can be spread through arthropods or insects that bite shared needles or syringes or just contact with the blood so key concepts take away from this chapter there are several factors required for microbes to cause disease so after the microbe gains entry and enters the host most pathogens have to adhere to the host tissue and evade the host defenses pathogens are usually going to leave the body via the same portal of exit where they entered initially