This week we look at cellular pathogens. This lecture covers bacteria, fungi, unicellular protozoa, and worm parasites. All of the pathogens in this lecture are full fledged organisms. They are all made up of one or more cells bound by a plasma membrane. They all use DNA as their genetic material, transcribe it into RNA and translate the messenger RNA into proteins on ribosomes. They may have cell walls or other extracellular material external to the plasma membrane. Most live outside the cells of the host they infect, but a few are obligate intracellular parasites. They all have more information than mammalian viruses and thus more opportunities to code for infectious strategies and techniques for deflecting your immune system. We're beginning with bacteria. Bacteria are prokaryotes. They come in a wide variety of shapes and metabolic strategies. They've been around the longest of any organism, and we had the most innate mechanisms for recognizing them. Many TLR combinations target bacterial components from either the interstitial fluid outside the cell or from the interior of the phagolysosome. Bacterial components activate both the lectin and the alternative complement pathways which guard the circulatory systems. Both systems some in neutrophils and macrophages to phagocytize and digest bacteria. Now, this explains in part why it takes more than one bacterium to initiate an infection. If 99.9 percent of bacteria are caught by these systems, you're going to need a lot of bacteria to get a few through by chance. Our first strategy is simply that there are so many of them. Some bacteria can live in the soil on their own, but happily cause opportunistic infections if they can enter a nice, warm, nutrient-rich human body. In addition, bacteria have individual strategies and those common to a related group and that they use to thwart the immune system. Anthrax has a couple of strategies it shares with many bacteria and a few uniquely dirty tricks of its own. Our second strategy, hideout as a spore until opportunities present themselves. A third strategy, wrap yourself up in a capsule, making it hard for phagocytes to capture you, unless of course your capsule is coated with antibodies. Encapsulated versions of bacteria are far more virulent than unencapsulated. That was the basis of the Avery experiment demonstrating that DNA was the genetic material coating for the enzymes that produce the capsule. Let's look and see how they infect you. Anthrax is a gram positive prokaryote with capsule, which makes it more difficult for macrophages and neutrophils to phagocytize them. You can see here that they enter the host as spores, hatch, multiply, and often kill the host, but they're not contagious. That is, you're unlikely to get anthrax from someone infected with it. Spores may enter at the skin, which would be cutaneous anthrax producing a black lesion. Spores may enter the lungs inhalation, which is typically lethal. Inside a host, spores activate and produce three toxins designed to kill host cells, allowing bacteria to absorb their components. There's a delivery protein called protective antigen that binds a protein receptor on the host cell surface. That helps to pick up the other two toxins from outside the cell, transport them into an endosome, and then the endosome releases the toxins into the cytosol. One is called Edema factor, which hijacks calmodulin, resulting in the production of excess cyclic AMP and preventing the regulation of metabolic pathways. The other factor, lethal factor hydrolyzes a MAP kinase kinase. This is an enzyme in a signaling pathway necessary for cell survival, and that also helps to kill the cells. So basically, the cell can no longer function and dies by necrosis, hence the black skin lesions and the name of the disease, anthrax which comes from black. Once the bacteria have essentially used up all of the host resources, they have ineffective avoidance strategy to hang out and survive. That is, each bacterium produces a long-lived endospore. The bacterium duplicates its DNA, sets one copy aside in its own plasma membrane, and then deletes some of its genes and synthesizes a very resistant spore coat around the bacterium's DNA and the plasma membrane. The rest of the cell then decays leaving behind one infected spore per cell, as we can see here where the B.subtilis is doing the same thing. So a couple of fun facts before we move on. The anthrax attacks led to the development of forensic pathogen genetics, which currently allows us to pinpoint the source very specifically of any strain of a pathogen. Anthrax was the first disease study by Koch is used to formulate his disease identification postulates. The vaccines to protect sheep and cattle against anthrax were the very first veterinary vaccines developed. Currently for therapies, we have both active and passive immunization against anthrax, something we'll discuss in a later lecture. So that's it for anthrax. Our second example is tuberculosis. Mycobacterium tuberculosis is the version that infects humans. There are also other species that affect different mammals. In this case, they all have a common strategy. They produce a very thick resistant cell wall. This cell wall is not only resistant to phagocytes, it tends to stick the cells to each other in impenetrable clumps and the clumps are referred to as courting, shown here in these two different pictures. If we look at the cell wall, we can compare the cell wall to that of the anthrax. Anthrax has of course a gram-positive cell wall at an exterior capsule. Tuberculosis also has a thick peptidoglycan gram-positive wall, but in addition to that, it has a layer of something called a arabinogalactan which is a highly branched polysaccharide and connects this wall to mycolic acid. This is the thick waxy layer that's hard to penetrate. This thing is so hard to penetrate, it even has porins in it to allow bacteria to let material in and out. Here is a representation of the different varieties of mycolic acid that can be found in outside highly protective layer. They also produce anti-lysosome compounds that block lysosome maturation, reduce the pH in the endosomes and also block NF-kappa B activation and thus TH1 responses. So when a macrophage that has been essentially compromised in such a way magnetizes a bacterium, they may put it in their lysosomes, but the lysosomes don't digest the bacteria and the bacteria will literally hitch a ride in macrophages to various places around the body. This is strategy Number 5. We blocked the TH1 response, in so doing we render the lysosomes less effective at hydrolyzing the bacteria, and the bacteria then hitch you right in the macrophages. A review of the disease process typically begins when a person breathes in droplets infected from another infected person who's coughing, you can also get them from contaminated food. Hosts will usually respond to a TB threat with an all out TH1 response, DTH response, which usually kills the bacteria. The DTH response may trigger the formation of granulomas are large tubercles. However, the lytic enzymes from these tubercles often destroy neighboring lung tissue, resulting in caseous or cheese like lesions that calcify as they heal, and actually that is like the diagnostic trait that you get when you look at these things in an X-ray. In primary TB disease, 1-5 percent of the cases this occurs soon after infection. But in the majority of cases you get a latent infection with no obvious symptoms. These are maybe 2-23 percent of the latent cases, the dormant bacilli will eventually overcome immune suppression and produce active tuberculosis and is when a person is in an inactive case that they are actually contagious. That is, they have a chronic cough, bloody sputum, and weight loss, which is the source of the term consumption, which suggests the person is consuming themselves from within. While tubercular lesions are typically found in the lungs, they can be found throughout the body having hits derived with those macrophages and can attack bone and brain and other organs. The activity of the DTH T cells that resists tuberculosis form the basis of the tuberculin skin test. We discussed that in course 3 lecture 5. Immunosuppression, of course, increases the risk of getting an active lethal cases of TB and HIV has prompted a resurgence of this disease. So epidemiology, surprisingly, one-third of all humans are or were infected with TB. TB provides a significant of selective force on human populations in terms of designing an immune prevention for it. Four-fifths of all Africans are, or were infected. Even in the US, 5-10 percent of the population is infected. That is, they have had received a TB, probably overcome it and produce a positive skin test. Your immune defenses often specifically target that cell wall. For example, we have a version of CD1 that presents mycolic acid to Th cells. This has been specifically designed to pick up this component. Some gamma delta T cells specifically target this lipid during their gene rearrangement. We have NKT cells that also target TB infected cells and of course we have the formation of granulomas, which we discussed before, where we attempt to wall off macrophages that are harboring TB bacteria. So looking at prevention and treatment here again is a picture of an electron micrograph that shows the coding in the TB bacteria. People really aren't contagious until they are actively coughing. So one thing we do is look for TB bacteria in sputum. In that case, somebody is contagious and they need to be treated. In the post-World War II period, we developed antibiotics that were good at treating tuberculosis, but the treatment took over a year and if people stopped doing it, they began to develop resistant strains and today, multi-drug resistant strains of TB are a bane of treatment. The older forms of treatment, pre-antibiotic were to give people good nutrition, rest and outdoors. In particular, it's thought that the sunshine and the vitamin D were also helpful in fighting off the disease. There are also vaccines for both TB and Anthrax, and we will discuss them later. For now, I would like to emphasize that bacteria have an endless array of ways to get around our immune system. Gram-negative bacteria, including Cholera, shown in the background, produce a very versatile array of toxins. In an additional [inaudible] to your whole defense system, they even produce toxins that mimic cytokines. So strategy number 6, produce cytokine analogues that overstimulate the immune system, leading to self destruction. There is the converse strategy, strategy 7, produce cytokine analogs and encourage a Th2 response which is inadequate to get rid of the bacteria. Bacteria have other strategies and this is certainly not an exhaustive list of the ways they can kill you. Just remember, these are ancient pathogens. They are not your friends and they continue to evolve ways to get around your defenses.
