In this section, we're going to do a quick survey of some of the bad guys. That is, things that would like to make you the meal, pathogens. We're going to look at them in five different categories because in some ways, that's a good way to make sense about how they are structured, and therefore, what kind of attack you can level with them. So, in the first place, we're going to start out with this, our least favorites, that is viruses. Here we have a selection of unpleasant viruses. If you look at them, they all have in common, that they have some nucleic acid in the middle, but they're not cellular. They're not alive. They have protein coats on the outside. Well, those protein coats are there to get into you. When they get into you, your body has to find some way of telling them apart from you. We're going to see one of the principal ways we do that, is by looking at their information. So, basically, these guys are an information packet in a delivery system and well, anybody's only purpose in life is to reproduce. So, that's your viruses. They are not cellular. The next category is bacteria. This is a picture of a spirochaete, are organisms that we call prokaryotic. That is, if we look at their structure in detail, spirochaetes, for example, are the things that cause syphilis. Okay. Here is a diagram of a bacterium. You can see that it has its DNA not surrounded by a nuclear membrane. You probably can't tell this, but its ribosomes are different from yours and that's why we have antibiotics that work because they attack protein synthesis in prokaryotes and not eukaryotes. They have flagella that help move them around but they're nothing like your flagella, and one of the things that they have is a lot of stuff on the surface that allows you to tell them from you. This is a surface cut through the plasma membrane, here, and cell wall of a gram-positive bacteria. This thick green section is the peptidoglycan cell wall. This cell wall has biochemical traits that are nothing like yours. Also, the lipids on the surface are nothing like yours. So, you can use that to help you decide that you've got some kind of pathogen. Even worse, this is a picture of a gram-negative pathogen. These guys also are very dangerous. They have a peptidoglycan wall and they have two membranes, an inner one, the regular plasma membrane, and an outer one. In this outer one are lipopolysaccharides that you really will just respond strongly if you run into them. Nonetheless, these are among the more deadly bacteria and we're running out of really good antibiotics to treat them. So, paying attention to how this one differs from you is really one of the first steps in designing some ways to protect you from them. The next group that we look at is fungi, and this is a candida shot. Fungi are eukaryotic, so their ribosomes are a lot like yours. They have an organized nucleus, but they do have a cell wall on the outside. It's nothing like the cell wall around bacteria. It's also completely different from that of plants. It has characteristics that you can sometimes use to distinguish them and maybe even attack them. This is another view of candida, a little bit different magnification, slightly higher. You can see that it has both hyphae around itself. This is a different fungal infection that's inside the tissue. Another type of fungal infection that we have here would be something called Cryptococcus gattii, and that one is dangerous. So, fungal infections include things like athlete's foot, ringworm, toenail fungus, yeast infections. All of these things are obnoxious and annoying, but not usually lethal. There is, however, a new fungus out called Cryptococcus gattii, and this thing is really bad. It can actually grow in your lungs and you sometimes have to have surgery to take out masses of the fungus because it can grow so large before it's treated, that it's almost like having a tumor. Okay. So, in general, fungi are not something that you can't deal with, but again, it's like anything else. Sometimes you wind up with a particularly lethal case, especially, if you're immune compromised. Now, this next category are those of unicellular eukaryotes. These are eukaryotes like we are. They have nuclei and ribosomes that resemble ours. They have a plasma membrane, and they don't have a cell wall. So, that is something that makes these guys sometimes quite difficult to treat. They include things like Giardia, Trichomonas, various trypanosome diseases including chagas and sleeping sickness, Leishmania. All of these guys are really not closely related to each other. These guys are all very different. For each one of them, you have to develop a specific strategy for that general type of pathogen. Again, the strategy development is quite difficult because you want something that kills the pathogen and not the patient. These are similar enough to your own physiology that finding the thin end of the wedge to get in between them can be quite difficult. One of the worst versions is malaria. This is a drawing of malaria, showing how malaria will grow inside of red blood cells, swell and eventually break them. It's really interesting because the red blood cells are among your smaller cells and yet they can grow 15-20 of these malarial parasites before they burst open. So, malaria has a number of strategies to keep you from killing it, and we'll look at how we try to get back to them later on. Our last category is that of the multicellular worm. Worm parasites are, let us face it, achy. These are two Schistosoma cercariae that is larval forms. They're among the few parasites or pathogens that can penetrate intact skin. These things look relatively harmless, but they're not. They can get in. Again, they will grow, take up residence in your bladder rectum and live for 20 years making little copies of themselves. Oh, gross. This is a picture of Trichinella. This is one of the infections you get from eating under-cooked pork, and what you've got here are cyst of these worms that are stuck in muscle tissue. So, they're in-cysted in the tissue where they've got a nice little hard covering around them that makes it very hard for your immune system to get to. Finally, and I'm not sure if you can get a good look at the mouth on this thing, this is the attachment site of a hookworm. These things, of course, will sometimes attach to the intestinal wall. They'll occasionally even travel to your lungs. They're really nasty. In general, these guys like to live for years, and years, and years, and years in you so that they tend to do chronic infections, and you need ways of recognizing them to keep them down.
