Microbiology: Microbial Pathogenesispg. 1

Sheila Morris

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Whenever a microbe is able to cause disease symptoms we refer to it as a pathogen. Microbial pathogenesis is the study of the interaction between the pathogen and the host. The pathogen is trying to find a place to live; the host is trying to convince it to live somewhere else.

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In this process innate immunity and antigen specific adaptive immunity, like Ab and Ag-specific T-cells, protect the host from pathogens and tumors. The microbe has virulence factors, special properties it develops to enable it to infect the host. It’s amazing virtually every different pathogen has a somewhat different strategy to do this. They probably evolve to come into places where the host wasn’t very good at protecting against them and gradually moved into those spots, the host has to evolve to protect itself. They all have quite different strategies, which explains why immunology is so complicated. There are lots of things that go on we can talk about in immunology that we really don’t understand really well. They’re evolved to fight some pathogen that was a really big problem thousands of years ago, but not so much anymore. We still have all of those abilities.

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By understanding microbial pathogenesis, understanding the mechanisms allows us to know drug targets. Also it allows us to develop vaccines or in some cases like AIDS, to understand why the vaccines don’t work. As we learn more about these interactions it will be more and more effective to use cytokines to treat patients, we’ll develop better supportive therapies and preventative measure.

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Antibiotics have been used since WWII to control bacterial infections. At first they worked incredibly well, and then the bacteria, who are specialists at evolving, learned to get around them. They were originally developed from natural molecules that the microbes in fact made themselves, because when they’re living in soil or in your gut or throat, they’re competing with each other, all the normal flora compete with each other. They have made molecules that kill off each other; the first antibiotics developed were simply recognizing that these molecules were made. Penicillin was made by certain molds and fungi that attacked the bacterial cell wall so they could get a large number of them. The antibiotics we use today all block essential microbial specific functions. We don’t want them to block essential functions in us. It’s easy in bacteria they have stuff like cell walls that we don’t have. Efforts were made to make antifungal and antiviral antimicrobials, that’s more challenging because a lot of the pathways they use are similar to pathways the host use. That’s why they were developed later and many of them have deleterious effects to the host. It’s hard to sort that out.

Antimicrobial resistance is the result of mutations in genes or acquisition of genes from other bacteria. If you take a large dose of a certain antibiotic everyday, the normal flora will gradually develop the ability to defend itself against that antibiotic. If it’s a gram negative flora, and you get infected with a gram negative organism, the antibiotic resistance, which is frequently encoded on a plasmid, will move at a fairly low frequency, but since there’s millions of bacteria it does a good job of jumping into the new pathogen. Now it’s resistant to that same antibiotic that you’ve been taking. It’s not as big of a problem here because we need prescriptions for antibiotics, in much of the world you go to the drug store and buy what you want. There people have considerable problems because they have a lot of, especially intestinal flora, that make people sick frequently. They buy antibiotics to prevent what’s in them, then they get something bad and they can’t treat it because they’ve used the latest antibiotics.

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It’s especially important in microbial pathogenesis to do studies in vivo. You can study a lot of things in vitro but you don’t really know if it plays a role or if it plays the role you think it does unless you can do the study in the host. One of the ways this can be done is if you have a drug that blocks a specific pathway in the bacteria or host, you administer it in the mouse you’re infecting and see what the effect is. You can use cytokines, specific Ab, various gene products that target viral species or host mechanisms.

One of the nicer ways to do these studies is to make specific mutations in the pathogen. If you think acquiring iron is essential for this particular pathogen to grow in the host and cause disease, you can knock out the ability to acquire iron, and then ask what happens in the host. You can also knock out the ability of the host to tie iron up and ask what happens in the disease.

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Talking about classifications of microbes, this is a little bit different classification scheme than normal with gram -/+. First there are those that don’t colonize or infect a person, that’s most of the microbes in the world. We don’t worry about those. Then we have those that are opportunistic flora that only cause disease in us when we’re sick with something else, or our immune system is not properly developed or we have an immunodeficiency disease, such as AIDS. These are organisms that would not normally infect you or me, but if we have some other disease, they may cause a problem, like COPD, cystic fibrosis, etc.

There’s another group called normal flora. These are commensal, symbiotic bacteria. The fact that we call them normal flora comes from the fact that they don’t cause symptoms. They’re there a lot or all of the time. S. mutans on the teeth, the specific strain you acquire when they erupt, you have the rest of your life. They hang on so tightly that even other S. mutans can’t colonize. They are well evolved organisms. One thing that makes their life so successful is that they don’t do anything to hurt their host. They don’t have to worry about the host dying and getting to someone else.

Then there’s the disease producing pathogens. Some are maintained in the population by colonization, like pneumococci, staphylococci, meningococci. Only occasionally cause disease. Frequently group A streptococci isn’t causing disease, they’re just colonizing our airways. If there’s some perturbation these can go in and cause disease, sometimes life threatening.

There are other pathogens, like shigella, which virtually always cause disease. It’s just takes 10 organisms to cause infections in the lower intestine. It causes a bloody erosion in the gut, it’s fairly severe, if you’re malnourished anyways and can’t replenish fluids it can be fatal. When you get one of these guys, you invariably get sick. It’s much more common that disease comes from things hanging around, but we also get infections this way. The flu virus is something we don’t carry, but when we get it, we get sick. Cold viruses, we don’t ever carry those, they cause infection when we get them. They move through the population from person to person.

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Normal flora can exist in the gut, especially in the large intestine. There are more bacteria in the large intestine than there are cells in the body. Most of the live things we carry with us aren’t us. Oral cavity has a lot of normal flora, 1000s of different species. Upper airways, genital and urinary tract, skin. Much of the urinary tract tends to be more sterile, but you can get organisms living in there that will live for a long time, a person is colonized permanently.

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Normal flora cause no clinical symptoms, they’re often beneficial. Their most important role is that they have worked so hard to evolve to live in their niche in our upper airways, mouth, nose and gut that it’s difficult for pathogens to get in there. They have to compete with normal flora that occupies that space. You probably all at some time, may not remember it, were given a heavy dose of antibiotics as a child to get rid of something and had diarrhea for days to a week afterwards because your normal flora were removed along with what the physician was after. Pathogens could come in, that normally wouldn’t have caused trouble, and made you sick. Gradually the normal flora comes back and pushes them out. Some of the normal flora produces vitamins for us, vitamin K we get primarily from normal flora.

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A pathogenic microbe has to do several things. This is a fairly important list. They have to be something that can be acquired by the host. You have to be able to catch it. It has to have the ability to exploit a niche or environment. This would be true also of normal flora. The things that makes pathogens a little different is that rather than just trying to live on the surface of things relatively away from the immune system, they have decided that competing with normal flora is tough and decide to invade, at least a little bit or a lot, into host tissue. They go in an actually try to occupy host tissue. They try to do that in a way to avoid the immune system, invariably the immune system reacts to protect you against them. Inflammation is caused and you get symptoms. Symptoms are invariably, not always, the result of the inflammation produced in the war between the pathogen and you. It’s not a bad war, if you lose you die. The symptoms are good symptoms. Fever, redness, swelling is your body trying to take care of itself.

To get into sterile tissue they have to partially evade host defenses. If they completely evaded we would be dead unless they have a system to shut themselves down before they kill us. More and more bacteria, which colonize for long periods and only rarely make us sick, have evolved strategies to colonize a little into sterile tissue but not kill us. They’re trying not to kill us because when we die, they die. They have to partially evade defenses to stay there. Anthrax completely evades, we die.

They have to multiply in the host. If they don’t multiply, they don’t get transferred to other people. They have to increase their numbers. We wouldn’t call them a disease if they didn’t produce symptoms. Those are basic attributes of pathogenic microbes.

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The evolutionary success of a pathogen is measured by its ability to infect/colonize additional hosts and not necessarily its ability to kill.

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We talked about this already.

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HIV is an example of a pathogen (pathogen’s aren’t clever, they don’t write out a game plan) that came from some related virus in monkeys that wasn’t quite as hard in monkey as HIV is on us. It learned to evolve fairly well with monkeys, they could be sick and have it for a long time, sometimes die but usually okay. In people, it was growing in the CD4 t-cells, which are the cells we use to protect ourselves from viruses. It wasn’t particularly successful at doing this; it took it a long time to kill a person unless they had a t-cell deficiency or some other infection like TB that would kill them. Because these people were very inefficiently killed, they lived for a long time. There was a good opportunity that it would go to the next person. It turned out to just have happened on this incredibly good strategy to go through the human population. It took advantage of things in human behavior that worked out really well for it.

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A couple of pathogens that don’t mind if they kill you: one is Vibrio cholera, the strains around 20-3- years ago, and Anthrax. Anthrax evolved to kill ungulates, hoofed animals that live in savannahs in Africa, not us. Cholera comes in orally, multiplies in the gut, secretes cholera toxin that affects the sodium pump and causes mass secretion of water into the intestine. All of the fluid in the body and electrolytes goes into the intestine. The cholera is in there growing like mad, all of the water goes out and with it comes the cholera, like 10^9 bacilli/mL of fluid that comes out of people. You can take almost all of the fluid in your body and transform it into this fluid that you’re excreting. When it comes out in this tremendous fluid volume, there’s a very good chance if you’re in an area without sanitation that someone else is going to get it. Then they put all their fluid into the environment with their cholera, and it will move on. It doesn’t mind if you die, because in the process of killing you it goes on.

Anthrax has a similar strategy. When an animal inhales anthrax, it rapidly causes death but it grows so fast that it produces billions of anthrax bacilli. It manages to produce these before the animal completely collapses and dies, at which point other microbes come in and compete. The anthrax then makes spores, it recognizes that the food supply has stopped and makes spores that lay there on the savannah where the animal died, for years, waiting for another ungulate to come along, sniff some in, gets Anthrax, dies, makes another pile of spores. This is a pathogen that also doesn’t mind killing its host, that’s just part of its system. Fortunately for us, there are not a lot of pathogens that work that way.

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We’re going to look at host defense mechanisms that are involved. These are all mechanisms that you learned about in immunology, but we’re going to talk about them more in their interactions with pathogens.

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One of the primary defense mechanisms is mechanical. This is the barriers of the skin, tight junctions between epithelial cells, all of the mucus covered epithelial (everything from your mouth all the way through) which is being moved by cilia. From the lungs mucus moves up and you swallow it, it goes into the stomach, all of the upper airway mucus too, more is produced in the gut and eventually it moves out. This mucus carries with it microbes that would otherwise be able to readily attack that surface. The cilia moving the mucus turns out to be really important and you can see part of this importance in people who smoke. They gradually kill off their cilia, old people lose their cilia over time, then the mucus stops flowing as well and begins to pool and you’re able to start getting more lung infections with things like S. pneumonia, group A strep, haemophilius.

The acid in the stomach kills the bacteria that go through it. Most of the stuff you eat gets killed in the stomach. Enteric pathogens that make you sick living in your gut either have a strategy to protect themselves from the acid, like shigella, others just count on very large numbers coming in and a small fraction making it through. It’s easier with a large food meal than if they come in by themselves or in water.

When we take things to suppress stomach acid to prevent ulcers, it actually should be enhancing gastric disease. If we lived in a place where we had a lot more of that disease it would be a problem. In this country the ulcer is usually worse than what you might have a higher frequency of getting without the acid.

Phagocytes: macrophages, kupffer cells in the liver, neutrophils in the blood and invade tissue, all of these can phagocytize bacteria. They can do it in part because of structures on bacteria that phagocytes can recognize as unique to pathogens. They do it in part because complement is better deposited on pathogens than our own cells because we inhibit complement deposition. Pathogens that cause us most trouble have evolved their own inhibitors of complement deposition. Complement decorates bacteria and causes clearance by phagocytes to increase.

C reactive protein is a protein we make during inflammation, bacterial or other, it has a number of properties. One is that it attaches the surface of many respiratory bacteria because they have phosphocholine on their surface, decorating the surface. The c reactive protein then fixes complement and is recognized by phagocytes.

These are the non-specific or less specific immune protection systems.

Antibody is generated to antigen specific determinants. The first time you’re infected with most bacteria, you won’t have very much antibody that recognizes that. There may be a little bit of antibody that was made to something else and cross reacts a little bit but it won’t do too much good. The first time you actually have to get sick and these things on the right side are what are keeping you alive to make an antibody/t-cell response. Once antibodies are made and bind the surface of the bacteria they activate complement extremely efficiently, the phagocytes work much better.

T-cells are produced and cause cell mediated immunity. It takes a little longer to produce than antibody but once you produce this you can mount a cell mediated response which will protect you against pathogens that have gotten inside of cells.