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Introduction to bacteria. Created by Sal Khan.

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Video transcript

I think we've all heard of the word bacteria. And we normally associate it with negative things. You say bacteria, those are germs. So we normally associate those with germs, and they indeed are germs, and they cause a whole set of negative things. Or at least from the standard point of view, people believe that they cause a whole bunch of negative things. So let's just list them all just to make sure we know about them, we're all on the same page. So the bad things they do, they cause a lot of diseases: tuberculosis, Lyme disease. I mean, I could go on and on. You know, pretty much any time-- well, I'll be careful here. Whenever people talk about an infection, it's often caused by a bacteria. It can also be caused by a virus. An infection is, in general, anything entering you and taking advantage of your body to kind of replicate itself, and in the process, making you sick. But bacterial infections, let me write that down. And this whole perception of bacteria being a bad thing is probably a good reason why almost any soap you see now will say antibacterial on it. Because the makers of the soap know that in conventional thinking, bacteria are viewed as a negative thing. And you're like, OK, Sal, I know where you're going with this. Bacteria isn't all bad. There are some good traits of bacteria. For example, I could stick some yogurt in some-- or I could stick some bacteria in some milk and it'll help produce some yogurt, sometimes spelled yoghurt. And that's obviously a good thing. It's a delicious thing to eat. You say, well, I know I have bacteria in my gut. It helps me digest food. And these are all true, but you're like, look, you know, on balance, I still think bacteria is a bad thing. I'm not going to take sides on that debate, as I tend to avoid taking sides on debates in these science videos. Maybe I'll do a whole playlist where I do nothing but take sides on debates, but here I won't take any sides on that. But I'll just point out that you are to a large degree made up of bacteria. It's not just your gut. It's not just the gut or the yogurt you might eat or the plaque on your teeth, which is caused by bacteria. It's this kind of film that's created by bacteria that eventually causes cavities and whatever else. And it's not just the pimples on your face. Bacteria actually represents a majority of the cells on your body. So for every-- and this is kind of an astounding fact. For every one cell on the human body, every one human cell-- so these are all cells that all have your DNA in them and they'll have nucleuses, and I'll talk about that in a second-- you have 20 bacteria. Now, your response there is to say, OK, that's fair enough, but these bacteria must be much smaller than the human cell, so it must be a very small fraction of my mass. And you're right. It's not like we're mostly bacteria by mass, although we are mostly bacteria by actual cells. But even if you were to take out all of the water in your body, then by mass, bacteria is going to be roughly 10% of your mass. So I weigh about 150 pounds, I've got 15 pounds of bacteria walking around with me. So we always kind of think of ourselves as like the bacteria is riding on us, but to a large degree, we're kind of in symbiosis. We're kind of two creatures, or not just two creatures, two types of creatures living together, because I don't have just one type of bacteria on me. I have thousands of types of bacteria on me. There's a huge amount of diversity, and we're just scratching the surface in terms of the number and types and diversity of bacteria that exist. So I've talked a lot about bacteria, and hopefully, this fact right here will make you realize that they're super important to just our everyday existence. Just to make sure we understand the magnitude of this, in a previous video, and I looked this up again, we have on the order of 10 to a 100 trillion cells, human cells. So for every one of these we have 20 bacteria, we're talking about having on the order of 200 to 2,000 trillion bacteria on us at any time. And I'm a hygienic person. I take showers daily, and that's even me. It's not like you can somehow eliminate them. And even more, it's not like you even want to eliminate them. But that's fair enough. You're probably asking, OK, Sal, I'm convinced that bacteria are important. What do they actually look like? And they're these small unicellular organisms. That's my bacteria right there. And they're different from the cells that make up us. When I say us, I'll throw in all plants, animals and funguses, fungi. And the big difference, or the one that people noticed first, is that all of the Eukarya, which includes plants, animals and fungi, all of their DNA is inside of a nucleus, a cellular nucleus. So that's the nucleus right there. And all of our DNA, it's normally in its chromatin form. It's all just spread around something like that. In bacteria, which are what people originally just classify it on whether or not you have a nucleus, in bacteria, there is no membrane surrounding the DNA. So what they have is just a big bundle of DNA. They just have this big bundle of DNA. It's sometimes in a loop all in one circle called a nucleoid. Now, whenever we look at something, and we say, oh, we have this thing; it doesn't; there's this assumption that somehow we're superior or we're more advanced beings. But the reality is that bacteria have infiltrated far more ecosystems in every part of the planet than Eukarya have, and there's far more diversity in bacteria than there is in Eukarya. So when you really think about it, these are the more successful organisms. If a comet were to hit the Earth-- God forbid-- the organisms more likely to survive are going to be the bacteria than the Eukarya, than the ones with the larger-- not always larger, but the organisms that do have this nucleus and have membrane-bound organelles like mitochondria and all that. We'll talk more about it in the future. Bacteria, for the most part, are just big bags of cytoplasm. They have their DNA there. They do have ribosomes because they have to code for proteins just like the rest of us do. Some of those proteins, they'll make some from-- bacteria, they'll make these flagella, which are tails that allow them to move around. They also have these things called pili. Pili is plural for pilus or pee-lus, so these pili. And we'll see in a second that the pili are kind of how the bacteria are able to do one form of introducing genetic variation into their populations. Actually, I'll take a little side note here. I'm pointing out bacteria as not having a cell wall. There's actually another class that used to be categorized as type of a bacteria, and they're called Archaea. I should give them a little bit of justice. They're always kind of the stepchild. They used to be called Archaea bacteria, but now people realize, they've actually looked at the DNA, because when they originally looked at these, they said, OK, these guys also have no nucleus and a bunch of DNA running around. These must be a form of bacteria. But now that we've actually been able to look into the DNA of the things, we've seen that they're actually quite different. But all of these, both bacteria and Archaea, are considered prokaryotes. And this just means no nucleus. No nucleus, and more generally, this is what most people refer to, but more generally, they don't have these membrane-bound organelles that our cells have. Now, the next question you might say is, well, how do these bacteria reproduce? And for the most part, they do something not completely different from mitosis, although I want to call it mitosis. We call it binary fission. I'm not going to go into the deep mechanism here, but the idea is fairly simple. I have a bacteria right here. It replicates its DNA, so it'll have two of these nucleoids here, and then the cytoplasm essentially splits, or it's kind of a form of cleavage right there. It splits and then you have two of them. You have two of them then. And then each of them, they can code for the proteins necessary to produce all of their extra appendages, the flagellum, which is this long tail-like thing that can help it move. And it's actually fascinating because it's operating at such a small scale, but you can still kind of get this motor movement going on. Even at this very, very small scale, using very primitive-- I won't say primitive, because that's making a value judgment on these things, but using-- you know, these flagellum are on the order of several nanometers, on the order of tens of nanometers wide. So you don't have a lot of atoms to deal with, but you're still able to get this kind of wave-like motion that can move the bacteria around. Now, you're saying, hey, Sal, in that first video on evolution, you told me that we see evolution every day and bacteria is one example. When we use antibiotics, we think it'll help eliminate bacteria, but that one bacteria that has some type of resistance, it'll survive, so it is more fit. How did these guys get variation? Well, the one way, and this is the way everything can get variation, is they can get mutations. And bacteria replicate so quickly, they reproduce so quickly that even if you have a mutation that's one in every thousand times, by the time you have a million bacteria, you'll have a thousand mutations. So they have mutations, but they also have this form. I don't want to call it sexual reproduction, because it's not sexual reproduction. They don't form gametes and the gametes don't fertilize each other and then produces a zygote. But two bacteria can get near each other and then one of their piluses-- I'll do that right here. So the piluses are these little structures on the side of the bacteria. They're these little tubes, really. One of the piluses can connect from one bacteria to another, and then essentially you have a mixing of what's inside one bacteria with another. So let me draw their nucleoids. And then they have these other pieces of just DNA that hangs out called plasmids. These are just circular pieces of DNA. Maybe this guy has got this extra neat plasmid. He got it from someplace, and it's making him able to do things that this guy couldn't do. Maybe this is the R plasmid, which is known for making a bacteria resistant to a lot of antibiotics. And what happens is, that bacteria-- and actually, there's mechanisms where the bacteria know that, hey, this guy doesn't have the R plasmid. And we're just beginning to understand how it actually works, but this will actually replicate itself and give this guy a version of the R plasmid. You could also have these things, transposons, and I should make a whole video on this because we have transposons, too. But there's parts of DNA that can jump from one part of a fragment of DNA to another, and these can also end up in the other one. So what you have is kind of-- it's not formal sexual reproduction, but what you essentially have is a connection, and these bacteria are just constantly swapping DNA with each other and DNA is jumping back and forth, so you can imagine all sorts of combinations of DNA happen even within what you used to call one bacterial species and very quickly can turn to multiple species and become resistant to different things. If this makes it resistant to an antibiotic, then it can kind of spread the information to produce those resistant proteins or whatever to the other bacteria. So this is kind of a form of introducing variation. And so when you transfer stuff via this pilus, or the plural is pili, this is called conjugation, bacterial conjugation. Now, the last thing I want to talk about, because it's something that you've heard a lot about, are antibiotics. A lot of people, they get sick. The first thing they want to get is an antibiotic. And an antibiotic is just a whole class of chemicals and compounds, some of them naturally derived, some of them not, that kill bacteria. So now if someone is undergoing a surgery and they get a cut, instead of them having to worry about getting an infection, they'll take some antibiotics to prevent the bacteria from growing on them. But the question is how was this discovered or where does it come from? It actually came from Alexander Fleming. Let me write him down. Very important, because the discovery of antibiotics is, in my opinion, the most important discovery in medicine so far. So, Alexander Fleming. He was studying-- I think it was Staphylococcus. I forget which bacteria it was, but it was in a Petri dish. He was using a Petri dish. Let me draw a Petri dish. There's a little circle. There's some nutrients that the bacteria can grow on. So let's say the bacteria, you know, it's growing on this Petri dish. And he went out, and he came back into the room, and he saw that some mold, some fungus had grown on this, kind of a bluish-greenish fungus had grown on the center of his Petri dish. And the bacteria, there was kind of this space around it, and the bacteria couldn't get close to it. And this mold, this fungus was called Penicillium, the Penicillium fungus. He was able to figure that out. He took a sample of this and then he cultured it, which means letting it grow and then seeing what it is. This was Penicillium. And he figured out that, gee, this fungus must have something, some chemical that it's emitting that's essentially killing the bacteria around it, that's not allowing the bacteria to get near it. And so that led to the discovery of penicillin. This was in the late 20s, 1920s. By the time World War II came around, now people had gunshot wounds and they had to get things amputated, whatnot, but for the first time, they could actually give people antibiotics and not worry about-- or they probably still worried about it, but didn't have to worry about this thing as much as they did before. And now, you know, if you have bacteria, if you have tuberculosis or Lyme disease or anything, the treatment all involves taking antibiotics. And there are many, many more types of antibiotics now coming from many, many more different sources, but the general idea is the same. You want to kill bacteria. Although you don't want to kill all bacteria, because some of it's good. In fact, we are made up a whole lot of bacteria. I don't know if I even mentioned this earlier in the video. There's bacteria in our skin that helps take up oil and moisturizes and make our skin nice and supple. So, you know, the way you think about it, you could view them as negative or you could view them as positive or you could view them as something in between, but the really amazing thing, at least in my mind, is that we're living in symbiosis with them. I remember I saw a Star Trek episode once where you had these people. You had these people, and they were some alien race. Jean-Luc Picard had-- they ran into them. They looked very humanoid like that. But it turns out-- let me draw this human-- that they had these little bugs in their brain stem. So they had these big insects in their brain stem and these insects started infecting the crew of the Enterprise, and they were controlling their brains and making them act weird and whatever not, and this seemed like a very bizarre alien concept of some creepy-crawlie living in us and affecting our brains and affecting us in some ways. But if you really think about it, we are doing this, and it's not just with one little bug, it's on the order of trillions. Hundreds of trillions of bugs are with us every day and they make us us. I mean, I'm here recording videos along with-- or maybe I should even say the bacteria is recording videos or it's maybe partially responsible for controlling bacteria. And it's known that the bacteria can even affect our mental state. There's a whole bunch of research now that certain types of bacteria can cause schizophrenia. Actually, syphilis does. Bacteria can cause depression. Lyme disease, it's known that when you go into later phases of Lyme disease, it can affect the mental condition of the person who has the infection, so it affects every part of who we are. I mean, it would be hard to even talk of being a human being without the 10% of our mass or the 2,000 trillion cells or 2,000 trillion bacteria that really make us us.