So I'm going to start with my first question. What are bacteria? So bacteria, as shown here, are very, very, very small. This is the head of a pin, and this shows bacteria coated on the head of the pin at increasing magnification. So bacteria are microorganisms, meaning they're tiny, made up of usually single cells. So unlike people, people have lots and lots of cells in one organism. Bacteria usually only have one. Bacteria are extremely diverse. These are two examples. This shows a coli. This is a staff-orius, cephalococcus-orius. It's not really purple. It's been colored in this picture. There is an estimate to be between 40,000 and 1 billion different species of bacteria in the world. It's a huge range. And there's huge uncertainty about this number, because there's probably on the order of thousands that have been studied, not billions. If we think about this number and look at an evolutionary tree, you can't see anything here. Of course, it's an evolutionary tree. These are all the different organisms, or types of species in the world. If you zoom in, that little branch includes all animals, plants, worms, and the insects. Almost everything else is microorganisms. And everything in blue is bacteria. So when we're talking about microorganisms and bacteria, we're talking about huge diversity. So it's very difficult for me to talk about any particular bacteria in very concrete terms, because there's a huge diversity. So you will hear me say in this course, most bacteria like this, and most are like that, because I can't talk about all of them. There are always exceptions. So when you have this huge diversity, how do you even get started in characterizing? If you go to a hospital with an infection, how do they even get started? Well the first thing we look at is cell shape. So in this show's different shapes of bacteria. Okay. So what you have up in the left corner here, these are called coxae. They're circles. They're spheres. For example, a staphylococcus orius is a spherical bacteria. So it's a circle, as I've drawn here. Then you can have something called diplococci, which are two of the coxae stuck together with each other. Then you have strutococci, which grow in long chains of spherical bacteria. And then there's a few other variants here. And then below are the bicillae. Bicillae and coxae are the most common type of bacteria shape. And again, you have a bicillis, which is a broad shape. It's longer than it is wide. And then you have diplo-bicillae, which is two stuck together, or strutoccibicillae, which grow in chains. So these make up the vast majority of bacterial shapes. But then you got a lot of different odd things. For example, down here, an example is colobacter, has a stalk sticking out of a large appendage on the cell. You also have vibrio, which are shaped like a coma. And you have, really can't say this, a corkscrew shape. There's actually like that. This is the first way that we would, if I get an unknown bacteria, my first thing to do is to look at what is a shape. And that starts to let me figure out what type of bacteria it is. I want to mention quickly, bacteria are now viruses. Viruses are different from bacteria. Viruses grow inside cells, and they aren't actually cells themselves. They don't have all the machinery to be able to produce new reproduce, new cells. Some viruses, and you'll hear this next week, actually infect bacteria. So just why I make clear, viruses are not bacteria. So in this wide diversity, I want to point out also that we mostly time think of bacteria is bad. They cause disease, there's a problem. That is not really generally true. Most bacteria are perfectly fine. We need them. We need them to digest food. We need them to help plants grow, etc. We need bacteria all around us. So we only usually talk about a handful of bacteria. In fact, there's only around 100 bacteria types of bacteria species. They cause disease. As shown here. So about 1% at most can cause disease. Some examples that do cause disease are micro-bacterium tuberculosis, listeria, and salmonella. So next, in the next section, I'm going to start to describe the structure of a bacteria. So now I want you to pause again for a minute and think about what do you think a bacteria needs to be able to grow and survive? What are the basic needs of a bacteria? Think about that for a minute.