 Carl Linnaeus in 1735 came up with a plan, and he placed all organisms, including microbes, into five kingdoms. And those kingdoms include manila, which had bacteria and archaea in them, protista, fungi, plantae, and anemilia. Microbes were placed in the first three kingdoms that include manila, protista, and fungi. But later studies, sophisticated tools from the field of molecular biology became available, and scientists started studying DNA and RNA sequences. They came to know that previous five kingdom classification did not answer all the questions, and they found a variation that led to another system. So the knowledge that was created by use of electron microscope, biochemical and physical characteristics of microbes, and nucleic acid and protein sequencing made the previous classification kind of obsolete, and microbiologists or taxonomists came with another scheme. That was largely based on the ribosomal RNA sequences. What they found, that bacteria and archaea, they belong to two different groups. So in 1977, Carl Woos was an American microbiologist. He proposed that the word domain should be introduced into this classification, and now all organisms are put into three domains. That include bacteria, like true bacteria, and archaea, and then all those eukaryotic cells are placed in eukarya, which then include protists, fungi, or fungi, and plants, and animals. So this slide simply indicates what domains contains, what kingdom. For example, domain bacteria has kingdom bacteria, and domain archaea, the kingdom is also archaea, and for domain eukarya, there are four kingdoms that include protista, fungi, plantae, and animalia. Now let's just talk a little bit more about what these different microorganisms are. Bacteria, for example, they're prokaryotes. They do not have well-defined nucleus. They are usually single-celled organisms, and they have a cell wall, and the cell wall contains what we call peptidoglycan. This is a picture that shows a scanning electron image of a bacteria. Archea, they're also prokaryotes. They also have cell wall, but their cell wall does not contain peptidoglycan. Instead, they have special lipids in their cell walls. And also, they vary in their RNA, ribosomal RNA sequences, and they live in extreme conditions. By extreme conditions, I mean like in hot water that has 100 degrees Celsius temperature, and these organisms survive in those harsh conditions as well. Similarly, they can survive in very high salt concentrations, and those archaea are called halophiles. Eukarya include protists, fungi, animals, and plants. This slide shows various protists, and they include algae, which have cellulose in their cell wall. In their photosynthetic, they make oxygen, protozoa, they're a material, and they are unicellular organisms, and slime molds. Slime molds behave like protozoa at one stage of their life, and they also behave like a fungus at another stage of their life. Fungi could be unicellular, and if they're unicellular, we call them yeast. And when they become multicellular, environment can change them into either yeast or mold. But when they acquire multicellular characteristics, we call them mold or mushrooms. This is an example of a moldy bread. As you can see that the mold is grown on the bread loaf. There are other organisms that do not fall into any of the categories that we have already studied, and these are called viruses. Viruses are acellular organisms. They do not have a defined nucleus or a cell. Their composition is very simple. They have protein capsid or covering in which the nucleus sits. And the nucleus basically is not a nucleus, it's just a DNA molecule or RNA molecule. There is another variation of these viruses, and that is called viroids. They look like viruses, but the difference is that they only have circular RNA with no protein capsid. There is another word which we call viresoids. These are single-stranded RNA molecules, and they also have no protein capsid. And one of their features is that they, by themselves, are not infectious. That means that they cannot cause a disease by themselves. They need another virus for causing infection. Then the last object is called prions. And interestingly, these are protein in nature, but they are infectious.