 Hi everybody, Dr. O here. This video we're going to cover Cook's postulates. So the name after Robert Cook, one of the most important historical figures in microbiology, we covered him way back in the beginning with the microbiology history lesson stuff. So he's really famous for a couple of things. These postulates, but also how he's kind of given credit to be the father of creating aseptic laboratory techniques and you'll see why he had to in order for his postulates to work. So he would have first used these postulates to prove that the causative agent of anthrax was the bacterium known as bacillus anthracis, so but we can use it in quite a few cases. But I really want to focus in on when we can't use it as well. So Cook's postulates are a way to try to prove that organism A causes disease Y, whatever. So you'll see here at the top that first thing is an organism that causes a disease should be absent from healthy or animals. So you see here that the healthy rodent has the, there is the organism is absent. So that's like kind of rule number one. And then rule two to actually run through his postulates, you take a diseased animal or organism and you, you take samples and you remove the organism that you think is the causative agent of disease. So number one, the suspected causative agent must be absent from all healthy organisms but present in all diseased organisms and that's very important. And number two, you take that and you isolate it and grow it in pure culture. So number two, the causative agent must be isolated from the diseased organism and grown in pure culture. So you take it out and you isolate it. So it's the only organism around. If you take just a sample from a diseased animal and put it in a healthy animal and it gets sick or dies, all you know is one of the organisms that you, that you transmitted or some compound was what caused it. So, so isolating pure culture is very important. He took this very seriously. He would have done rounds and rounds of isolating pure cultures, maybe six, eight different rounds of isolating pure cultures before he put them in the next animal. So step three, the cultured agent must cause the same disease when inoculated into a healthy susceptible organism. So he took that organism that he isolated in pure culture and gave it to another animal and it got the same disease or died the same way if it led to death. And then the last step in Cook's postulates, the same causative agent must then be re-isolated from this second animal and obviously be the same organism. So this is how you're able to prove that a certain organism is the causative agent of a certain disease. And this is very important and this was a huge deal back when Robert Cook came up with these postulates and actually did it, right? He proved the causative agent of several diseases. But I wanna talk about the ways this won't work. So number one, you keep seeing this term pure isolating, pure culture. Some organisms can't be grown in pure culture. So there are some organisms we just cannot culture. We're learning a lot more about that now. When you analyze samples genetically, you realize there might be four, six, 10, 20 times more organisms there than what you can actually grow on a petri dish somewhere. One good example would be like mycobacterium lepre. They have to grow it in the fat pads of rodents or like armadillos to get it to grow because there isn't an easy way to grow it in a laboratory and some organisms just cannot be grown. So if you can't isolate an organism in pure culture, then you really can't use these postulates. That's number one reason it won't work. Number two, if multiple organisms cause the same disease, you're gonna have a problem here. So think about like the cold or really lots of different things. Yes, you can prove that one organism is a causative agent of a certain disease, but you can't say it's the causative agent if there's multiple organisms that can cause the same disease. And then the last reason this won't work very well or when it won't work is if one organism can lead to multiple diseases. So you could take, let's say this first lab rat here died, let's say they have strepococcus pneumonia and they died of pneumonia. So you extract it, you isolate the organism, you grow it in pure culture and give it to another rodent and it dies of sepsis. And you give it to a third rodent and it dies of meningitis. See how one organism could cause multiple diseases. So those so cooks postulates very important. They were especially important back in the early history of microbiology, but you will see those are the three reasons that it doesn't always work. All right, so that is cooks postulates and the exceptions to those rules. I hope it helps. Have a wonderful day. Be blessed.