Microbiology, part 31: Growth - Culture Media

Hi. I'm Cathy with Level Up RN. In this video, we are going to talk about different types of culture media. And at the end of the video, I'm going to give you guys a little quiz to test your understanding of some of the key points I'll be covering, so definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcard on culture media so you can follow along with me.

A culture medium is a liquid, solid, or semi-solid substance designed to support the growth of microorganisms in a laboratory. Let's go over some different types of media that you definitely need to know for your microbiology class and exams. A chemically defined medium is a medium where the exact chemical composition of the medium is known. So this would be like a cook who follows a recipe to the T. So we know exactly what went into the dish. On the other hand, with a complex medium, the exact chemical composition is not known and varies from batch to batch. So this would be like a cook who puts a little of this and a little bit of that into a dish and doesn't really measure anything. So we don't know exactly how much of each ingredient went into the dish.

So nutrient agar is an example of a complex medium. It is a general-purpose medium that is used to cultivate a large variety of microorganisms. A selective medium supports the growth of wanted microorganisms but inhibits the growth of unwanted microorganisms. An example of a selective medium is MacConkey agar. It contains bile salts and crystal violet, which inhibits the growth of gram-positive bacteria but enhances the growth of gram-negative bacteria. MacConkey agar also serves as a differential medium. A differential medium is a medium that allows different microorganisms to be visually distinguished based on a change in the color of the colonies or of the medium.

So MacConkey agar contains lactose and a pH indicator. So bacteria that ferment lactose, such as E. coli, produce acid, which lowers the pH of the agar, which turns the pH indicator a bright pink color. So on the plate displayed on the screen, lactose-fermenting colonies are on the left, and non-lactose-fermenting colonies are on the right. And finally, an enrichment medium is a medium that contains growth factors, vitamins, and other nutrients that promote the growth of fastidious microorganisms. So fastidious microorganisms are basically microbes that are super fussy or particular about their nutrients and/or environment. An example of an enrichment medium is blood agar. This type of agar is often used to cultivate pathogenic organisms that produce enzymes that cause hemolysis, or destruction of red blood cells. And on the agar shown here in the image, you can see a zone of hemolysis around the colonies.

All right. It's quiz time, and I have four questions for you. In this particular quiz, I'm going to provide a description, and I want you to name that culture medium. You guys ready? Number one, this medium allows different microorganisms to be visually distinguished based on a change in the color of the colonies or of the medium. The answer is, a differential medium. Question number two, the exact chemical composition of the medium is not known. The answer is, a complex medium. Number three, this medium supports the growth of wanted microorganisms but inhibits the growth of unwanted microorganisms. The answer is, a selective medium. And number four, this is a medium where the exact chemical composition of the medium is known. The answer is, a chemically defined medium.

All right. That's it for this quiz. I hope you did great, and I hope you found this video to be helpful. Take care and good luck with studying.