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Learn the toughest concepts covered in Microbiology with step-by-step video tutorials and practice problems by world-class tutors.

10. Dynamics of Microbial Growth


Selective Media

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in this video, we're going to begin our lesson on selective media. And so selective media is a specific type of microbial growth media that is designed to promote only the growth of the species of interest. And therefore it's also designed to inhibit the growth of other species that are not of interest. And so a classic example of a selective media is McConkey auger. And so McConkey auger is a specific type of selective media that is commonly used in the medical field to isolate gram negative intestinal bacteria. And so this McConkey auger actually contains crystal violet, which is a specific dye that is going to inhibit gram positive cell growth, therefore promoting gram negative cell growth. And the McConkey auger also contains bile salts which are going to inhibit non intestinal bacteria, helping to promote intestinal bacterial growth. And so between the crystal violet and the bio salts they are helping to promote or select for the growth of gram negative intestinal bacteria. And so if we take a look at this image down below, we can see how using McConkey agar can help to isolate gram negative bacteria that grow in the intestine and so notice on the left hand side over here what we have is a tube that contains a liquid broth of a mixture of bacterial cells. So it's a mixed culture of bacterial cells. And so this mixed culture of bacterial cells includes gram positive gut bacteria. And so uh what we can do is we can take this mixture of bacterial cells and transfer some of it to this McConkey auger plate. And because the McConkey agar plate contains crystal violet and bio salts, it will inhibit the growth of gram positive cells and it will also inhibit the growth of non intestinal bacteria. And so it's helping to select for gram negative intestinal bacteria. And so what you can see here is on the McConkey agar plate, the only colonies that form our colonies that are intestinal gram negative bacteria and all of the other cells are going to be inhibited. And so they will not appear or grow on the McConkey agar plate. And so the McConkey agar plate is a type of selective media because it is selecting for a very specific species of interest to grow and inhibiting the growth of other speeches that are not of interest. And so this year concludes our brief lesson on selective media and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

A type of culture medium that induces the growth of one particular organism in a mixed population of cells is: