In this video, we're going to begin our lesson on selective media. Selective media is a specific type of microbial growth media designed to promote only the growth of the species of interest and to inhibit the growth of other species that are not of interest. A classic example of selective media is MacConkey agar, a specific type of selective media commonly used in the medical field to isolate gram-negative intestinal bacteria. MacConkey agar contains crystal violet, a specific dye that inhibits gram-positive cell growth, therefore promoting gram-negative cell growth. It also contains bile salts, which inhibit non-intestinal bacteria, helping to promote intestinal bacterial growth. Between the crystal violet and the bile salts, they promote or select for the growth of gram-negative intestinal bacteria.
If we take a look at this image below, we can see how using MacConkey agar can help isolate gram-negative bacteria that grow in the intestine. Notice on the left-hand side over here, what we have is a tube containing a liquid broth of a mixture of bacterial cells, a mixed culture of bacterial cells including gram-positive gut bacteria. We can take this mixture of bacterial cells and transfer some of it to this MacConkey agar plate. Because the MacConkey agar plate contains crystal violet and bile salts, it will inhibit the growth of gram-positive cells and non-intestinal bacteria. It is helping to select for gram-negative intestinal bacteria. What you can see here on the MacConkey agar plate is that the only colonies that form are colonies of intestinal gram-negative bacteria. All other cells are going to be inhibited, and so they will not appear or grow on the MacConkey agar plate. The MacConkey agar plate is a type of selective media because it is selecting for a very specific species of interest to grow while inhibiting the growth of other species that are not of interest.
This here 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.