∞ generated and posted on 2016.01.29 ∞
Microbiological growth medium that prevents some but not all organisms from growing.
Note that the idea of selective medium is not identical to that of differential medium. The reason for making this point is a combination of the two concepts not necessarily being obviously different and that often selective media is also differential media, though for different reasons. In distinguishing these concepts, keep in mind that the idea of "selection" in fact does not necessarily exclude that of differentiation. That is, it is quite common for a selective medium to also be a differential medium, though typically for different reasons and to different ends.
Examples of selective media (which also typically are differential media) include endo agar, eosin agar, mannitol salt agar, MacConkey agar, phenylethyl alcohol agar, etc. Note how these are all agar-based media, which often is a matter of convenience as anything.
Note also the concept of enrichment culture, which is a kind of selective medium though one that is typically used for reasons other than organism identification.
One means of differentiating selective and differential media is in terms of the concepts of artificial and natural selection. With artificial selection, as strictly considered, what is going on is a differentiation among organisms in terms of some trait that in some manner is observable to the breeder, i.e., us. That is, this is a trait that is not intrinsically selective within the environment that the organism resides but instead is selective only because we choose to use it as a means of differentiating among individuals.
With natural selection, by contrast, the selective agent is an intrinsic component of the selective environment. In particular, it impacts directly the ability of an organism to survive and/or reproduce. Thus, with selective media, rather than simply giving rise to a change in the appearance (defined broadly) of a culture, instead the organisms involve fail even to produce substantial growth. As noted, though, it is possible to include ingredients in media that give rise to differences in microorganism growth, microorganism appearance, or even both at the same time. However, differences in growth is a selective effect (selective media) whereas differences in appearances is a differential effect (differential media). That some substances can effect both at the same time is simply a function of the chemistry of the substance along with the physiology of the organisms involved.
The following video gest off to a slow start but good introduction to the use of MSA plates, a type of both differential and selective media:
The following video is a little amateurish but nonetheless does a good job of introducing MacConkey medium: