Selective media used for the growth of only selected microorganisms. For example, if a microorganism is resistant to a certain antibiotic, such as ampicillin or tetracycline, then that antibiotic can be added to the medium in order to prevent other cells, which do not possess the resistance, from growing.
On the other hand, selective media used to isolate pure cultures of bacteria from mixed populations.
Then why researchers used selective/differentiation agar plates to enumerate the pure cultures? For enumeration why don't used common agar like nutrient or LB agar?
Selective media are necessary for quantitation of a single species (or subset of species) from complex samples with multiple bacterial and/or fungal species present. Meaningful quantitative data from culture can only come from evenly-plated direct inoculation of an original sample. If there are multiple species in the original sample it is very difficult to count colonies of only a single colony morphology, especially if other species present in the sample grow faster, exhibit swarming motility, etc. Thus, if the selective medium does not significantly inhibit the growth of the target species being quantified, that colony morphology can be counted from serial dilutions while the colonies of other species (greatly reduced) are ignored. If one is only interested in total counts, then there is no reason to use selective or differential media.
Selective media with antibiotics allow selection for the resistant population. Depending on the resistance mechanism, subculture on non-selective medium may cause the loss of the resistance mechanism. If you cultivate for instance ESBL for a number of generations on standard media, the bacteria are at risk of losing the plasmid. The same holds true for selection of metabolic characters. Once the selection pressure is ‘off’ (i.e. the bacteria grow on a nutrient reach medium) they may adapt by changing their metabolism.
In addition, the continuing use of selective media makes it easier to detect contaminations that are still within a culture or may come into the culture during experimental manipulations.
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