Selective & Differential Medium. Introduction. In addition to general-purpose media, which allow the growth of most types of bacteria, microbiologists use specialized media to identify and/or isolate specific groups of bacteria .
On the other hand, selective inhibition of some types of microorganisms can be achieved by adding dyes, antibiotics, salts or specific inhibitors which affect the metabolism or enzyme systems of the organisms. For example, media containing potassium tellurite, sodium azide or thallium acetate (at concentrations of 0.1 - 0.5 g/l) will inhibit the growth of Gram-negative bacteria.
and the differential agent is what causes the bacteria look
Some media are both selective and differential, that is, they are able to select against the growth of certain organisms while the organisms that do grow may exhibit some differential growth characteristics.
On MSA, pathogenic Staphylococcus aureus produces small colonies surrounded by yellow zones. The reason for this change in color is that S. aureus ferments the mannitol, producing an acid, which, in turn, changes the indicator from red to yellow.
The eosin and methylene blue dyes cause lactose fermenters to have pink colonies. E. coli incorporates so much of the dye that the dyes precipitate in the cells and give the colonies a metallic green sheen.
The dark colonies produced on EMB agar is a result of the acid produced during lactose fermentation precipitating the dyes in the media.
Coliform bacilli produce acid from lactose fermentation causing the colonies to turn red from the pH indicator.
Yussuf M. AbdElalHanan I. Abu Hasira
Yasser M. AldahdohZeina A. Shubair
Islam F. Hasouna
Dr. Abdelraouf A. Elmanama