Hope you are now aware of Screening techniques - Introduction, dealt by our Sir(http://www.pharmainfo.net/dr-girija-sankar/blog/introduction-screening-t...
). Let us proceed a bit further.
Take a common example. Collect a soil sample and dilute it to a concentration. This solution is sprayed, spread or applied on to the agar medium and incubated. After incubation, what will you observe??? Yes!!! Aliquots of colonies of microorganisms.
We can come to a conclusion of what type of products is being produced by these microbes using primary screening. Microorganisms producing organic acids, amines from various Carbon substrates can be detected by the addition of pH indicating dyes such as neutral red, bromo thymol blue into the agar medium. The production of the above compounds can be detected by change in the colour of the dye in the vicinity of the colony. Similarly the dyes can be substituted with Calcium Carbonate. In the case the production of the above compounds can be detected by cleared zone of dissolved Calcium carbonate around the colony..
Well you cant judge or finalize the production of organic acids or amines using the afore said methods. The major con is that inorganic acids and bases may also be produced during microbial growth. For example commonly used Nitrogen source in the production is Ammonium salts - Ammonium sulfate. During the growth the organism may utilize Ammonium ion leaving behind sulfate ion as sulfuric acid which is indistinguishable from organic acid production.
Therefore, the cultures giving positive reaction must be tested further to assure that organic acid or base actually has been produced. This is done by using chromatographic techniques.
The screening approach is now-a-days has been employed in search for microorganisms producing antibiotics. The technique we use is the "crowded plate"
procedure. Through this method we will know the microorganism producing the antibiotic without regard to what type of microorganisms may be sensitive to the antibiotic. In this case, we measure inhibition zone.
Crowded plate technique carry some limitations since the activity is not measured against specific microorganism. Therefore, the screening is improved by the incorporation of "test organism" i.e., an organism used as an indicator for the presence of specific antibiotic activity. Dilutions of soil sample are applied to the surfaces of agar plates so that well isolated colonies will develop. The plates are incubated until the colonies are a few millimeters in diameter and so that antibiotic production will have occurred for those organisms having this potential. A suspension of test organism is sprayed/applied on the surface of agar and incubated further. The antibiotic activity is determined by measuring the inhibitory zone diameter in addition a rough approximation of relative amounts of antibiotic produced can be known by measuring diameters of zones of inhibited test organism growth. 
I have discussed this example since everyone is aware of antibiotic assays. Thus primary screening allows the detection and isolation of microorganisms that possess industrial applications. This is followed by Secondary screening which will be dealt in the upcoming blogs.
1. Industrial Microbiology by L.E.Casida , Jr. Reprint edition 2007, NEW AGE INTERNATIONAL (P) LIMITED PUBLISHERS, Page no: 55-63
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