1. Prepare a 5 h broth culture of the organism to be selected and a 5 h mixed broth culture of the organism(s) to be
inhibited.
2. Take three sterile tubes and label 1 to 3. Using a sterile Pasteur pipette, place in each tube, 5 drops of the broth
culture containing the organism to be selected.
3. Using a second sterile Pasteur pipette, add to each tube the following drops of the mixed broth culture containing
the organisms to be inhibited:
Tube 1: 5 drops
Tube 2: 10 drops
Tube 3: 15 drops
4. Divide a plate of the medium to be tested into three segments and label 1, 2, and 3. Using a small sterile loop, inoculate the appropriate segments of the plate with a loopful of the organism suspension from the tubes (i.e. 1 loopful from tube 1 in segment 1, etc).
5. Inoculate a second plate of the medium with a loopful of the pure 5 h broth culture of the organism to be selected.
6. After overnight incubation at 35–37 C, record the degree of selectivity of the medium (from the segmented plate)
and the size and appearance of the colonies of the selected organism (from the pure culture plate). Compare with the results of previous performance tests.
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