Figure  3. ( above) Modified Kirby Bauer Plates containing a lawn of test bacteria. The dark spots are agar plugs cut from 2 week old starch agar streak plates (see Fig. 1) of Streptomyces isolated from different soil samples. The zones of inhibition surrounding  five of the agar plugs are produced by the diffusion of antibiotics from the agar plugs into the Kirby Bauer agar plate.

Figure 4.  Inhibition zone diameters produced by agar plugs from Streptomyces isolates (on the left) compared to the inhibition zones produced by commercial antibiotic disks (on the right) in the Kirby Bauer Assay.

• Week 7 or 8.  The agar streak plates from which the agar plugs were taken are flooded with 3 to 5 ml of sterile 25% glycerol to prepare spore suspension stock cultures. Spore suspensions are stored in a freezer at either  -20 C or  -70 C.


• The students are placed in groups to continue the project during scheduled laboratory class hours but additional time outside of class is usually required. Many students volunteer to work on the project on their own time at this point.

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