1. Make a mark on the bottom of the DNA plate, dividing it in half.
2. Heavily inoculate one side of the plate with Escherichia coli by rubbing growth from the slant in a circular area about the size of a quarter.
3. Inoculate the other side of the plate with Serratia marcescens, in the same manner.
4. Incubate the plate at 35°C for 24 hours.
5. Examine the plate for growth.
6. Drop 1 N HCl or 0.1% toluidine blue onto the agar surface until it is thinly covered with fluid.
7. Examine the areas around the growth on both sides of the plate for evidence of clearing or opacity, or a pink color if toluidine blue was used.
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