Question

You have decided to determine how many microbes are living on the lettuce in the salad bar at your favorite restaurant. You place 1 gram of lettuce and 99 mls of water in a blender and blend the mixture. This is sample A. You then transfer 1 ml of this dilution into to another that contains 9 mls of water. This becomes sample B. You next transfer 1 ml of sample B into a separate container that contains 9 mls of water. This is sample C. Next you transfer 1 ml each from samples B & C onto separate nutrient rich agar plates (samples-B & C to be transferred on two separate plates), swirl, let harden and incubate at 37oC. When you examine the plates after 48 hours you find 110 colonies growing on plate C. (For your information 1 gram = 1 ml) find

a) what was the final dilution?

b) if 230 colonies would have been counted, would this have been a valid count?

c) if you counted 30 colonies on one plate, how many cells/g would have been in the original sample of lecture?

Answer 1