Question

You have decided to determine how many microbes are living on the lettuce in the salad bar at your favorite restaurant. You place 5 grams of lettuce and 45 ml of water in a blender and blend the mixture. This is sample A. You next make a 1/10 dilution of sample A. This becomes sample B. You then transfer 1 ml of this dilution (sample B) into to another that contains 4 ml of water. This is sample C. Next you transfer 100 ul from sample C onto a TY plate, spread plate, and incubate. When you examine the plates after 48 hours you find 168 colonies growing. (For your information 1 gram = 1 ml)

A) What was the bacterial count per ml in SAMPLE A?

B) Would you expect to find more colonies growing on a plate that was inoculated from sample B or sample C?

C) How many microbes are living on 1 gram of lettuce.

Answer 1

A. The colony count per ml of the sample A is, 840,000.

B. Yes, we expect more colonies growing if we plated from sample B because it is more concentrated than the sample C. There would be more bacteria present in sample B.

C. In one gram of lettuce, there are 168,000 bacteria are living. This we get by dividing CFU by 5 , as we added 5 grams of lettuce.