Serial Dilution Practice Problems

In each case, you have a stock culture followed by 6 serial dilution tubes, and from each of those a sample is taken and plated as a lane on an agar plate.

The first three are straightforward.

1. You serially transfer 100 µl through the 6 tubes and from those transfer 10 µl samples onto the respective lanes on a nutrient agar plate. You get these results for the six lanes: TMTC - TMTC - TMTC - 64 - 7 - 1. How many cfu/ml in stock culture #1?

2. You serially transfer 100 µl through the 6 tubes and from those transfer 10 µl samples onto the respective lanes on a nutrient agar plate. You get these results for the six lanes: TMTC - 86 - 9 - 0 - 0 - 0. How many cfu/ml in stock culture #2?

3. You serially transfer 100 µl through the 6 tubes and from those transfer 10 µl samples onto the respective lanes on a nutrient agar plate. You get these results for the six lanes: TMTC - TMTC - TMTC - TMTC - 42 - 3. How many cfu/ml in stock culture #3?

   These next three have difficulties. You should be able to detect errors in your own work so that you will have confidence when things work out right.

4. You serially transfer 100 µl through the 6 tubes and from those transfer 10 µl samples onto the respective lanes on a nutrient agar plate. You get these results for the six lanes: TMTC - 98 - 95 - 80 - 76 - 50. What is it about these that prompts you to question the correctness of your performance? What did you probably do wrong?

5. You serially transfer 100 µl through the 6 tubes and from those transfer 10 µl samples onto the respective lanes on a nutrient agar plate. You get these results for the six lanes: TMTC - 98 - 42 - 21 - 10 - 5. What is it about these that prompts you to question the correctness of your performance? What did you probably do wrong?

6. From a cloudy stock culture, you serially transfer 100 µl through the 6 tubes and from those transfer 10 µl samples onto the respective lanes on a nutrient agar plate. You get these results for the six lanes: 0 - 0 - 0 - 3 - 41 - TMTC. What did you probably do wrong?

   These next have a change of procedure. Otherwise, they are straightforward. If you can do them, you must rather thoroughly understand.

7. From a stock culture, you serially transfer 1 ml through 6 tubes each containing 1 ml. From these 10 µl is plated in lanes on MacConkey agar. Lane 1 gives rise to 96 colonies. How many colonies should you expect to find in the other lanes?

8. If the first six problems pertain to 10-fold serial dilutions. What -fold serial dilutions were made in the previous question?

9. You serially transferred 1 ml through a series of six tubes each containing 2 ml. Then 10µl was transferred to a petri plates lanes. The next day you find these results - TMTC - TMTC - 82 - 27 - 9 - 3. How many cfu/ml in stock culture #9?

10. (This is a very hard problem: good mathematical minds will get it; see the light and say it is easy.) You made a line of 9 serial dilution tubes, each containing 2.16 ml, and then from the stock culture, you serially transferred 1 ml through the 9 tubes. Then you plated 10 µl samples onto lanes. You obtained these results:
    TMTC - TMTC - TMTC - 99 - 31 - 10 - 3 - 1 - 0 .
    First, what "fold" serial dilution did you make? Second, how many cfu/ml in the stock culture. (Bright mathematicians can do this in their heads, but duller mathematicians can yet slug through it!)