Bacteriology/Food Science 324:
SAMPLE PLATING PROBLEMS
to go along with Exercise 1 and Appendix F.1
in the 1999 edition of the lab manual

Also click here for another review of dilution plating.
Solutions to the problems below are posted here.

1. (This is Example No. 2 on page 88.) One ml of a bacterial culture was pipetted into a 9 ml dilution blank. One-tenth ml of this dilution was pipetted into a 9.9 ml dilution blank. From this dilution, one-tenth ml was plated with 25 ml of culture medium. 219 colonies arose after incubation. How many colony-forming units were present per ml of the original culture? By whatever method you work it through, you should result in this answer: 2.19 X 10⁶ CFUs/ml – which is better rounded off to 2.2 X 10⁶. (Does the amount of medium in the plate matter?)
   
   
2. Five grams of yogurt were mixed with 45 ml of sterile diluent. Two successive 1/100 dilutions were then made. One-tenth ml was plated from the last (most dilute) dilution onto each of two plates of an all-purpose medium. After incubation, 75 colonies were counted on one plate and 73 were counted on the other. Calculate the number of CFUs per one gram of the yogurt.
   
   
3. If the first dilution made in the problem above involved one gram of yogurt being added to nine ml of diluent, would there be any change in the answer to the problem? Briefly explain why or why not.
   
   
4. You are given 10 ml of a 10^–1 dilution of sauerkraut juice. You wish to do a "total aerobic plate count" such that each plate is inoculated with 0.1 ml, and the following plated dilutions are made: 10^–2, 10^–3, 10^–4 and 10^–5. Diagram the procedure you would use to accomplish this, and clearly indicate the amounts of diluents and inocula.
   
   
5. In the setup for the problem above, you obtain 120 colonies on the plate inoculated with 0.1 ml of the 10^–2 dilution of juice. What would be the no. of CFUs per ml of the undiluted sauerkraut juice?
   
   
6. You took 5 ml of a sample and added it to a 64 ml dilution blank. You would then express the dilution as which one of the following:
   1/10    5/64    5/69    1/100
   
   
7. One should expect the same number of CFUs in 10 ml of a 10^–2 dilution of a hamburger sample as in      gram(s) of the undiluted sample.
   
   
8. The same dilution can be obtained in each of the following situations:
   
   
   1. The addition of 1 ml of a sample to 9 ml of sterile diluent.
      
      
   2. The addition of      ml of the same sample to 99 ml of diluent.
      
      
   3. The addition of 10 ml of the same sample to      ml of diluent.
      
      
9. Eleven grams of cheese were mixed with 99 ml of sterile diluent. A 1/100 dilution was then made. One-tenth ml was plated from the last (most dilute) dilution onto each of 3 plates of PCA. An average of 50 colonies per plate was counted. Calculate the number of CFUs per gram of the cheese.
   
   
10. One gram of cheese was added to a 99 ml dilution blank. One 1/100 dilution was then made. From the last dilution made, 0.2 ml was plated onto each of two plates of MacConkey Agar. After incubation, one plate had 90 red colonies and 192 white colonies, and the other plate had 94 red colonies and 188 white colonies. Determine the number of gram-negative, lactose-fermenting CFUs per gram of the cheese.
    
    
11. One gram of yogurt was added to 99 ml of sterile diluent. Four decimal (1/10) dilutions were then made, and one-tenth ml was plated in duplicate on Plate Count Agar. After incubation, the following colony counts were made:
    

    dilution made of the yogurt	colony count
    initial dilution	TNTC, TNTC
    first 1/10 dilution	TNTC, TNTC
    second 1/10 dilution	442, 432
    third 1/10 dilution	52, 48
    fourth 1/10 dilution	7, 3

    
    Using this data, calculate the number of CFUs per gram of the yogurt.
    
    
12. One-tenth ml of milk was plated, resulting in 42 colonies. What was the number of CFUs per ml of the milk?