Bacteriology/Food Science 324:
SAMPLE INDICATED NUMBER
AND MPN PROBLEMS
to go along with Exercise 2 and Appendix F.2
in the 1999 edition of the lab manual

Also click here for another review of the MPN method.
Solutions to the problems below are posted here.

  1. In each of the following, a food product was decimally diluted, and one ml from each dilution was inoculated into a tube of an all-purpose broth medium. After incubation, the tubes showing growth were marked with a +. State the indicated number of organisms per gram of food product in each of the following:


    1. dilution: 10–2 10–3 10–4 10–5
      result: + + – –


    2. dilution: 1/10 1/100 1/1000 1/10,000 1/100,000
      result: + + – + –

  2. One gram of food was blended with 9 ml of diluent. Three additional 1/10 dilutions were made. From each of these four serial dilutions of the food, tubes of an all-purpose broth medium were inoculated as shown in the table below. After incubation, the following results were obtained:

    Dilution of the food 10–1 10–2 10–3 10–4
    Amount of dilution inoculated into
    each of 3 tubes of broth    		 1.0 ml 1.0 ml 1.0 ml 1.0 ml 0.1 ml
    Number of tubes showing growth 3 3 3 2 0

    Using a 3-tube MPN table, determine the most probable number (MPN)of microorganisms per gram of the food.

  3. Five grams of food were blended with 45 ml of diluent. Three additional 1/10 dilutions were then made. From each of these four serial dilutions, one-tenth ml was inoculated into each of three tubes of an all-purpose broth medium. The following results were observed after incubation:

    Dilution of the food 10–1 10–2 10–3 10–4
    Number of tubes showing growth 3 3 1 0

    Using a 3-tube MPN table, determine the MPN of microorganisms per gram of the food.

  4. A sample of raw hamburger was divided aseptically, and weighed portions were added to flasks of appropriate amounts of pre-enrichment broth utilized in the FDA Salmonella isolation and identification procedure (Exp. 16). The amounts utilized are indicated in the table below. The entire FDA procedure for the enrichment, isolation and identification of Salmonella was done from each flask, and the number of flasks in which Salmonella was confirmed present is indicated below.

    Amount of hamburger added to
    each of three flasks of broth    		 50 g 5.0 g 0.5 g
    No. of flasks from which
    Salmonella was isolated    		 3 3 0

    Calculate the Salmonella count per gram of the hamburger.

Return to the 1999
Bact./Food Sci. 324
Home Page. 		Solutions are here.
Page last modified on 7/20/00 at 6:00 PM, CDT.
John Lindquist, Department of Bacteriology,
University of Wisconsin – Madison