Bacteriology/Food Science 324:

Given October, 1996

Answers to the questions below are posted here.

I.  MULTIPLE TRUE/FALSE (5 points). In the blank by each statement, place a + for a true statement or a O for a false statement. There can be any number of true or false statements. Do not change or qualify the wording of any statement in any way. Each is either true or false as stated. (1/2 point for each blank.)

Example: The following are plural terms which should never (not ever) be used as singular terms:

  +    bacteria
  O    coccus
  +    media

1.  Regarding the pseudomonads:

        Refrigeration prevents the growth and spoilage of foods by these organisms.
        The acidic reaction associated with their growth in Glucose O/F Medium is an indication of glucose fermentation by these organisms.
        They can grow on selective media used routinely for the isolation of enterics.
        They are like enterics in being gram-negative, catalase-positive rods.

2.  Regarding the enterics:

        They are defined by microbiologists as being any organism whose primary habitat is the intestinal canal of warm-blooded animals and humans.
        They all ferment glucose.
        The enteric group includes fecal coliforms and Salmonella.

3.  Regarding the lactics:

        The group name "lactics" refers to the fact that they all ferment lactose.
        They are all gram-positive organisms which can be rods or cocci.
        As a rule, they are all capable of respiration and fermentation.

II.  MATCHING (14 points).

1.  (4 points) Match the bacteria in column a with the associated category of food-related organisms in column b. Only one letter per blank. Any letter may be used any number of times or not at all. (1 point for each blank.)


        sauerkraut bacteria
        yogurt bacteria
        fecal coliforms
        fluorescent pseudomonads

A.  starter culture
B.  wild fermentation
C.  indicator organism
D.  spoilage organism (i.e., one which
causes a defect which renders a food
item undesirable)

2.  (10 points) Place the letter of the correct item from column b in the blank by each statement in column a. Only one letter per blank. Any letter may be used any number of times or not at all. (1 point for each blank.)


        Nitrite burn is associated with this pigment.
                Two physiological types of lactics.
        Test which differentiates between these two types.
                Two physiological types of enterics.
        Test which differentiates between these two types.
        Term associated with a particular organism's need for lots of growth factors.
        The presence of citrate in a medium used for this test can cause a false-positive reaction.
        The benzidine test is needed to tell if a positive catalase reaction is due to true catalase or this enzyme.

A.  fastidious
B.  homofermenters
C.  mixed-acid fermenters
D.  myoglobin
E.  methyl red test
F.  oxidase
G.  Durham tube
H.  butanediol fermenters
I.  fluorescein
J.  hot-loop test
K.  pseudocatalase
L.  heterofermenters

III.  SHORT ANSWER (15 points).

1.  (2 points) The controversy over the addition of nitrite in cured meat products involves what two conflicting effects of this preservative?

2.  (2 points) Name and briefly explain two important factors in the use of indicator organisms.

3.  (6 points) What are the three steps in the coliform detection and isolation procedure? Briefly explain the purpose of each step.

4.  (3 points; 1/2 point per item)

a.  A certain amount of salt (NaCl) is included in the formulation of some fermented food products such as sauerkraut and sausage. List two effects of the addition of NaCl.

b.  List three ways in which acid produced by the fermentation of organisms in sausage can affect the product.

c.  What group (genus or group of genera) of organisms is responsible for any desired acid production in sausage?

5.  (only 2 points) A little medium thought question: You wish to exploit certain properties of the difficult-to-isolate bacterium Excalibacterium (an enteric) in order to help you detect and isolate it from certain food samples which are highly contaminated with other enterics. You decide to start with MacConkey Agar which you know contains lactose as the only fermentable sugar. Peptone is another medium ingredient which you recall; it contains a mixture of various amino acids – none in any especially high amount. Following is a table showing what has been found out about the organisms associated with this particular food product:

genusfermentation ofdecarboxylation of
other enterics+++ or –+ or –+ or –+ or –+ or –

a.  On MacConkey Agar, what would you expect the net pH reaction would be for any of the three genera specifically listed on the table above?

(Circle one)    ACID    ALKALINE

b.  As these 3 genera don't ferment or respire lactose, how can they grow (think about generating energy and obtaining carbon) on MacConkey Agar?

c.  What would be the best choice for a sugar to add to MacConkey Agar which will assist greatly in the differentiation of Excalibacterium colonies from the other organisms on the table?


d.  If lysine were to be included in the medium in a relatively large amount, what effect would this have on the pH reaction associated with Excalibacterium colonies?


IV.  PROBLEMS (6 points).

1.  (4 points) An one gram sample of hamburger was added to 9 ml of sterile saline and blended vigorously. Four, subsequent 1/10 dilutions were then made. One ml amounts from the dilutions were plated in duplicate with Plate Count Agar (PCA). From the same dilutions, a 3-tube most probable number analysis was set up with a broth medium formulated to support the growth of only gram-negative bacteria; each tube was also inoculated with one ml . After appropriate incubation, the results were recorded as follows:

Dilution made of the original hamburgerfirstsecondthirdfourthfifth
Colony count on PCATNTC
No. of broth tubes showing growth31000

a.  Calculate the total number of colony-forming units per gram of the hamburger.

b.  Calculate the most probable number of gram-negative organisms per gram of the hamburger.

2.  (2 points) The same dilution can be obtained in each of the following situations:

a.  The addition of 1 ml of a sample to 9 ml of sterile diluent.

b.  The addition of         ml of the same sample to 27 ml of diluent.

c.  The addition of 10 ml of the same sample to         ml of diluent.

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John Lindquist, Department of Bacteriology,
University of Wisconsin – Madison