Lab Practicum - Sample Questions

1. The photo on the left shows 2 urea broth tubes that were inoculated with different bacteria.

A. Which of these 2 tubes contains urease-positive bacteria?

The tube on the right - dark red color

B. Give one example of how urease can be helpful to bacteria.

1. Survival in acidic environments; 2. To obtain nitrogen.

C. What are the breakdown products of urea?

Carbon dioxide and ammonium

D. Is the urease-positive broth more acidic or alkaline than the control?

The dark red color indicates alkaline reaction (due to ammonium)

2. The photo on the left shows 2 citrate slants that were inoculated with different bacteria.

 A. Which of these 2 slants contains bacteria that can use citrate?

The tube on the left - blue color

B. What do bacteria use citrate for?

As a carbon/energy source

C. What metabolic pathway is citrate used in?

The Krebs cycle

D. Is the citrate-positive slant more acidic or alkaline than the control?

The blue color indicates an alkaline reaction due to removal of citric acid and formation of sodium carbonate (due to the carbon dioxide)

3. The DNAse plate on the left was incubated with 2 bacteria and then treated with HCl.

A. Which of these 2 bacteria is DNAse-positive?

The one on the left (SA)

B. When DNA is broken down, what monomers are produced?

Nucleotides

C. Which of these 2 bacteria is likely to be more pathogenic?

SA - because DNAse aids these bacteria in evading host defenses

D. Why was hydrochloric acid (HCl) added?

To precipitate (make visible) the intact DNA (= cloudy)

4. Examine the SIM deeps pictured on the left.

A. Which of these deeps displays indole production?

The tube on the left (EC) - red color when Kovac's reagent is added.

B. Which of these deeps displays H2S production?

The tube on the right (PV) - black color throughout the tube.

C. Name the 2 sources of H2S in bacterial metabolism.

1. Anaerobic respiration using sulfur compounds as electron acceptor

2. Breakdown of amino aids (Cysteine - contains sulfur)

D. Where does indole come from?

Breakdown of the amino acid tryptophan

5. Examine these nitrate broths. (The tubes labeled "+Zn" had no color change before zinc was added).

A. Which of the 4 tubes shown on the left show evidence of bacterial nitrate reduction?

Tube 1 (EC) and 3 (SOIL)

B. Which of these 4 tubes shows evidence of denitrification?

Tube 3 (SOIL) - even though Zinc was added, no red color developed, showing that the bacteria had reduced nitrate beyond nitrite all the way to N2 gas (note the gas bubbles)

C. What can nitrate be reduced to?

nitrite, then nitrogen gas or nitrous oxide, then ammonium

D. During what metabolic process does this occur?

Anaerobic respiration

6. For the 3 Gelatin deeps pictured on the left:

A. Which of these 3 tubes contain(s) gelatinase-positive bacteria?

The tube on the right (liquid gelatin)

B. What is gelatin broken down into?

Amino acids, since gelatin is a protein

C. How can this enzyme help bacteria?

Gelatin (collagen) is found in connective tissues. Bacteria that hydrolyze it are more invasive/more pathogenic.

7. For the glucose fermentation tube seen on the left:

A. What type of fermentation (Base, negative, acid, gas) occurred?

Acid and Gas

B. Give some specific examples of fermentation products that can account for this.

lactic acid, CO2, acetic acid, formic acid, H2

C. What sugar(s) can this bacterium ferment?

Glucose. (Need to do a different experiment to find out about other sugars)

D. The ph of this medium is acidic.

8. For the 4 lactose fermentation tubes shown here:

A. Which of these bacteria is/are able to ferment lactose?

EC (acid & gas)

B. What causes the color in the tube marked "AF"?

production of base (ammonium from amino acid deamination)

C. Which of these organisms shows a neutral reaction?

SC - orange color

D. What enzyme does EC have that the other 3 organisms don't?

lactase (beta-galactosidase)

9. For the starch agar plate shown here:

A. Which of these 3 organisms can break down starch?

BS

B. What enzyme is needed to break down starch?

amylase

C. What causes the blue coloring in the agar?

Starch + Gram's Iodine --> blue

D. What is the major breakdown product?

Glucose

10. For the motility deeps shown here:

A. Which of these 3 organisms is/are motile?

EC and PV

B. What part of the cell is necessary for motility?

Flagella

C. What other culture medium can be used to determine motility?

SIM deep

11. For the milk agar plate shown here:

A. Which of these 3 organisms can break down milk protein?

PA and BS (weakly)

B. What is the name of this protein?

casein

C. What is the protein broken down into?

amino acids

D. What is the name of the enzyme that breaks down this protein?

casease (protease)

12. For the Methyl Red test seen here:

A. Which of these 2 tubes shows a positive test?

The right tube (red): E. coli

B. What type of fermentation do MR+ bacteria carry out?

Mixed acid fermentation 

C. What is the pH of these 2 tubes?

Left tube: pH = 6; right tube: pH = 4

D. Name some specific fermentation products you might find in the positive tube.

Succinic acid, acetic acid, formic acid, lactic acid)

13. For the two MRVP broths shown here (30 minutes after adding Barritt's reagent):

A. What is this test called?

Voges-Proskauer test

B. Which of these 2 tubes shows a positive test result?

The right tube (red color): Klebsiella

C. What fermentation pathway can this test detect?

Butanediol fermentation

D. How is this test related to the Methyl Red test?

The MR test detects mixed acid fermenters, which do NOT produce butanediol. The VP test detects butanediol fermenters, which are, in turn, MR-negative.