MCB 101 Introductory Microbiology Lab
Practice Exam 3
Spring 2008
For each question, choose the one best answer.
There are 25 multiple choice questions that are worth 4 points each.
1) In Experiment 15: Contamination of Hamburger, we used 3M Petrifilm cultures to
count the bacteria in the diluted meat samples. What volume of liquid is used to
inoculate a 3M Petrifilm culture?
A. 0.1 ml
B. 0.3 ml
C. 1.0 ml
D. 3.0 ml
E. 10.0 ml
2) A hamburger sample is tested for coliform bacteria by spreading samples on standard
Petri dish cultures of EMB agar. After incubation the dark purple-green colored
coliform colonies are counted. A 0.10 ml aliquot of a 1 x 10-2 dilution of the sample
gives 90 coliform colonies. What is the concentration of coliform bacteria in this sample
of hamburger?
A. 9.0 x 101
D. 9.0 x 104
B. 9.0 x 102
E. 9.0 x 105
C. 9.0 x 103
3) According the U.S. Department of Agriculture (USDA) what is the maximum
permissible number of bacterial cells in hamburger?
A.
0.0
B. 1.0 x 101
C. 1.0 x 103
D. 1.0 x 105
E. 1.0 x 107
bacterial cells/gram
bacterial cells/gram
bacterial cells/gram
bacterial cells/gram
bacterial cells/gram
4) On a 3M Petrifilm culture for E. coli/coliforms, such as those we used in
Experiment 15, a colony of a fecal coliform appears:
A.
B.
C.
D.
E.
red and not associated with a gas bubble.
white and not associated with a gas bubble.
blue and not associated with a gas bubble.
red and associated with a gas bubble.
blue and associated with a gas bubble.
5) Food safety experts suggest hamburger should be heated to _______ to kill
all harmful bacteria.
A. 100oF
D. 212oF
B. 130oF
E. 350oF
C. 160oF
6) What is the term for microorganisms that can grow and cause food spoilage in the
refrigerator?
A. acidophillic
D. mesophillic
B. psychrophillic
E. thermoduric
C. thermophillic
7) What type of microorganism is Penicillium?
A. Fungus
D. Archaea
B. Bacteria
E. Virus
C. Protozoa
8) The food preservative sodium nitrite is:
A. an organic acid.
B. a phenol derivative.
C. a fat soluble antioxidant.
D. an inhibitor of pyruvate-ferrodoxin oxidoreductase.
E. produced as a waste product of microbial fermentation of sugar.
9) Nitrites are added to meat because it gives meat a pink color and inhibits:
A. Pseudomonas aeruginosa
C. Clostridium botulinum
E. Esherichia coli
B. Bacillus subtilis
D. Salmonella typhi
10) How do propionate and benzoate prevent microbial growth?
A.
B.
C.
D.
E.
They attack proteins, prosthetic groups, sugars and nucleic acids in microbes.
They inhibit two enzymes; pyruvate-ferrodoxin oxidoreductase and ferrodoxin.
They inhibit ripening enzymes that are naturally found in food.
They denature protein destroying the cytoplasmic membrane of microbes.
They increase the proton concentration of the cytoplasm in microbes.
11) Which one of the following substances is not classified as a disinfectant?
A. alcohol
D. tetracycline
B. bleach
E. hydrogen peroxide
C. phenol
12) An antiseptic is:
A.
B.
C.
D.
an antimicrobial substance used to kill microorganisms on an operating table.
an antimicrobial substance that is used to kill or inhibit microorganisms on the skin.
an antimicrobial substance added to a food or beverage to inhibit spoilage.
a drug derived from a natural product that is used fight microorganisms within
a patient's body.
E. a drug that is a completely man-made chemical used to fight microorganisms
within a patient’s body.
13) Which two of the following statements about disinfectants and antiseptics are true?
i. In a diffusion test of an antimicrobial agent, the size of the zone of inhibition is
not affected by the temperature of incubation or the composition of the medium.
ii. Phenol was used as a disinfectant in hospitals for many years.
iii. Chemical disinfectants are more lethal to bacteria at higher concentrations.
iv. In a diffusion test of an antimicrobial agent, a more resistant species of bacteria
will show a larger zone of inhibition.
A. i. and ii.
B. i. and iii.
C. i. and iv.
D. ii. and iii.
E. ii. and iv.
14) The Snyder Test is used to:
A. check for high numbers of glucose-fermenting, acid-tolerant bacteria in saliva.
B. check for the presence of lactose fermenting bacteria in saliva.
C. check for the presence of lactic acid in saliva.
D. determine the effectiveness of a disinfectant solution.
E. determine the coliform count in a sample of water.
15) Which one of the following statements pertaining to Snyder Test Agar is true?
A.
B.
C.
D.
E.
If the Snyder Test Agar turns blue it indicates the presence of fermentative bacteria.
Snyder Test Agar contains lactose as the fermentable sugar.
Snyder Test Agar will turn fully yellow only if the pH drops below 3.8.
The starting pH for Snyder Test Agar is 6.8, which is close to neutral.
Snyder Test Agar is incubated for not more than 22-25 hours before checking for
the final results.
16) Which one of the following statements about lactic acid and ethanol fermentations
is false?
A. The predominant sugar found in grape juice is sucrose.
B. Both ethanol and lactic acid can inhibit the growth of undesirable microorganisms
in food products.
C. In a 100% efficient fermentation, lactic acid bacteria can convert 100 grams of sugar
to 100 grams of lactic acid.
D. In ethanol fermentation yeast produce one molecule of CO2 for every molecule of
ethanol they make.
E. Oxygen is needed for fermentation reactions to take place.
17) Experiment 18b: Yogurt Production
Which two of the following statements about setting up a yogurt culture are true?
i. Boil your milk until it starts to turn brown and then stir in some good yogurt
to inoculate the culture while it's still steaming hot.
ii. The incubation temperature for the yogurt culture is 43oC
iii. Before adding yogurt to the skim milk, the milk should be cooled to below 10oC
iv. The production of yogurt is an example of a microbial succession that primarily
involves two species of bacteria.
A. i. and ii.
B. i. and iii.
C. i. and iv.
D. ii. and iii.
E. ii. and iv.
18) A particular batch of grape juice contains 22% sugar. (This is in units of weight per
volume, or 22 g of sucrose per 100 ml.) If 2000 liters of this grape juice is turned into
wine, what is the maximum amount of ethanol that could be produced in a 100% efficient
fermentation?
(Hint: First, determine how much sugar you start with.)
Overall reaction:
formula weights:
(grams/mole)
A.
B.
C.
D.
E.
C12H22O11 + H2O  4 HOCH2CH3 + 4 CO2
342
209.1 kg of ethanol
224.9 kg of ethanol
236.7 kg of ethanol
460.0 kg of ethanol
23670 grams of ethanol
18
4 x 46 = 184
4 x 44 = 176
19) Yogurt production primarily involves:
A.
B.
C.
D.
E.
the fermentation of a sugar to produce lactic acid.
hydrolysis of protein to produce short peptides and free amino acids.
putrifying bacteria.
boiling the milk for 30 minutes.
bacteria that grow best in the refrigerator.
20) Which two of the following bacteria are used to make yogurt?
i.
ii.
iii.
iv.
Lactobacillus bulgaricus
Pseudomonas aeruginosa
Staphylococcus aureus
Streptococcus thermophilus
A. i. and ii.
B. i. and iii.
C. i. and iv.
D. ii. and iii.
E. ii. and iv.
21) Redox Reactions Involving Nitrogenous Compounds
Which two of the following chemical changes are oxidation reactions?
i.
ii.
iii.
iv.
nitrite is converted to nitrate
ammonia is converted to nitrite
nitrate is converted to elemental nitrogen (N2)
elemental nitrogen (N2) is converted to ammonia
A. i. and ii.
B. i. and iii.
C. i. and iv.
D. ii. and iii.
E. ii. and iv.
22) Which one of the following statements about the nitrogen cycle is false?
A.
B.
C.
D.
E.
Nitrosococcus bacteria can produce energy by oxidizing ammonia to nitrite.
The oxidation of ammonia to nitrate is called denitrification.
Some strains of Pseudomonas bacteria can reduce nitrates to nitrogen gas.
The enzyme urease breaks down urea to ammonia and CO2.
The breakdown of amino acids and other nitrogen-containing organic compounds
to release ammonia is called mineralization.
23) Trommsdorf reagents were used in both Experiment 19b: Denitrification
and in Experiment 19c: Nitrification.
Which one of the following statements about Trommsdorf reagents is false?
A. Trommsdorf reagents produce a dark-blue color when they react with nitrite (NO2-).
B. Trommsdorf reagent II can burn your skin or cause holes to appear in your clothing.
C. For the nitrate reductase test, a color change to blue after the addition of zinc
indicates a NEGATIVE test result (nitrate was not reduced by the bacteria).
D. For the ammonia oxidation test, a color change to blue after addition of zinc
indicates a POSITIVE test. (Bacteria oxidized the ammonia completely to nitrate.)
E. Trommsdorf reagents produce a blue color when they react with ammonium (NH4+).
24) Which one of the following statements about Nitrification is false?
A. Nitrification is also called ammonia oxidation.
B. Nitrification is a two-step process. Nitrosococcus carries out the first step while
Nitrobacter completes the process.
C. Nitrification is an aerobic process.
D. Nitrification requires an energy input, it consumes a lot of ATP and reducing power.
E. Nitrification is the conversion of ammonia to nitrate.
25) Experiment 19: Soil Bacteria and the Nitrogen Cycle
Which one of the following statements about Experiment 19 is false?
A.
B.
C.
D.
E.
Denitrification is a form of anaerobic respiration.
Mammals excrete excess nitrogen primarily in the form of ammonia.
The main carbon source in the ammonia broth used in Experiment 19C is carbonate.
The carbon source in nitrate broth (Ex. 19b) is a mixture of organic compounds.
The only organisms that can convert elemental nitrogen (N2) to organic forms are
certain prokaryotes.