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Lab Quiz: Serial Dilutions

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In which of the following foods would you normally expect to find a high number of bacteria?
a) Salad dressing
b) Yogurt
c) Lunchmeat
d) Canned peaches
b) Yogurt
Where would you expect to find members of the genus Pseudomonas?
a) On the hands and in the respiratory tract of food handlers
b) On eating utensils in a restaurant
c) In the soil
d) In the guts of mammals
c) In the soil
Where would you be most likely to find the fungus Penicillium?
a) In soils and water
b) Growing on animal hides
c) In the gut of humans
d) In the respiratory tract and on the skin of food handlers
a) In soils and water
Where would you be most likely to find members of the genera Escherichia?
a) On the hands and in the respiratory tract of food handlers
b) In soil and water
c) In the guts of mammals
d) All available options are likely to contain Escherichia.
c) In the guts of mammals
How does bacterial enumeration help to determine if a food is safe for consumption?
a) It will determine the number of organisms responsible for spoilage that are present in a sample.
b) It will determine the number of pathogens present in the food.
c) It identifies microorganisms that are pathogenic or will cause food spoilage.
d) This will give a general idea of the quality of the food.
d) This will give a general idea of the quality of the food.
In which of the following food types would you expect to see high numbers of microbes?
a) Hamburger
b) Prepackaged salad
c) Cheese
d) None of the available answers
c) Cheese
What does the presence of enteric organisms in milk indicate?
a) Fecal contamination of the milk occurred somewhere in the production process.
b) The milk was contaminated by microbes on the skin of the cows.
c) The milk was contaminated by microbes on the skin of the people working in the processing plant.
d) The milk was contaminated with soil at some point in the process.
a) Fecal contamination of the milk occurred somewhere in the production process.
Which of the following terms best describes the enumeration of bacteria as described in your lab?
a) None of the available answers.
b) A direct enumeration
c) Enteric
d) An indirect enumeration
b) A direct enumeration
You found 67 colonies on the ground beef plate with the 10^-2 dilution. What is the concentration of bacteria per milliliter of beef?
a) 6.7 × 10^2 CFU/ml
b) 6.7 × 10^4 CFU/ml
c) 6.7 × 10^3 CFU/ml
d) 67
c) 6.7 × 10^3 CFU/ml
Which of the following is indicative of the presence of E. coli?
a) colonies on EMB agar with a metallic sheen
b) growth on EMB agar
c) colonies on EMB agar with a white precipitate
d) inhibited growth on EMB agar
a) colonies on EMB agar with a metallic sheen
If you had fresh lettuce containing 9.3 × 10^3 organisms/ml, how many colonies would you expect to see on a plate that represents a 10^-2 dilution?
a) 9
b) 93
c) 930
d) 9300
b) 93
Which of the following bacterial concentrations would produce a TFTC plate at the 10^-4 dilution?
a) 2 × 10^8 organisms/ml
b) 2 × 10^6 organisms/ml
c) 2 × 10^7 organisms/ml
d) 2 × 10^5 organisms/ml
d) 2 × 10^5 organisms/ml
Which of the following would produce a countable plate?
a) A solution containing 2.1 × 10^4 at a 10^-1 dilution
b) A solution containing 1.9 × 10^4 at a 10^-2 dilution
c) A solution containing 1.6 × 10^7 at a 10^-6 dilution
d) A solution containing 2.5 × 10^7 at a 10^-2 dilution
b) A solution containing 1.9 × 10^4 at a 10^-2 dilution
After running this experiment, you find that the carrots you tested had only 15 bacteria/ml. Your lab partner states that this number is so low that the carrots do not need further treatment. Why should you disagree?
a) None of the available answers are correct.
b) Since these organisms are Gram negative, they are definitely pathogenic, so even a small dose could cause disease.
c) The 15 bacteria are resistant to the erythromycin, methicillin, and bacitracin found in EMB agar.
d) A low number of organisms does not indicate the absence of pathogens.
d) A low number of organisms does not indicate the absence of pathogens.
Why must you perform multiple dilutions for each food type?
a) You do not know if the organisms in the food are pathogenic.
b) You do not know the initial concentration of bacteria in the food.
c) You need to get one TFTC plate, one countable plate, and one TNTC plate for each food.
d) You do not know if the organisms in the food are Gram positive or Gram negative.
b) You do not know the initial concentration of bacteria in the food.
Why are the coliform plates incubated at 37° C?
a) Coliforms, by definition, are thermophiles.
b) They grow best at body temperature.
c) The nutrients will not be released from EMB agar at temperatures below approximately 35° C.
d) None of the available answers is correct.
b) They grow best at body temperature.