Question 1

Which of the following best describes an acidophile?

Accepted Answer

A microorganism that grows best at pH 0–5.4

Question 2

A student inoculates E. coli into broths of pH 3, 5, 7, and 9, incubates at 37°C, and measures turbidity at 600 nm. Which pH is most likely to show the highest turbidity for E. coli?

Accepted Answer

pH 7

Question 3

Why do most enzymes in bacteria have an optimum pH range between 6.5 and 7.5?

Accepted Answer

Because the protein structure of enzymes is most stable near neutral pH

Question 4

Saccharomyces cerevisiae is incubated at 25–30°C in broths of pH 3, 5, 7, and 9. Based on its classification, at which pH would you expect the greatest growth?

Accepted Answer

pH 5

Question 5

Which of the following equations correctly represents the calculation of optical density (O.D.) from percent transmittance (%T) in a spectrophotometer?

Accepted Answer

$$O.D. = -\log_{10} \left( \frac{\%T}{100} \right)$$

Question 6

A student finds that Alcaligenes faecalis grows best at pH 9. How should this organism be classified based on its pH preference?

Accepted Answer

Alkaliphile

Question 7

If a spectrophotometer reading at 600 nm shows a high %T and a low O.D., what does this indicate about the microbial growth in the sample?

Accepted Answer

There is little microbial growth in the sample

Question 8

Which of the following best explains why molds and yeasts are often found in acidic environments such as fruit juices?

Accepted Answer

They prefer and grow best at lower pH values

Question 9

A student averages the O.D. readings from three tubes of the same pH. What is the main reason for using the average rather than a single reading?

Accepted Answer

To minimize the effect of experimental error and increase reliability

Question 10

Which of the following scenarios best demonstrates the application of pH tolerance in food preservation?

Accepted Answer

Pickling vegetables in vinegar to inhibit bacterial growth

Question 11

If a bacterium is classified as a neutrophile, which of the following pH ranges would most likely inhibit its growth?

Accepted Answer

pH 2.0

Question 12

During an experiment, a student labels the test tube caps instead of the tubes themselves. What is the primary reason for this practice?

Accepted Answer

To prevent misidentification if tubes are reused or washed

Question 13

Which of the following is a likely consequence if E. coli is incubated at 25°C instead of 37°C in the pH experiment?

Accepted Answer

Reduced growth rate and lower turbidity readings

Question 14

A researcher wants to determine the effect of pH on a newly discovered microbe. Which experimental design would best allow for this analysis?

Accepted Answer

Inoculate identical broths at different pH values and measure growth using a spectrophotometer

Question 15

Which of the following best explains why the spectrophotometer is set to 600 nm for measuring microbial growth?

Accepted Answer

600 nm is optimal for detecting turbidity caused by microbial cells

Question 16

If a microbe shows equal growth at pH 5.5 and pH 8.0, but not at pH 3 or pH 9, how should it be classified?

Accepted Answer

Neutrophile

Question 17

Which of the following is a plausible reason why some bacteria can survive in a pH range of 4–9?

Accepted Answer

They possess enzymes with broad pH stability

Question 18

A student observes that the O.D. readings for Saccharomyces cerevisiae are highest at pH 5 and lowest at pH 9. What does this suggest about its pH preference?

Accepted Answer

It prefers acidic environments

Question 19

Which of the following best describes the relationship between enzyme structure and pH?

Accepted Answer

Extreme pH values can denature enzymes by altering their protein structure

Effect of pH on Microbial Growth: Experimental Study Notes

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