Question 1

Which of the following best explains why thermophilic microbes have a higher proportion of charged amino acids (such as lysine and arginine) in their proteins?

Accepted Answer

Charged amino acids form strong ion pairs and salt bridges, stabilizing protein structure at high temperatures.

Question 2

A food scientist wants to preserve cucumbers using a method that inhibits most spoilage bacteria. Which of the following pH values would be most effective for this purpose, and why?

Accepted Answer

pH 4.0, because acidic conditions denature bacterial proteins and inhibit growth.

Question 3

Which adaptation allows hyperthermophilic Archaea to maintain membrane integrity at extremely high temperatures?

Accepted Answer

Formation of a lipid monolayer with ether-linked lipids.

Question 4

A psychrophilic bacterium is transferred from its native glacier environment to a refrigerator at 4°C. Which membrane adaptation would most likely help it survive?

Accepted Answer

Increasing the proportion of polyunsaturated fatty acids in its membrane.

Question 5

Which equation best describes the relationship between water activity ($a_w$) and solute concentration in a microbial environment?

Accepted Answer

$a_w = \frac{P}{P_0}$, where $P$ is the vapor pressure of the solution and $P_0$ is the vapor pressure of pure water.

Question 6

Why are heat-stable enzymes from thermophiles, such as Taq DNA polymerase, essential for the polymerase chain reaction (PCR)?

Accepted Answer

They can withstand repeated heating cycles without denaturing, allowing DNA to be copied efficiently.

Question 7

A microbe is classified as a facultative anaerobe. Which of the following best describes its growth behavior in the presence and absence of oxygen?

Accepted Answer

It grows best with oxygen but can also grow without it.

Question 8

Which of the following is a key reason why organic acids (like acetic acid) are more effective than strong mineral acids for food preservation?

Accepted Answer

Organic acids can cross microbial membranes in their undissociated form and acidify the cytoplasm.

Question 9

Which adaptation would you expect to find in a halophilic microbe living in a salt lake?

Accepted Answer

Accumulation of compatible solutes such as glycine betaine to balance osmotic pressure.

Question 10

A bacterium is exposed to a sudden increase in salt concentration. What is the most immediate effect on the cell?

Accepted Answer

Water leaves the cell, causing plasmolysis and reduced cell volume.

Question 11

Which of the following best explains why extreme pH conditions can be lethal to microbes?

Accepted Answer

Extreme pH denatures proteins and disrupts enzyme activity, halting essential cellular processes.

Question 12

Which physical method of microbial control is most effective for sterilizing heat-sensitive liquids such as vaccines?

Accepted Answer

Filtration through a membrane filter.

Question 13

Which of the following best describes the effect of low temperature (refrigeration) on microbial growth?

Accepted Answer

It slows down metabolism and growth but usually does not kill microbes.

Question 14

Which of the following is a correct match between a microbe and its preferred environment?

Accepted Answer

Acidophile – pH 3.0

Question 15

Which adaptation is most likely to be found in a psychrophilic microbe to maintain enzyme activity at low temperatures?

Accepted Answer

Enzymes with more flexible structures and open active sites.

Question 16

Which of the following best describes the role of chaperone proteins in thermophilic microbes?

Accepted Answer

They refold damaged proteins and prevent aggregation at high temperatures.

Question 17

A food manufacturer uses modified atmosphere packaging (MAP) to preserve fresh meat. Which combination of gases is most likely to inhibit aerobic spoilage bacteria?

Accepted Answer

Low oxygen and high carbon dioxide.

Question 18

Which of the following best explains why lactic acid bacteria are important in yogurt production?

Accepted Answer

They ferment sugars to produce lactic acid, lowering pH and inhibiting spoilage microbes.

Question 19

Which of the following is the best definition of sterilization in the context of microbial control?

Accepted Answer

The complete destruction or removal of all forms of microbial life, including spores and viruses.

Environmental Limits on Microbial Growth: Temperature, pH, Osmolarity, and Oxygen

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