Oxygen requirements of microorganisms MCQs With Answer

Introduction: This quiz set on the oxygen requirements of microorganisms is designed specifically for M.Pharm students studying Bioprocess Engineering and Technology. It covers classifications (obligate aerobes, obligate anaerobes, facultative anaerobes, microaerophiles, aerotolerant organisms), biochemical defenses against oxygen toxicity, laboratory tests and media used to determine oxygen preference, and practical implications for bioreactor operation such as oxygen transfer and measurement. Questions emphasize mechanistic understanding and applied reasoning relevant to fermentation processes, sterile culture techniques, and process optimization. Use these MCQs to test and deepen your grasp of how oxygen availability shapes microbial physiology and how engineers measure and manipulate oxygen in bioprocesses.

Q1. What defines an obligate aerobe?

• Organisms that require oxygen for growth and cannot grow without it
• Organisms that grow only in the absence of oxygen
• Organisms that grow equally well with or without oxygen
• Organisms that require very low oxygen concentrations

Correct Answer: Organisms that require oxygen for growth and cannot grow without it

Q2. What characterizes an obligate anaerobe?

• It requires oxygen at high concentrations to grow
• It cannot tolerate molecular oxygen and grows only in its absence
• It grows better in oxygen but can ferment without it
• It tolerates oxygen but does not use it for metabolism

Correct Answer: It cannot tolerate molecular oxygen and grows only in its absence

Q3. Which statement best describes a facultative anaerobe?

• Grows only when oxygen is completely absent
• Requires a microaerobic atmosphere for growth
• Can grow with or without oxygen, often growing faster with oxygen present
• Tolerates oxygen but derives energy only by fermentation

Correct Answer: Can grow with or without oxygen, often growing faster with oxygen present

Q4. What is an aerotolerant anaerobe?

• An organism that requires oxygen and dies without it
• An organism that grows only at the air–liquid interface
• An organism that tolerates oxygen but does not use it for growth
• An organism that requires reduced oxygen levels to grow

Correct Answer: An organism that tolerates oxygen but does not use it for growth

Q5. Microaerophiles are best described as:

• Organisms that cannot survive in the presence of oxygen
• Organisms that require oxygen at concentrations lower than atmospheric levels
• Organisms that grow equally well at all oxygen concentrations
• Organisms that use oxygen only for detoxification

Correct Answer: Organisms that require oxygen at concentrations lower than atmospheric levels

Q6. Which enzyme is generally used as a simple biochemical test to indicate an organism’s ability to detoxify hydrogen peroxide?

• Urease
• Catalase
• Beta-galactosidase
• Lysozyme

Correct Answer: Catalase

Q7. What is the primary function of superoxide dismutase (SOD) in aerobic microbes?

• Converts hydrogen peroxide into water and oxygen
• Converts superoxide radicals into hydrogen peroxide and oxygen
• Reduces oxygen to water in the respiratory chain
• Breaks down organic peroxides

Correct Answer: Converts superoxide radicals into hydrogen peroxide and oxygen

Q8. Which medium is classically used to determine the oxygen requirements of microorganisms by providing an oxygen gradient?

• MacConkey agar
• Thioglycollate broth
• LB broth
• Sabouraud dextrose agar

Correct Answer: Thioglycollate broth

Q9. Which redox indicator in thioglycollate medium turns pink in the presence of oxygen?

• Phenol red
• Resazurin
• Neutral red
• Methylene blue

Correct Answer: Resazurin

Q10. In a thioglycollate tube, where would you expect microaerophilic bacteria to form the heaviest growth?

• At the very bottom of the tube where oxygen is absent
• At the top surface exposed to air
• Just below the surface where oxygen concentration is reduced
• Evenly throughout the tube

Correct Answer: Just below the surface where oxygen concentration is reduced

Q11. Which of the following organisms is a classic example of an obligate anaerobe?

• Escherichia coli
• Staphylococcus aureus
• Clostridium botulinum
• Bacillus subtilis

Correct Answer: Clostridium botulinum

Q12. Which organism is typically a facultative anaerobe commonly encountered in pharmaceutical microbiology?

• Streptococcus pneumoniae
• Escherichia coli
• Clostridium perfringens
• Campylobacter jejuni

Correct Answer: Escherichia coli

Q13. Oxygen toxicity in microbes is mainly caused by which species?

• Reactive oxygen species (ROS) such as superoxide and hydrogen peroxide
• Nitrogen gas accumulation
• Excess molecular oxygen acting as a nutrient
• Formation of ozone in culture media

Correct Answer: Reactive oxygen species (ROS) such as superoxide and hydrogen peroxide

Q14. Which instrument is commonly used to measure dissolved oxygen in a fermenter?

• Spectrophotometer
• pH meter
• Clark-type polarographic oxygen electrode
• Refractometer

Correct Answer: Clark-type polarographic oxygen electrode

Q15. Henry’s law is most relevant to which aspect of oxygen handling in bioprocesses?

• Relation of oxygen solubility in liquid to its partial pressure in the gas phase
• Enzymatic detoxification of oxygen radicals
• Microbial oxygen uptake kinetics at high cell density
• Rate of oxygen diffusion through cell membranes

Correct Answer: Relation of oxygen solubility in liquid to its partial pressure in the gas phase

Q16. What does the term KLa represent in aerobic fermentation?

• The rate of oxygen consumption per cell
• The volumetric mass transfer coefficient describing oxygen transfer from gas to liquid
• The oxygen concentration at half-maximal growth
• The maximum dissolved oxygen concentration achievable

Correct Answer: The volumetric mass transfer coefficient describing oxygen transfer from gas to liquid

Q17. Which process change will most directly increase KLa in a stirred tank bioreactor?

• Reducing medium viscosity by adding polymers
• Increasing agitation speed to enhance gas–liquid interfacial area
• Lowering the temperature to increase gas solubility
• Switching to anaerobic headspace gas

Correct Answer: Increasing agitation speed to enhance gas–liquid interfacial area

Q18. Oxygen uptake rate (OUR) in a culture is defined as:

• The rate at which oxygen dissolves from gas into the liquid phase
• The rate at which oxygen is produced by photosynthetic microbes
• The rate at which cells consume dissolved oxygen per unit reactor volume per time
• The equilibrium concentration of oxygen in the medium

Correct Answer: The rate at which cells consume dissolved oxygen per unit reactor volume per time

Q19. The Pasteur effect refers to which metabolic observation?

• Increase in fermentation rate in the presence of oxygen
• Decrease in fermentation (glycolysis) rate when oxygen becomes available
• Complete inhibition of respiration by oxygen
• Shift from aerobic respiration to anaerobic respiration under oxygen excess

Correct Answer: Decrease in fermentation (glycolysis) rate when oxygen becomes available

Q20. How do many bacteria adapt at the molecular level to persist under low oxygen tension?

• They synthesize oxygen-scavenging toxins
• They express high-affinity terminal oxidases and alternative electron transport components
• They eliminate respiratory chains entirely and rely on passive diffusion
• They increase membrane permeability to let oxygen in freely

Correct Answer: They express high-affinity terminal oxidases and alternative electron transport components

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