Question

Which one of the following cannot tolerate oxygen?

• A. Cl. botulinum
• B. S. aureus
• C. E. coli
• D. Shigella spp.

Hint:

Clostridium botulinum is a strict obligate anaerobe; it requires anaerobic conditions ($E_h < -150\,\text{mV}$) to grow and produce toxin.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Microorganisms are classified based on their oxygen requirements and antioxidant enzyme defenses (catalase and superoxide dismutase).

Step 2: Detailed Explanation:
Clostridium botulinum is an obligate (strict) anaerobe that lacks superoxide dismutase and catalase enzymes.
It cannot tolerate molecular oxygen ($\text{O}_2$) because toxic reactive oxygen species (superoxide radicals $\text{O}_2^{\bullet-}$ and hydrogen peroxide $\text{H}_2\text{O}_2$) accumulate and cause cell death.
Staphylococcus aureus, Escherichia coli, and Shigella spp. are facultative anaerobes that grow in both the presence and absence of oxygen.

Step 3: Final Answer:
Hence, Cl. botulinum cannot tolerate oxygen.