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MCAT Biology Question — Clostridium

A researcher places several colonies of Clostridium, an obligate anaerobe, in a nutrient-rich broth but accidentally leaves the caps off the test tubes. Returning the next day, she notices that none of the tubes have shown bacterial growth. Which of the following is the most likely explanation for the lack of growth?

A. Oxygen from the air diffused into the broth and the lack of enzymes capable of breaking down the superoxide anion lead to death by oxygen toxicity.
B. Fungal spores in the air landed in the broth and the subsequent fungal growth consumed the available nutrients.
C. The ambient temperature of the air cooled the broth below the optimal growing temperature of 37ºC, causing the Clostridium to die off.
D. The failure to cap the test tubes was irrelevant and the Clostridium must have died due to some unknown factor.

Click for Explanation

Clostridium is an obligate anaerobe, as the question states. In the presence of atmospheric oxygen, the bacteria will die off. There are number of various factors that create oxygen toxicity, but one of the major ones is the absence of enzymes necessary to deactivate toxic oxygen species. Thus (A) is the correct answer.

B: The question makes no mention of fungal growth, and we have no reason to suspect that fungal spores in the air would be of a species capable of out-competing the bacteria for nutrients.

C: The ambient temperature in the room is unlikely to kill off bacteria. Cooler than optimal growing conditions may slow bacterial growth, but room temperature is unlikely to be fatal.

D: See explanation above for (A).

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