Why do these streptococci stop metabolizing?

Why do these streptococci stop metabolizing?

• A. Sensitive to the acid they produce
• B. They run out of substrate
• C. They don't have enzymes
• D. They are killed by oxygen

Answer: (A)

Streptococci stop metabolizing because the acid they produce during fermentation accumulates and lowers the surrounding pH, which inhibits the enzymes and transport systems that drive glycolysis and other metabolic pathways. This self-produced acid acts as a feedback signal: at acidic pH, enzyme activities decline and metabolism stalls even if substrates are still present. The other ideas don’t fit as well—running out of substrate would halt metabolism for that reason, these bacteria do have the necessary enzymes, and they’re generally not killed by oxygen and can tolerate it, so oxygen isn’t the culprit here.

Streptococci stop metabolizing because the acid they produce during fermentation accumulates and lowers the surrounding pH, which inhibits the enzymes and transport systems that drive glycolysis and other metabolic pathways. This self-produced acid acts as a feedback signal: at acidic pH, enzyme activities decline and metabolism stalls even if substrates are still present. The other ideas don’t fit as well—running out of substrate would halt metabolism for that reason, these bacteria do have the necessary enzymes, and they’re generally not killed by oxygen and can tolerate it, so oxygen isn’t the culprit here.