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S mutans Aciduricity — MFDS Part 1 MCQ

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HardMicrobiologyS mutans AciduricityMFDS Part 1

Following repeated exposure to fermentable carbohydrate, Streptococcus mutans remains metabolically active as plaque pH falls. Which cellular mechanism most directly enables it to maintain intracellular pH under these acidic conditions?

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Correct answer: D — ATP-dependent extrusion of cytoplasmic protons by membrane F-type H+-ATPase

Explanation lettering: D = shown as A · C = shown as B · B = shown as C · A = shown as D

A is correct. The aciduricity of S. mutans depends partly on its membrane F-type H+-ATPase, which hydrolyses ATP to expel protons from the cytoplasm and thereby supports intracellular pH homeostasis. Experimentally reducing expression of an F-ATPase catalytic subunit selectively impairs growth and survival at low, but not neutral, pH. Increased proton permeability would instead accelerate cytoplasmic acidification. Lactate dehydrogenase supports fermentation but does not protect the cell by consuming extracellular lactic acid. S. mutans does not respond by switching to cytochrome-mediated respiration, and extracellular glucans promote adherence and biofilm structure rather than directly neutralising intracellular protons. Other stress adaptations contribute to acid tolerance, but F-type H+-ATPase is the listed mechanism most directly responsible for proton extrusion.

Reference: Sekiya M, et al. F-type proton-pumping ATPase mediates acid tolerance in Streptococcus mutans. Journal of Applied Microbiology. 2023;134(4):lxad073. https://pubmed.ncbi.nlm.nih.gov/37055370/