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Streptococcus mutans Virulence — MFDS Part 1 MCQ

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HardMicrobiologyStreptococcus mutans VirulenceMFDS Part 1

An isogenic Streptococcus mutans mutant has normal growth, lactate production, acid tolerance and sucrose-independent binding to salivary pellicle. After repeated exposure to dietary sucrose, however, it fails to accumulate as a tightly adherent, cohesive biofilm on smooth enamel. Loss of which virulence function most directly accounts for this phenotype?

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Correct answer: E — Polymerisation of the glucose moiety of sucrose into water-insoluble glucans by GtfB and GtfC

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

A is correct. GtfB and GtfC synthesise predominantly water-insoluble glucans from sucrose. These extracellular polymers create binding sites and a cohesive matrix that promotes bacterial clustering and stable accumulation on enamel. Normal pellicle binding does not exclude failure of this subsequent sucrose-dependent phase. The F1F0-ATPase primarily supports acid tolerance, while lactate dehydrogenase mediates acid production; both functions are stated to be intact. Intracellular polysaccharide permits continued metabolism when exogenous carbohydrate is unavailable but does not form the adherent extracellular matrix. Fructans provide an extracellular carbohydrate reserve, but they are less important than insoluble glucans for tight biofilm adhesion and cohesion.

Reference: Bowen WH, Koo H. Biology of Streptococcus mutans-derived glucosyltransferases: role in extracellular matrix formation of cariogenic biofilms. Caries Research. 2011;45(1):69-86. https://pubmed.ncbi.nlm.nih.gov/21346355/