Which bacterium is the PRIMARY etiological agent in the initiation of enamel caries?

Correct answer: B. Streptococcus mutans

• A. Lactobacillus acidophilus is more important in the progression and deepening of carious lesions (in dentinal caries) rather than initiation. It thrives after the niche is established by S. mutans.
• B. CORRECT. Streptococcus mutans is the primary cariogenic organism in the initiation of enamel caries. It produces acid (lactic acid) from sucrose fermentation, synthesizes glucan polymers that promote adhesion to tooth surfaces, and thrives in acidic environments. S. mutans is the key initiating pathogen in the caries process.
• C. Actinomyces viscosus (and A. naeslundii) are primarily associated with ROOT caries in elderly patients — they colonize exposed cementum surfaces, not enamel initiation.
• D. Fusobacterium nucleatum is associated with periodontal disease (bridging organism) and is not a primary cariogenic pathogen.