A cervical restoration is colonised by Streptococcus mutans. Which organism would you MOST expect to find alongside it in root-surface caries?

Correct answer: A. Actinomyces viscosus

• A. CORRECT. ACTINOMYCES species — particularly A. viscosus and A. naeslundii — are the organisms most characteristically associated with ROOT-SURFACE CARIES, alongside S. mutans and lactobacilli. They are gram-positive filamentous rods that colonise exposed cementum and dentine in older patients with gingival recession, and they are prominent early colonisers of the pellicle at the cervical margin.
• B. AGGREGATIBACTER ACTINOMYCETEMCOMITANS is the organism classically linked to localised aggressive (juvenile) periodontitis — periodontal, not cariogenic.
• C. PORPHYROMONAS GINGIVALIS is a red-complex PERIODONTAL pathogen — gram-negative, anaerobic, associated with attachment loss, not with caries.
• D. TREPONEMA DENTICOLA is a spirochaete and another red-complex periodontal pathogen found in deep pockets, not a cariogenic organism.

Note: Keep the two ecologies separate: CARIES organisms are gram-positive, acidogenic and aciduric (S. mutans, lactobacilli, Actinomyces). PERIODONTAL pathogens are gram-negative, anaerobic and proteolytic (red complex, A. actinomycetemcomitans).