Micro & Immunology · patient case question
Which organism is most responsible for the initiation of these lesions?
Patient case: Rampant caries on a full-mouth series
Patient: Female, 29 years old
Chief complaint: “A piece of my back tooth broke off.”
History: No systemic disease. Frequent sugared energy drinks. Last dental visit six years ago. Brushes once daily, no floss.
Findings: Full-mouth series shows occlusal and proximal caries in 14 teeth. Tooth #3 has gross coronal destruction extending 2 mm subgingivally on the mesial. No periapical radiolucencies. Salivary flow is normal.
Show answer and explanation
Correct answer: B. Streptococcus mutans
- A. Porphyromonas gingivalis is a red-complex PERIODONTAL pathogen, an asaccharolytic anaerobe that degrades protein. It has no role in caries.
- B. CORRECT. Streptococcus mutans is the principal INITIATOR of coronal caries. It adheres to the pellicle, synthesises extracellular glucans from sucrose via glucosyltransferase to build the biofilm matrix, is strongly acidogenic, and is ACIDURIC — it keeps producing acid at pH values that suppress competing species.
- C. Actinomyces species, particularly A. viscosus and A. naeslundii, are chiefly implicated in ROOT-SURFACE caries, where cementum and dentine rather than enamel are the substrate.
- D. Lactobacilli are associated with PROGRESSION of established lesions rather than initiation. They are poor adherents to smooth enamel but thrive in the retentive environment of an open cavity, so their counts track lesion activity.
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