Micro & Immunology · patient case question
Which organism is most closely associated with ROOT SURFACE caries?
Patient case: Root caries in a dry mouth
Patient: Male, 70 years old
Chief Complaint: “My teeth are decaying at the gumline and my mouth is always dry.”
Background: Takes amlodipine, sertraline and oxybutynin. Generalised gingival recession following treated periodontitis.
Vital Signs: BP 136/80 mmHg · HR 70 bpm
Findings: Multiple soft, leathery lesions on the exposed root surfaces of several teeth. Unstimulated salivary flow is markedly reduced. One molar has long-standing deep occlusal caries; a section of an extracted tooth from the same mouth shows a translucent zone beneath the lesion and dark bands within the dentine.
Show answer and explanation
Correct answer: C. Actinomyces viscosus
- A. Prevotella intermedia is likewise a periodontal organism, associated with pregnancy gingivitis and necrotising disease, and is not a caries pathogen.
- B. Porphyromonas gingivalis is a red-complex PERIODONTAL pathogen. It is asaccharolytic and proteolytic, meaning it does not ferment carbohydrate to acid at all, so it cannot drive a carious process. Its presence alongside bone loss is a distractor.
- C. CORRECT. ACTINOMYCES VISCOSUS, with Actinomyces naeslundii, is the organism classically named for ROOT SURFACE caries. These are gram-positive filamentous rods that colonise exposed cementum and dentine. The underlying reason the flora differs is worth knowing: dentine and cementum demineralise at a HIGHER critical pH — about 6.2, against 5.5 for enamel — so a less strongly acidogenic organism is sufficient to initiate a root lesion.
- D. S. mutans is the organism of CORONAL caries INITIATION — acidogenic, aciduric, and able to synthesise extracellular glucans that let it adhere to smooth enamel. It is recovered from root lesions too, but it is not the association the question is testing.
Note: mutans is the key for coronal initiation. Here it is the answer. The triad to memorise: coronal INITIATION → S. mutans; deep lesion PROGRESSION → Lactobacillus; ROOT SURFACE → Actinomyces. Note that this patient's three medications are all xerostomic, which is the actual driver of his risk.
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