Micro & Immunology · single question
A tooth is negative for cold testing, positive to percussion and palpation, with no sinus tract or swelling in the buccal vestibule. What type of microorganism most likely caused this pulpal pathology?
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Correct answer: D. Obligate anaerobe
A necrotic pulp (negative to cold) without drainage (no sinus tract) creates a low-oxygen, low-pH environment. Obligate anaerobes thrive in these conditions. The key pathogens in root canal infections (Porphyromonas gingivalis, Prevotella intermedia, Fusobacterium) are obligate anaerobes. The absence of a sinus tract indicates a CHRONIC infection brewing in an anaerobic environment — obligate anaerobes are the predominant organisms.
- A. INCORRECT. Obligate aerobes require oxygen to survive. A necrotic pulp canal provides little to no oxygen — these bacteria cannot thrive there.
- B. INCORRECT. Facultative anaerobes (like Streptococcus) can survive with or without oxygen and are associated with early acute infections, not the established chronic infection implied here.
- C. INCORRECT. Facultative aerobes can use oxygen when present but survive without it. They are more common in early (acute) canal infections.
- D. CORRECT. Obligate anaerobes (Porphyromonas gingivalis, Prevotella intermedia, Fusobacterium nucleatum) thrive in the oxygen-depleted environment of a necrotic root canal. They are the predominant bacteria in asymptomatic chronic periapical lesions.
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