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Endodontics · single question

What are the most common bacteria found in the primary root canal (initial infection, not failed RCT)?

  1. Gram-positive aerobic cocci
  2. Gram-negative obligate anaerobes
  3. Streptococcus mutans
  4. Enterococcus faecalis
Show answer and explanation

Correct answer: B. Gram-negative obligate anaerobes

  • A. Gram-positive aerobes are more common in early caries and supragingival infections — not primary root canal infection.
  • B. Correct. Primary (initial) root canal infections are polymicrobial and predominantly composed of GRAM-NEGATIVE OBLIGATE ANAEROBES (Bacteroides, Fusobacterium, Prevotella, Porphyromonas, Treponema, Peptostreptococcus). These thrive in the low-oxygen environment of an infected pulp.
  • C. Streptococcus mutans is associated with dental caries — not primary root canal infections.
  • D. Enterococcus faecalis is associated with FAILED root canal treatment (persistent/secondary infection) — not primary infection.

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