A mandibular first molar has a completed root canal treatment and an occluso-proximal amalgam restoration. The adjacent tooth is missing, and the site bleeds on probing. Which description best characterises the microbial flora that would be recovered from this location?

Correct answer: A. Three or more anaerobic species

• A. CORRECT. Bleeding on probing signals an inflamed, deepened, low-oxygen site. A plaque-retentive overhang or open contact next to a missing tooth accelerates that shift. The mature subgingival biofilm at such a site is POLYMICROBIAL and dominated by MULTIPLE (three or more) obligate ANAEROBES — the red complex (Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola) alongside orange-complex species such as Fusobacterium nucleatum and Prevotella intermedia.
• B. Aerobic species do not increase as a pocket deepens. Oxygen tension FALLS as the pocket deepens and the redox potential drops, which selects against aerobes and progressively favours obligate anaerobes.
• C. The shift here is anaerobic, but it is not a one-or-two-organism picture. Periodontal disease is characteristically POLYMICROBIAL — a whole community shifts, rather than a single pathogen taking over the way it does in a monomicrobial infection.
• D. Aerobic organisms in small numbers describe a shallow, healthy sulcus, not an inflamed site. Early colonisers such as Streptococcus and Actinomyces are facultative and aerotolerant, and they dominate only where the sulcus is shallow, well oxygenated and free of bleeding.

Note: The whole point of this item is the ecological shift: healthy sulcus → gram-positive, facultative, non-motile, few species. Diseased pocket → gram-negative, obligately anaerobic, motile, MANY species. Anything that traps plaque — a restoration overhang, an open contact from a missing neighbour — speeds that transition along.