Which type of bacteria colonizes subgingival plaque?

• A. Spirochetes
• B. Aerobic
• C. Anaerobic
• D. Actinomyces

Answer: (A)

The key idea is that subgingival plaque exists in a low-oxygen environment, so the bacteria that colonize it are anaerobic. The sulcus or periodontal pocket has little oxygen, allowing anaerobic or microaerophilic organisms to thrive and become the dominant players in that biofilm. While spirochetes can be found in diseased pockets, they represent a subset of the anaerobic organisms present there rather than the overall type defining subgingival plaque. In contrast, many subgingival pathogens are anaerobic Gram-negative bacteria such as Porphyromonas, Prevotella, and Fusobacterium; these organisms thrive because the environment is oxygen-poor. Early plaque colonizers on exposed tooth surfaces might include facultative organisms like Actinomyces, but as you move deeper into the subgingival space, anaerobes predominate. So, describing subgingival plaque colonizers as anaerobic best reflects the biology of that site.