Which type of bacteria colonizes the deepest layers of dental plaque?

• A. Anaerobes
• B. Aerobes
• C. Spores
• D. Micrococci

Answer: (A)

An oxygen gradient within dental plaque explains why the deepest layers are populated by anaerobes. The outer surface of the biofilm is exposed to saliva and air, so aerobes (and facultative anaerobes that tolerate oxygen) dominate there. As these bacteria metabolize, they consume oxygen and the biofilm structure limits diffusion, creating a low-oxygen, anoxic environment in the deeper layers. In this built environment, obligate anaerobes thrive and become the predominant colonizers. Spores or micrococci don’t define the deepest layer in this biofilm pattern, since aerobes and other non-anaerobic forms are more associated with the surface or transitional zones.