Microbiology and Immunology 6400 Oral Intermicrobial Interactions Practice Test

Oxygen is highest at the outer surface of dental plaque and drops off with depth. Oxygen diffuses in from saliva, but it is rapidly consumed by the bacteria at the exterior, so the outermost layer remains oxic or only mildly reduced, while deeper layers become increasingly anaerobic due to ongoing consumption and limited diffusion.

This vertical gradient shapes which microbes dominate where: aerobes and microaerophiles tend to occupy the surface where they can access oxygen, facultative anaerobes appear across several layers as they can tolerate either condition, and strict anaerobes flourish in the deeper, oxygen-poor zones. With depth, metabolism shifts from oxygen-based respiration to fermentation and anaerobic respiration using alternative electron acceptors. The result is a stratified, diverse community where metabolic interactions—facilitation by surface microbes producing fermentable substrates that feed deeper anaerobes—help maintain the layered structure and overall plaque ecology.