Microbiology and Immunology 6400 Practice Test 2026 – Complete Exam Prep

In dental plaque, microbial succession happens as the biofilm matures. Early colonizers attach to the acquired pellicle and establish the base layer, creating conditions that allow other organisms to join. As the biofilm thickens and becomes more anaerobic, late colonizers—the organisms typically associated with periodontal disease—establish themselves. Porphyromonas gingivalis and Treponema denticola are classic examples of these late colonizers; they thrive in the mature, anaerobic environment of the plaque and are strongly linked to disease progression.

The other organisms listed are more characteristic of earlier stages or bridging roles. Streptococcus sanguinis and Streptococcus oralis are well-known early colonizers, as are Streptococcus mutans and Actinomyces spp. Fusobacterium and Veillonella can appear after the initial colonizers and can act as bridges or workers in the transitional stages, but the best match for late-stage colonization in this set is Porphyromonas gingivalis and Treponema denticola.