In dental plaque biofilms, arginine breakdown to ornithine, ammonia, and CO2 is primarily performed by which organisms?

• A. Dental plaque streptococci
• B. Actinomyces naeslundii
• C. Streptococcus mutans
• D. Porphyromonas gingivalis

Answer: (A)

Arginine breakdown in dental plaque is driven by bacteria that possess the arginine deiminase system, which converts arginine into ornithine, ammonia, and CO2. This pathway is a common feature of dental plaque streptococci, the early and abundant inhabitants of the biofilm, and it provides a way to neutralize acids produced from sugar fermentation by releasing ammonia, thereby helping to maintain a higher local pH. That buffering role is why dental plaque streptococci are the best fit for this process. The other organisms listed are more associated with different roles in the biofilm: Porphyromonas gingivalis is a periodontopathogen that thrives in proteolytic conditions rather than contributing to arginine metabolism for pH buffering; Streptococcus mutans is primarily known for acid production from carbohydrate fermentation; Actinomyces naeslundii participates in early colonization and some metabolism but is not the main arginine-degrading contributor in plaque.