Which organism is NOT inhibited by the reactive nitrogen species generated by oral commensal streptococci?

• A. S. mutans
• B. Aggregatibacter actinomycetemcomitans
• C. Enterococcus faecalis
• D. Porphyromonas gingivalis

Answer: (D)

Reactive nitrogen species from oral commensal streptococci create nitrosative stress that can inhibit many nearby bacteria by damaging DNA, proteins, and crucial metabolic processes. Different oral bacteria have varying abilities to detoxify NO and related species. Porphyromonas gingivalis stands out because it has effective defenses against nitrosative stress, allowing it to survive in environments where NO and related species are present. This resilience comes from its redox systems and nitrosative stress response mechanisms that help neutralize reactive nitrogen species, maintain cellular integrity, and keep metabolism running under stressful conditions. The other organisms—Streptococcus mutans, Aggregatibacter actinomycetemcomitans, and Enterococcus faecalis—generally lack the same level of nitrosative detoxification capability in this context, making them more susceptible to inhibition by the reactive nitrogen species produced by neighboring streptococci.