Several genera of oral bacteria contain ________ that can convert dietary nitrate into nitrite.

• A. Nitrate reductase genes (Nar)
• B. Nitrite reductase genes (Nir)
• C. Nitrate transporter genes
• D. Nitric oxide synthase genes

Answer: (A)

Nitrate to nitrite conversion in oral bacteria is carried out by nitrate reductase enzymes. These enzymes, encoded by nitrate reductase genes (nar), catalyze the reduction of dietary nitrate (NO3−) to nitrite (NO2−), initiating the nitrate–nitrite–nitric oxide pathway that can influence oral chemistry and host physiology. The nitrite produced can then be converted further to nitric oxide under acidic conditions or through other microbial pathways. In contrast, nitrate transporter genes merely import nitrate without chemical reduction, nitrite reductase would reduce nitrite to other species, and nitric oxide synthase makes nitric oxide from arginine rather than from nitrate.