Nitrite Supplementation of Drinking Water Does Not Impair the Resistance of Mice to Intragastric Inoculation with Listeria monocytogenes

Introduction:  There has been public controversy regarding the use of nitrite in cured meat products. Many vegetables, including leafy greens, are high in nitrate. Once ingested the nitrate is reduced to nitrite by bacteria residing in the oral cavity or gastrointestinal tract. Recent evidence suggest that nitrite is an important signaling molecule in the cardiovascular tract. One reactive nitrogen intermediate (nitric oxide) is also an antimicrobial effector molecule and can have a selective effect on regulatory T lymphocytes.

Purpose:  In this project, we examined the effect of nitrite supplementation on host defense using a mouse model of gastrointestinal listeriosis. Our hypothesis was that dietary supplementation of drinking water with sodium nitrite will not  affect resistance of mice to gastrointestinal infection with Listeria monocytogenes.

Methods:  Mice were given graded amounts of nitrite (25 to 50 mg/l) in their sole source of drinking water. Control mice received water without nitrite. Although we observed a dose-dependent decrease in water intake by mice given drinking water supplemented with NaNO₂, water consumption remained within the normal range for mice. We then assessed the effects of NaNO₂ supplemented drinking water on the severity of infection in C57BL/6 mice inoculated intragastrically with 10⁶ CFU L. monocytogenes. Mice received the indicated amount of NaNO₂ in their drinking water for 4 days before inoculation with L. monocytogenes, and throughout a 3-day period of infection. The mice were then euthanized, their tissues removed and the listerial burden in tissues evaluated by plating on blood agar. In one experiment we exposed pregnant mice (10 to 14 days gestation) to nitrite-supplemented drinking water before inoculation with L. monocytogenesas described above.

Results:  Supplementation of mouse drinking water with NaNO₂ ( 25 or 50 mg/l) had no significant deleterious effect on resistance to listeriosis, as quantified by the recovery of viable L. monocytogenes from the spleen and liver at 3 days after inoculation. Nor did nitrite supplementation of drinking water ( 25 mg/l) have an adverse effect of resistance to listeriosis in pregnant mice. We recovered similar numbers of L. monocytogenesfrom the spleen, liver and fetoplacental units of pregnant mice given water nitrite-supplemented or control water.

Significance:  Sodium nitrite supplementation of drinking water does not have an adverse effect on resistance of mice to gastrointestinal infection with Listeria monocytogenes. The results of this study provide additional information to inform the risk/benefit debate regarding nitrites in the diet.