Quantifying translocation of Listeria monocytogenes in rats by using urinary nitric oxide-derived metabolites

The urinary nitric oxide metabolites NO(2)(-) and NO(3)(-) (summed as NO(x)) are a noninvasive, quantitative biomarker of translocation of salmonella from the intestinal lumen to systemic organs. Listeria monocytogenes is a food-borne gram-positive pathogen that can also cross the intestinal epithelium. In this study, we tested the efficacy of urinary NO(x) as a marker of listeria translocation. Rats (eight per group) were orally infected with increasing doses of L. monocytogenes; control rats received heat-killed listeria. The kinetics of urinary NO(x) and population levels of listeria in feces were determined for 7 days. Another group of rats was killed 1 day after infection to verify translocation by culturing viable listeria from systemic organs. Oral administration of increasing doses of L. monocytogenes resulted in a time- and dose-dependent increase in urinary NO(x) excretion. Translocation was a prerequisite for inducing a NO(x) response, since heat-killed L. monocytogenes did not elevate NO(x) excretion in urine. Fecal counts of listeria also showed dose and time dependency. Moreover, the number of viable L. monocytogenes cells in mesenteric lymph nodes also increased in a dose-dependent manner and correlated with urinary NO(x). In conclusion, urinary NO(x) is a quantitative, noninvasive biomarker of listeria translocation.