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Abstract
The lowest infective dose for Listeria monocytogenes in humans is as of yet unknown. Our study investigated the infectivity of various doses of L. monocytogenes labeled with green fluorescent protein (GFP). We used presence/absence of L. monocytogenes in the maternal liver and spleen, the placental tissue, and the fetal liver and brain in pregnant Cavia porcellus (guinea pigs) to indicate infection. Our goal was to investigate infectivity of L. monocytogenes at doses below the LD50 of 106 CFU that was isolated from a the 1985 outbreak resulting in 142 cases. We observed infectivity of doses of L. monocytogenes in the range of 108, 106, and 104 CFU. We found that a dose of 104 CFU as early as 2 days post inoculation resulted in all dams and 16 of 20 fetus testing positive for L. monocytogenes infection either by colony growth on Tryptic Soy Agar or after Fraser broth enrichment.