Molecular evidence in support of hematogenous dissemination of intraamniotic infection caused by Listeria monocytogenes in spontaneous preterm labor

Abstract Objectives Intraamniotic infection is a major cause of preterm labor. While ascending infection from the urogenital tract is well recognized, hematogenous transmission in which pathogens travel from the maternal circulation to the intraamniotic environment has been proposed. Listeria monocytogenes is a common bacterium causing Listeriosis and is believed to cause intraamniotic infection via the maternal bloodstream. However, molecular evidence supporting this route is limited. Methods A complete microbiologic workup was performed, including cultures from amniotic fluid, vagina, maternal and neonatal blood, and the placenta in a patient with intraamniotic infection. Whole genome hybrid sequencing was utilized to characterize the genomic features including virulence factors and antimicrobial resistance. Placental pathology, Brown–Hopps, Warthin–Starry staining, and calorimetric in situ hybridization were performed to detect the presence of Listeria in the placenta. Results L. monocytogenes was detected in both the amniotic fluid and chorioamniotic membranes but not in the vaginal fluid. Whole genome sequencing identified the two isolates as L. monocytogenes sequence type 1, clonal complexes 1, and serotype 4b. Placental pathology demonstrated extensive acute intervillositis with intracellular rod-shaped bacteria in the intervillous space of the placenta identified by Brown-Hopps and Warthin-Starry staining. Calorimetric in situ hybridization confirmed the presence of L. monocytogenes in the intervillous space. Conclusions This study provides the first molecular evidence supporting a hematogenous route of intraamniotic infection by L. monocytogenes Evidence in support includes the presence of acute intervillositis and the detection of L. monocytogenes in the amniotic fluid and intervillous space of the placenta combined with the absence of this organism in the vagina.