Listeriosis is a rare and severe foodborne infection caused by Listeria monocytogenes (Lm), which manifests as bacteremia, neurolisteriosis, and maternal–neonatal infections. Maternal–neonatal listeriosis accounts for 10%–20% of listeriosis cases across Europe and North America but can represent up to 50% of invasive cases, as reported in the South African outbreak, the largest to date. It is associated with major adverse outcomes including fetal death, preterm
birth, and/or early- or late-onset neonatal listeriosis in more than 80% of cases [5]. For maternal–neonatal listeriosis diagnosis, public health agencies require that Lm is isolated in any sample of maternal (blood, products of conception) or neonatal origin, including nonsterile sites. This definition is broader than for other maternal–fetal infections and reflects the pathophysiological steps that lead to maternal–fetal listeriosis after consumption of contaminated food, as
follows:
(1) Lm crosses the intestinal barrier
(2) maternal bacteremia occurs
(3) hematogenous placental seeding takes place and the placental barrier is crossed, resulting in (4) fetal/neonatal infection. Considering these steps and the fact that Lm is not a member of the maternal vaginal flora, its presence in any nonsterile neonatal sample is unlikely to reflect ascending colonization but rather invasive infection following hematogenous placentitis.

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