Microscopy of Intracellular Listeria monocytogenes in Epithelial Cells.

The pathogen Listeria monocytogenes is a facultative intracellular bacterium, which targets a large range of cell types. Following entry, bacteria disrupt the invasion vacuole and reach the cytoplasm where they replicate and use the actin cytoskeleton to propel themselves from cell to cell. Mammalian epithelial cells grown in vitro can be used to study the different steps of the intracellular life of Listeria. However, rapid multiplication and dissemination of bacteria can induce important cell death and detachment, resulting in the formation of lytic plaques. Thus, in vitro infections with L. monocytogenes are usually restricted to short time courses, from a few minutes to one day. Here, we present a method to study long-term L. monocytogenes infections in epithelial cells using epifluorescence microscopy. This protocol enables the observation of actin-based motility, intercellular dissemination foci, and entrapment of L. monocytogenes within vacuoles of persistence termed "Listeria-Containing Vacuoles" (LisCVs). We also describe a protocol to study the recruitment of cytoskeletal proteins at Listeria actin comet tails, as well as a method to assess the membrane integrity of intracellular bacteria using a LIVE/DEAD viability assay.

To Be Cytosolic or Vacuolar: The Double Life of Listeria monocytogenes.

Intracellular bacterial pathogens are generally classified into two types: those that exploit host membrane trafficking to construct specific niches in vacuoles (i.e., "vacuolar pathogens"), and those that escape from vacuoles into the cytosol, where they proliferate and often spread to neighboring cells (i.e., "cytosolic pathogens"). However, the boundary between these distinct intracellular phenotypes is tenuous and may depend on the timing of infection and on the host cell type. Here, we discuss recent progress highlighting this phenotypic duality in Listeria monocytogenes, which has long been a model for cytosolic pathogens, but now emerges as a bacterium also capable of residing in vacuoles, in a slow/non-growing state. The ability of L. monocytogenes to enter a persistence stage in vacuoles might play a role during the asymptomatic incubation period of listeriosis and/or the carriage of this pathogen in asymptomatic hosts. Moreover, persistent vacuolar Listeria could be less susceptible to antibiotics and more difficult to detect by routine techniques of clinical biology. These hypotheses deserve to be explored in order to better manage the risks related to this food-borne pathogen.

Listeria monocytogenes switches from dissemination to persistence by adopting a vacuolar lifestyle in epithelial cells.

Listeria monocytogenes causes listeriosis, a foodborne disease that poses serious risks to fetuses, newborns and immunocompromised adults. This intracellular bacterial pathogen proliferates in the host cytosol and exploits the host actin polymerization machinery to spread from cell-to-cell and disseminate in the host. Here, we report that during several days of infection in human hepatocytes or trophoblast cells, L. monocytogenes switches from this active motile lifestyle to a stage of persistence in vacuoles. Upon intercellular spread, bacteria gradually stopped producing the actin-nucleating protein ActA and became trapped in lysosome-like vacuoles termed Listeria-Containing Vacuoles (LisCVs). Subpopulations of bacteria resisted degradation in LisCVs and entered a slow/non-replicative state. During the subculture of host cells harboring LisCVs, bacteria showed a capacity to cycle between the vacuolar and the actin-based motility stages. When ActA was absent, such as in ΔactA mutants, vacuolar bacteria parasitized host cells in the so-called "viable but non-culturable" state (VBNC), preventing their detection by conventional colony counting methods. The exposure of infected cells to high doses of gentamicin did not trigger the formation of LisCVs, but selected for vacuolar and VBNC bacteria. Together, these results reveal the ability of L. monocytogenes to enter a persistent state in a subset of epithelial cells, which may favor the asymptomatic carriage of this pathogen, lengthen the incubation period of listeriosis, and promote bacterial survival during antibiotic therapy.