Density-dependent intracellular growth and cell-to-cell spread of listeria monocytogenes in an epithelial monolayer

Bacterial infection of mammalian hosts involves competition between the two parties on various fronts. Listeria monocytogenes is an intracellular bacterial pathogen that can cause spontaneous abortions in pregnant women and diseases such as meningitis in immunocompromised individuals. A clever way in which L. monocytogenes goes up against its host is by spreading from one cell to the other without exposure to the extracellular space. In this way, the bacterium evades detection by the host's humoral immune response, even as it travels from epithelial cells in the intestine, the initial site of infection, to distal organs such as the brain, spleen and placenta. In this work, we focus on three aspects of Listeria monocytogenes's life within epithelial monolayers: cell-to-cell spread, intracellular replication and host cell extrusion from the monolayer. First, we show that the host cell junction protein, E-cadherin, promotes intercellular spread at the recipient side of cell contacts. We find that expression of an E-cadherin mutant that cannot be ubiquitinated and is expected to undergo reduced internalization, limits bacterial cell-to-cell spread. Based on these observations, we hypothesize that L. monocytogenes is transferred from donor to recipient cell concurrent with E-cadherin internalization at the recipient cell and produce results that are consistent with caveolin-mediated E-cadherin internalization and macropinocytosis as mechanisms that promote L. monocytogenes cell-to-cell spread. Overall, cell-to-cell spread appears to be a domain in which the bacterium prevails over the host cell by exploiting a host cell process (transendocytosis) to disseminate through the epithelial monolayer. Next, we use live-cell microscopy and quantitative image analysis to characterize intracellular growth of L. monocytogenes within epithelial cells. We show that early in infection, bacteria grow exponentially at a constant rate, but that growth rate begins to slow after a certain number of bacteria per host cell volume is reached. Our results suggest that bacterial growth is limited by nutrient availability and similar to bacterial growth in culture, accumulation of bacteria within an enclosed space exhausts the environment of nutrients over time. While our results suggest that L. monocytogenes delays this reduction of growth rate by accessing additional nutrients through cell-to-cell spread, bacterial growth rate for the focus as a whole does decline over time in our experimental paradigm. This might represent a scenario in which L. monocytogenes attempts to overcome a limitation of inhabiting its intracellular niche but is not entirely successful in doing so. Lastly, we describe a phenomenon in which the host cell emerges as the victor and effectively limits bacterial infection. Here, at late time points in infection, innate immune signaling causes the collective extrusion of infected cells from the epithelial monolayer. Once infected cells are expelled from the monolayer, bacteria are no longer able to effectively disseminate to cells in the intact monolayer and face nutrient deprivation as their extruded host cell dies over time

Advisor: Dunn Alexander Robert
Monack Denise M.
O'Brien Lucy Erin