Salmonellosis is a foodborne gastroenteritic disease causing tenths of millions of cases per year. While the disease is commonly benign, it still leads today to hundreds of thousands of deaths per year, especially in young children and immunodepressed people. During my Ph.D. under the direction of Jost Enninga, I dissected the molecular and cellular strategies of Salmonella during its infection of epithelial cells.
Thanks to its flagella, Salmonella can move on the surface of intestinal cells and target specific host cells for infection. The selection criteria of the bacteria have been mostly unknown so far. We set up a pipeline of automatic acquisition and image analysis combined with mathematic modeling to forecast the probability of a given epithelial cell to be infected or not. We demonstrated that Salmonella can target specific host cells depending on features of their local environment, such as the local cell density. Besides, we found that the infection of an epithelial cell by Salmonella directly increases the vulnerability of the same cell to be reinfected, demonstrating the existence of a mechanism of long-term cooperation between bacteria.
To penetrate targeted cells, Salmonella induces membrane ruffles leading to its engulfment into a membrane-bound compartment. This compartment can maturate and form a replicative vacuolar niche called the “Salmonella-Containing Vacuole” (SCV). Alternatively, the bacteria can rupture its vacuole leading to Salmonella release into the cytosol. A discrepancy existed in the field regarding the nature of the early Salmonella-containing compartment. Coupling live fluorescence microscopy with correlative light electron microscopy, we revealed that Salmonella enters in epithelial cells in a tight compartment distinct from the surrounding macropinosomes formed at the infection site. As the bacterial infection induces the formation of those macropinosomes, we termed them “Infection- Associated Macropinosomes” (IAMs). Few minutes post-infection these endomembrane compartments can fuse with the SCV leading to its enlargement. Besides, the absence of fusion results in SCV destabilization and vacuolar escape. Thus, the interaction between IAMs and SCV determine the lifestyle of Salmonella.
To identify the host factors driving SCV-IAM interactions, we developed a highly specific fractionation method to isolate IAMs. After proteome analysis by mass spectrometry, we revealed the recruitment of specific SNAREs at the IAMs that could allow SCV-IAMs fusion.
Together, our work provides a new comprehensive model of the early steps of Salmonella invasion of epithelial cells.
Since November 2018, I work at the Leiden University Medical Center (the Netherlands) as postdoctoral fellow in the team of Pr. Sjaak Neefjes.