Recognition of host target cells by invasive bacteria is an essential step to infection, but the
impact of the host cell cycle on the invasion of bacterial pathogens is still largely unknown.
During these studies it was found that mitotic cells were more efficiently targeted for invasion by
Salmonella enterica Typhimurium (S. Typhimurium) than cells in other phases of their cell cycle.
This targeting was dependent on the SPI-1 (Salmonella pathogenicity island-1) T3SS (type 3
secretion system) translocase SipB, which binds to cholesterol at the surface of host cells.
Using the cholesterol binding dye filipin and a fluorescent ester of polyethylene glycolcholesterol,
which cannot flip through the plasma membrane, the levels of plasma membrane
and cell surface cholesterol along the cell cycle were measured and both found to be maximal
during mitosis. This increase was due to a transient loss in cholesterol asymmetry at the plasma
membrane during mitosis. It was found that cholesterol changed from a ~20:80 outer:inner
leaflet repartition during interphase to ~50:50 during mitosis. This explains the increase in
surface levels that make mitotic cells more susceptible to S. Typhimurium invasion and perhaps
to other viruses and bacteria that enter cells in a cholesterol-dependent manner. Additionally, it
was found that the change in cholesterol partitioning favours the recruitment of
phosphatidylinositol 4,5‑bisphosphate (Pi(4,5)P2) and activated ERM (Ezrin, Radixin, Moesin)
proteins to the plasma membrane. This recruitment was shown here to support cell rounding
during mitosis, as Pi(4,5)P2 and activated ERM are responsible for cross linking the plasma
membrane to the actin cytoskeleton during mitosis.
Some bacterial pathogens and viruses are known to interfere with the host cell cycle. Although
several reports have shown significant correlation between chronic infections and cancer
formation caused by Salmonella enterica Typhi, the impact of Salmonella on dividing cells was
unclear before this study. During this investigation it was found that intracellular S. Typhimurium
causes cytokinesis defects, which lead to binucleation of host cells. Intracellular replication of S. Typhimurium is modulated by the action of bacterial effectors translocated across the
Salmonella-containing vacuole (SCV) by the SPI-2 T3SS. In this study it was determined that
the cytokinesis failure induced by S. Typhimurium occurs during abscission and is caused by
the presence of a cluster of SCVs (microcolony) inside the dividing cell. Translocation of the
effectors SseF and SseG is important to induce cytokinesis failure, in a process that seems to
depend on the activity of the kinase Aurora B, known to regulate abscission. Importantly,
binucleation was also observed in infected cells in the intestinal epithelium of mice previously
inoculated with S. Typhimurium. Similarly to other enteric pathogens known to block the cell
cycle of the host, perhaps cytokinesis failure caused by S. Typhimurium delays the epithelial
cell turnover of the intestine, facilitating its colonization by the bacteria