Shigella sonnei O-antigen inhibits internalisation, vacuole escape and inflammasome activation

Two Shigella species, flexneri and sonnei, cause approximately 90% of bacterial dysentery worldwide. While S. flexneri is the dominant species in low-income countries, S. sonnei causes the majority of infections in middle and high-income countries. S. flexneri is a prototypic cytosolic bacterium; once intracellular it rapidly escapes the phagocytic vacuole and causes pyroptosis of macrophages, which is important for pathogenesis and bacterial spread. By contrast little is known about the invasion, vacuole escape and induction of pyroptosis during S. sonnei infection of macrophages. We demonstrate that S. sonnei causes substantially less pyroptosis in human primary monocyte-derived macrophages and THP1 cells. This is due to reduced bacterial uptake and lower relative vacuole escape, which results in fewer cytosolic S. sonnei and hence reduced activation of caspase-1 inflammasomes. Mechanistically, the O-antigen, which in S. sonnei is contained in both the lipopolysaccharide and the capsule, was responsible for reduced uptake and the T3SS was required for vacuole escape. Our findings suggest that S. sonnei has adapted to an extracellular lifestyle by incorporating additional O-antigen into its surface structures compared to other Shigella species.