Shigella sonneiO-antigen inhibits internalisation, vacuole escape and inflammasome activation

Abstract

15Two Shigella species, flexneri and sonnei, cause approximately 90% of bacterial dysentery 16 worldwide. While S. flexneri is the dominant species in low-income countries, S. sonnei causes 17 the majority of infections in middle and high-income countries. S. flexneri is a prototypic 18 cytosolic bacterium; once intracellular it rapidly escapes the phagocytic vacuole and causes 19 pyroptosis of macrophages, which is important for pathogenesis and bacterial spread. By 20 contrast little is known about the invasion, vacuole escape and induction of pyroptosis during 21 S. sonnei infection of macrophages. We demonstrate that S. sonnei causes substantially less 22 pyroptosis in human primary monocyte-derived macrophages and THP1 cells. This is due to 23 reduced bacterial uptake and lower relative vacuole escape, which results in fewer cytosolic 24 S. sonnei and hence reduced activation of caspase-1 inflammasomes. Mechanistically, the O-25 antigen, which in S. sonnei is contained in both the lipopolysaccharide and the capsule, was 26 responsible for reduced uptake and the T3SS was required for vacuole escape. Our findings 27 suggest that S. sonnei has adapted to an extracellular lifestyle by incorporating additional O-28 antigen into its surface structures compared to other Shigella species. 29 30 in this age group (1). Closely related to Escherichia coli, the genus is made up of four species; 34 S. flexneri, S. sonnei, S. dysenteriae and S.boydii. These are divided into serotypes based on 35 the O-antigen (O-Ag) structure. S. flexneri and S. sonnei are responsible for the majority of 36 infections however, dominance is highly dependent on the socioeconomic status of an area. 37 S. flexneri is associated with poor water sanitation and hygiene in developing countries. In 38 sub-Saharan Africa and Asia, S. flexneri accounts for 66% of cases and S. sonnei 24% of 39 cases (2). However, in areas with good socioeconomic conditions and a high gross domestic 40 product per capita, such as North America and Europe, S. sonnei is responsible for up to 80% 41 of infections (3). Transitional countries that have recently undergone socioeconomic 42 improvements show a shift from S. flexneri to S. sonnei as the dominant species (4-6). As a 43 number of large populous countries undergo this shift (eg. Brazil, India, China), S. sonnei is 44 emerging as an important pathogen. 45The pathogenesis of S. sonnei is poorly understood and generally assumed to be similar to S. 46 flexneri. The growing importance of S. sonnei has led to a re-evaluation of its pathogenesis 47 and has revealed some important differences from S. flexneri. These include a novel adhesin, 48 (7, 8), an antibacterial T6SS (9) and a group 4 capsule (G4C) which protects it from serum-49 mediated killing (10). Both species have a homologous T3SS that promotes secretion of 50 effectors into host cells. 51 Unlike other Shigella species which contain multiple serotypes, there is only one S. sonnei 52 serotype. The genes encoding biosynthesis and export of the O-Ag are encoded...

Shigella sonnei infection of zebrafish reveals that O-antigen mediates neutrophil tolerance and dysentery incidence

“…The S. sonnei 53G ΔO-Ag and its complemented strain ΔO-Ag +pSSO-Ag are described in [43]. Briefly, the O-Ag synthesis operon wbgT-wbgZ was replaced by a Kanamycin cassette using the λ Red recombinase approach as described above.…”

Very long O‐antigen chains of Salmonella Paratyphi A inhibit inflammasome activation and pyroptotic cell death

“…Here we found that the modification of LPS O‐antigen polysaccharide length, which is dispensable for caspase‐4 binding (Hagar et al, 2013 ; Kayagaki, Wong, et al, 2013 ), can prevent caspase‐4 activation and help bacteria evade detection in human cells. O‐antigen‐mediated evasion of caspase‐4 activation may also be used by S. sonnei (Watson et al, 2019a , 2019b ). Whether other bacteria that produce very long O‐antigen chains, such as S .…”