Influence of Shigella flexneri 2a O antigen acetylation on its bacteriophage Sf6 receptor activity, and bacterial interaction with human cells

Abstract

Shigella flexneri is a major causative agent of bacillary dysentery in developing countries where serotype 2a₂ is the prevalent strain. To date, approximately 30 serotypes have been identified for S. flexneri and the major contribution to the emergence of new serotypes is chemical modifications of the lipopolysaccharide (LPS) component O antigen (Oag). Glucosylation, O-acetylation and phosphoethanolamine (PEtN) modifications increase the Oag diversity providing benefits to S. flexneri LPS Oag acts as a primary receptor for bacteriophage Sf6, which only infects a limited range of S. flexneri serotypes (Y, X). It uses its tailspike protein (Sf6TSP) to establish initial interaction with LPS Oags that it then hydrolyses. Currently, there is a lack of comprehensive study on the parent and serotype variant strains from the same genetic background and the understanding of the importance of LPS Oag O-acetylations. Therefore, a set of isogenic strains [based on S. flexneri 2457T (2a₂)] with deletions of different Oag modification genes (oacB, oacD and gtrII) that resemble different naturally occurring serotype Y and 2a strains was created. The impact of these Oag modifications on S. flexneri sensitivity to Sf6 and the pathogenesis-related properties were then compared. We found that Sf6TSP can hydrolyse serotype 2a LPS Oag, identified that 3/4-O-acetylation is essential for resistance of serotype 2a strains to Sf6, and showed that serotype 2a strains have better invasion ability. Lastly, we revealed two new serotype conversions for S. flexneri thereby contributing to understanding the evolution of this important human pathogen.Importance The emergence of antibiotic resistance strains and lack of efficient vaccines have made Shigella a priority organism for the World Health Organisation (1). Therefore, bacteriophage therapy has received increasing attention as an alternative therapeutic approach. LPS Oag is the most variable part of LPS due to chemical modifications and is the target of bacteriophage Sf6 (S. flexneri-specific). We dissected the evolution of S. flexneri serotype Y to 2a₂, which revealed a new role for a gene acquired during serotype conversion and furthermore identified new specific forms of LPS receptor for Sf6. Collectively, these results unfold the importance of the acquisition of those Oag modification genes and further our understanding of the relationship between Sf6 and S. flexneri.