Expanded prevalence of group 4 capsule expression and its genetic requirements in a broad array of Shigella strains

The expression of a group 4 capsule (G4C) has been identified in Shigella sonnei and Shigella flexneri serotype 6 and demonstrated to play a significant role in pathogenic processes in S. sonnei. However, it has not been examined phenotypically in S. flexneri 6. Despite the impact of G4C, no systematic analysis of its expression or the presence of the genomic determinants has been performed across a large collection of S. sonnei and S. flexneri serotypes 2a and 6 or any of the other >46 Shigella strains and serotypes that cause disease. Our examination of the genomes of 1,219 geographically diverse Shigella isolates revealed the presence of intact gfc operons not only in the majority (≥98%) of S. sonnei and S. flexneri 6 strains but also, surprisingly, in the majority (≥97%) of Shigella boydii and Shigella dysenteriae strains. Approximately 95% of non-serotype 6 S. flexneri isolates contained a range of diverse gfc mutations preventing G4C expression; the remaining 5% contained an intact gfc operon yet had single-nucleotide polymorphisms within the promoter that resulted in lack of expression. These findings establish that no S. flexneri serotypes other than 6 express a G4C. Capsule expression was directly linked to reduced epithelial cell invasion for all strains examined, confirming differences in pathogenic processes between capsule-positive and capsule-negative Shigella strains. This work identifies a more widespread expression of capsule among Shigella strains than previously appreciated and identifies critical genetic requirements.

Shigella is the leading cause of moderate-to-severe diarrhea in 1- to 5-year-old children in low-resource settings, yet there is no licensed vaccine. Despite the impact of the group 4 capsule (G4C) on Shigella epithelial cell invasion, its expression has not been determined for the majority of Shigella species and serotypes that cause disease. This study investigated the presence of intact genes necessary for G4C expression in 1,219 geographically diverse Shigella isolates and examined phenotypes associated with G4C expression. We report that G4C is much more widespread than previously appreciated; it is expressed in the vast majority of Shigella sonnei and Shigella flexneri 6 clinical isolates, as well as Shigella boydii and Shigella dysenteriae strains. No S. flexneri serotypes outside of serotype 6 expressed a G4C. This study provides evidence that G4C expression in Shigella is much more widespread than previously recognized and emphasizes the importance of capsule expression in host-pathogen interactions.