Identification of Novel Protein Substrates and Chemical Inhibitors of the T3SA in Shigella

Abstract

Enteropathogenic bacteria, such as Shigella and Salmonella, are associated with diarrheal diseases, which remain a significant cause of infant mortality worldwide. The secretion of protein effectors by the type III secretion apparatus (T3SA) is used by these pathogens to invade human cells and modulate host cell functions. First, we used RNA-Seq to analyze the differential transcriptome of Shigella flexneri when the T3SA is active or inactive. This allowed us to identify two uncharacterized genes that were temporarily named gem1 and gem3 and whose expression was regulated by MxiE and IpgC as other late substrates of the T3SA. Finally, we pursued the characterization of gem1 and gem3 at the protein level and renamed them icaT and icaR, respectively, when we found their protein products were secreted by the T3SA. Furthermore, we find homologs of icaT and icaR with a conserved MxiE box in several E. coli phylogroups. We also demonstrated that these homologous genes could be reactivated when both MxiE and IpgC were introduced in these strains. This discovery paved a new perspective on the evolution of pathogenesis into the E. coli lineage as both commensal and pathogenic strains harbored these genes. Treating infections caused by Enterobacteriaceae is becoming more challenging due to growing antibiotic resistance and no vaccines are widely available. Accordingly, the World Health Organization (WHO) recognized that we entered the "post-antibiotic era," where new antibiotics or antivirulence drugs are urgently needed, including for Shigella. The T3SA is an attractive target for antivirulence drugs, which may become alternative to classical antibiotics. Through screening 3,000 compounds, we found two novel inhibitors of the T3SA. Our data suggested that one of these candidate inhibitors, a dipyridyl-containing compound, reduces the virulence of Shigella at the transcriptional level. Indeed, the virulence inhibition occurs via the repression of the transcriptional activator VirB by the small chromosomal RNA RyhB, which is upregulated by this compound through an unknown mechanism involving the pyridyl groups. The repression of VirB induced by this molecule reduce the expression of several genes encoding parts of the T3SA. In comparison, the second compound is a quinone that seems to affect the assembly of the T3SA.