Type VII secretion system and its effect on Group B Streptococcus virulence
Authors: Schindler Yulia , Rahav Galia , Nissan Israel , Valenci Gal , Ravins Miriam , Hanski Emanuel , Ment Dana , Tekes-Manova Dorit , Maor Yasmin 
Published: January 2023

GBS may cause a devasting disease in newborns. In early onset disease of the newborn the bacteria are acquired from the colonized mother during delivery. We characterized type VII secretion system (T7SS), exporting small proteins of the WXG100 superfamily, in group B Streptococci (GBS) isolates from pregnant colonized women and newborns with early onset disease (EOD) to understand better understand T7SS contribution to virulence in these different clinical scenarios.

GBS isolates were obtained from colonized mother prior to delivery and from newborns with EOD. DNA was analyzed for T7SS genes. A mutant EOD strain (ST17) was created by knocking out theessCgene encoding a T7SS protein.Galleria mellonellalarvae were used to compare virulence of colonizing, EOD, and mutant EOD isolates.

33 GBS genomes were tested, 17 EOD isolates and 16 colonizing isolates. The T7SS locus encoded 8 genes:essC, membrane-embedded proteins (essA; essB), modulators of T7SS activity(esaA; esaB; esaC) and effectors: [esxA(SAG1039);esxB(SAG1030). ST17 isolates encode two copies of theessCgene andesxAgene encoding putative effectors but were present only in 23.5% of isolates. In ST1 isolates three copies of esxA gene were identified, but in ST6 and ST19 isolates all T7SS genes were missing. EOD isolates demonstrated enhanced virulence inG. mellonellamodel compared to colonizing isolates. The 118659ΔessCstrain was attenuated in its killing ability, and the larvae were more effective in eradicating 118659ΔessCinfection.essCgene deletion was associated with reduced bacterial growth. We demonstrated that T7SS plays an essential role during infection and contributes to GBS pathogenicity.

Author Summary

Type VII secretion system (T7SS) is related to virulence in various bacteria but is not well characterized in Group B Streptococci (GBS). GBS may cause sepsis, meningitis, and death in newborns. The bacteria rarely cause disease in pregnant mothers. Newborns acquire GBS from the colonized mother during delivery. We studied the role of T7SS in GBS isolates obtained from newborns with GBS sepsis in the first week of life and in colonized pregnant mothers. By studying T7SS genes we discovered that the genetic structure of the T7SS differs between isolates causing severe disease and colonizing isolates. To study the virulence of different GBS isolates we injected them into larvae and monitored larvae survival. Isolates causing severe disease in the newborn caused a more severe disease in larvae compared to colonizing isolates. We then deleted T7SS genes in GBS isolates causing severe disease. The killing activity of GBS isolates without T7SS genes was attenuated. The larva responded to these bacteria similarly to the response found when injecting the larva with GBS isolates from colonized mothers. These results support our hypothesis that T7SS is important for causing severe infection in the newborn and that this system contributes to GBS pathogenicity.