A type VII secretion system in Group B Streptococcus mediates cytotoxicity and virulence.

Brady L Spencer,Prescilla E Nagao,Kelly S Doran,Jéssica C Mendonça,Michael Niederweis,Uday Tak,Michael R Wessels

doi:10.1371/journal.ppat.1010121

Abstract

Type VII secretion systems (T7SS) have been identified in Actinobacteria and Firmicutes and have been shown to secrete effector proteins with functions in virulence, host toxicity, and/or interbacterial killing in a few genera. Bioinformatic analysis indicates that isolates of Group B Streptococcus (GBS) encode at least four distinct subtypes of T7SS machinery, three of which encode adjacent putative T7SS effectors with WXG and LXG motifs. However, the function of T7SS in GBS pathogenesis is unknown. Here we assessed the role of the most abundant GBS T7SS subtype during GBS pathogenesis. In a murine model of hematogenous meningitis, mice infected with GBS lacking a functional T7SS or lacking the secreted WXG100 effector EsxA exhibited less mortality, lower bacterial burdens in tissues, and decreased inflammation in the brain compared to mice infected with the parental GBS strain. We further showed that this T7SS induces cytotoxicity in brain endothelium and that EsxA contributes to these cytotoxicity phenotypes in a WXG motif-dependent manner. Finally, we determined that EsxA is a pore-forming protein, thus demonstrating the first role for a non-mycobacterial EsxA homolog in pore formation. This work reveals the importance of a T7SS in host–GBS interactions and has implications for T7SS effector function in other Gram-positive bacteria.

Highlights

Bacteria utilize secretion systems to respond to changes in environment, defend against interbacterial killing, acquire nutrients, exchange genetic material, and promote virulence within the host [1,2]
Asymptomatically colonizing Group B Streptococcus (GBS) in the female genital tract can be transmitted to the fetus or newborn and can result in neonatal meningitis upon GBS disruption of the blood-brain barrier (BBB)
Using an example subtype I GBS strain, CJB111, we show that deletion of the ATPase-encoding gene, essC, mitigates virulence and GBS-induced inflammation in the brain, as well as cell death in brain endothelial cells, and that these phenotypes are dependent on the secreted T7SS substrate EsxA

Summary

Introduction

Bacteria utilize secretion systems to respond to changes in environment, defend against interbacterial killing, acquire nutrients, exchange genetic material, and promote virulence within the host [1,2]. While deletion of any one of these core components can abrogate T7SSb activity [13,19,20], the hexameric, membranebound ATPase EssC is considered the driver of T7SSb, of which the ATP-binding domains in the C-terminal region are required for substrate secretion and the C-terminal region is necessary for substrate recognition and specificity [15,21,22] It has been shown in several Gram-positive bacterial species that the C-terminal sequence of EssC is highly variable across strains, and each EssC variant is accompanied by a unique set of putative secreted effectorencoding genes [17,23,24]. It is likely that the function of a given T7SS in interbacterial competition or virulence is determined at the level of its secreted effectors

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