Hemolytic Membrane Vesicles of Group B Streptococcus Promote Infection

Abstract

Group B Streptococci (GBS) are β-hemolytic, Gram-positive bacteria associated with fetal injury, preterm birth, spontaneous abortion, and neonatal infections. A key factor promoting GBS virulence is the β-hemolysin/cytolysin, a pigmented ornithine rhamnolipid (also known as granadaene) associated with the bacterial surface. A previous study indicated that GBS produce small structures known as membrane vesicles (MVs), which contain virulence-associated proteins. Here, we show that GBS MVs are pigmented and hemolytic, indicating that granadaene is functionally active in MVs. Additionally, MVs from hyper-hemolytic GBS induced greater cell death of neutrophils, T cells, and B cells compared to MVs from isogenic non-hemolytic GBS, implicating MVs as a potential mechanism for granadaene-mediated virulence. Finally, hemolytic MVs reduced oxidative killing of GBS and aggravated morbidity and mortality of neonatal mice infected with GBS. Collectively, these studies reveal a novel mechanism by which GBS deploy a crucial virulence factor to promote bacterial dissemination and pathogenesis.

Production and composition of group B streptococcal membrane vesicles varies across diverse lineages

“…The presence and abundance of sialidase was variable: the ST-1 and ST-17 MVs all contained sialidase, but the ST-12 MVs lacked it. In two prior studies examining GBS MVs produced by a ST-7 strain, A909, neither C5a peptidase nor sialidase were identified (14,15), further highlighting differences across strains. However, we cannot rule out the possibility that the abundance of these virulence factors was beneath the detection limit in those studies.…”

“…Current knowledge regarding GBS derived MVs is restricted to one clinical strain (14,15) and hence, we sought to examine MV production and composition in a set of clinical strains with different traits. While no clear association was observed between clinical phenotype and the production or composition of MVs, we have demonstrated that the GBS MV proteome is STdependent.…”

“…In addition, TEM revealed that MVs displayed a spherical morphology containing a lipid bilayer and slightly electron dense interior (Figure 1C-D, Figure S2), which is typical of bacterialderived MVs (14,15).…”

“…Many bacteria produce membrane vesicles (MVs) of varying sizes (20-500 nm) containing toxins and other virulence factors that can modulate immune responses and influence pathogenesis (13). In addition, GBS can produce MVs that have been implicated in driving infection risk, though this remains an area in need of more research (14,15). While the exact role of GBS MVs in pathogenesis is not clear, intra-amniotic injection of GBS MVs produced by an invasive ST-7 strain induced preterm birth and intrauterine fetal death in mice (14).…”

“…While the exact role of GBS MVs in pathogenesis is not clear, intra-amniotic injection of GBS MVs produced by an invasive ST-7 strain induced preterm birth and intrauterine fetal death in mice (14). GBS MVs were also found to contain active virulence factors that could weaken murine gestational membranes, stimulate immune cell recruitment, and lyse host cells (14,15). Hence, an important, unanswered question is whether MVs derived from strains belonging to distinct phylogenetic lineages and clinical sources vary in composition and pathogenic potential.…”