M1T1 group A streptococcal pili promote epithelial colonization but diminish systemic virulence through neutrophil extracellular entrapment.

Group A Streptococcus is a leading human pathogen associated with a diverse array of mucosal and systemic infections. Cell wall anchored pili were recently described in several species of pathogenic streptococci, and in the case of GAS, these surface appendages were demonstrated to facilitate epithelial cell adherence. Here we use targeted mutagenesis to evaluate the contribution of pilus expression to virulence of the globally disseminated M1T1 GAS clone, the leading agent of both GAS pharyngitis and severe invasive infections. We confirm that pilus expression promotes GAS adherence to pharyngeal cells, keratinocytes, and skin. However, in contrast to findings reported for group B streptococcal and pneumococcal pili, we observe that pilus expression reduces GAS virulence in murine models of necrotizing fasciitis, pneumonia and sepsis, while decreasing GAS survival in human blood. Further analysis indicated the systemic virulence attenuation associated with pilus expression was not related to differences in phagocytic uptake, complement deposition or cathelicidin antimicrobial peptide sensitivity. Rather, GAS pili were found to induce neutrophil IL-8 production, promote neutrophil transcytosis of endothelial cells, and increase neutrophil release of DNA-based extracellular traps, ultimately promoting GAS entrapment and killing within these structures.

Recent advances in understanding the molecular basis of group B Streptococcus virulence.

Group B Streptococcus commonly colonises healthy adults without symptoms, yet under certain circumstances displays the ability to invade host tissues, evade immune detection and cause serious invasive disease. Consequently, Group B Streptococcus remains a leading cause of neonatal pneumonia, sepsis and meningitis. Here we review recent information on the bacterial factors and mechanisms that direct host-pathogen interactions involved in the pathogenesis of Group B Streptococcus infection. New research on host signalling and inflammatory responses to Group B Streptococcus infection is summarised. An understanding of the complex interplay between Group B Streptococcus and host provides valuable insight into pathogen evolution and highlights molecular targets for therapeutic intervention.

A group B streptococcal pilus protein promotes phagocyte resistance and systemic virulence.

Group B Streptococcus (GBS) is a major cause of invasive bacterial infections in newborns and certain adult populations. Surface filamentous appendages known as pili have been recently identified in GBS. However, little is known about the role of these structures in disease pathogenesis. In this study we sought to probe potential functional role(s) of PilB, the major GBS pilus protein subunit, by coupling analysis of an isogenic GBS pilB knockout strain with heterologous expression of the pilB gene in the nonpathogenic bacterium Lactococcus lactis. We found the knockout GBS strain that lacked PilB was more susceptible than wild-type (WT) GBS to killing by isolated macrophages and neutrophils. Survival was linked to the ability of PilB to mediate GBS resistance to cathelicidin antimicrobial peptides. Furthermore, the PilB-deficient GBS mutant was more readily cleared from the mouse bloodstream and less-virulent in vivo compared to the WT parent strain. Strikingly, overexpression of the pilB gene alone in L. lactis enhanced resistance to phagocyte killing, increased bloodstream survival, and conferred virulence in a mouse challenge model. Together these data demonstrate that the pilus backbone subunit, PilB, plays an integral role in GBS virulence and suggests a novel role for gram-positive pili in thwarting the innate defenses of phagocyte killing.

Group B streptococcal pilus proteins contribute to adherence to and invasion of brain microvascular endothelial cells.

Surface filamentous structures known as pili have been discovered recently in the gram-positive streptococcal pathogens that cause invasive disease in humans, including group B Streptococcus (GBS). We show that two GBS proteins involved in pilus formation, encoded by pilA and pilB, also facilitate the interaction of this important agent of central nervous system infection with endothelial cells of the human blood-brain barrier.

Blood-brain barrier invasion by group B Streptococcus depends upon proper cell-surface anchoring of lipoteichoic acid.

Group B streptococci (GBSs) are the leading cause of neonatal meningitis. GBSs enter the CNS by penetrating the blood-brain barrier (BBB), which consists of specialized human brain microvascular endothelial cells (hBMECs). To identify GBS factors required for BBB penetration, we generated random mutant libraries of a virulent strain and screened for loss of hBMEC invasion in vitro. Two independent hypo-invasive mutants possessed disruptions in the same gene, invasion associated gene (iagA), which encodes a glycosyltransferase homolog. Allelic replacement of iagA in the GBS chromosome produced a 4-fold decrease in hBMEC invasiveness. Mice challenged with the GBS DeltaiagA mutant developed bacteremia comparably to WT mice, yet mortality was significantly lower (20% vs. 90%), as was the incidence of meningitis. The glycolipid diglucosyldiacylglycerol, a cell membrane anchor for lipoteichoic acid (LTA) and predicted product of the IagA glycosyltransferase, was absent in the DeltaiagA mutant, which consequently shed LTA into the media. Attenuation of virulence of the DeltaiagA mutant was found to be independent of TLR2-mediated signaling, but bacterial supernatants from the DeltaiagA mutant containing released LTA inhibited hBMEC invasion by WT GBS. Our data suggest that LTA expression on the GBS surface plays a role in bacterial interaction with BBB endothelium and the pathogenesis of neonatal meningitis.