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

Abstract

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 antimi-crobial 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. Keywords Streptococcus agalactiae; pili; GBS; macrophage; neutrophil; antimicrobial peptidesThe gram-positive bacterium Group B Streptococcus (GBS) is a major cause of pneumonia, sepsis, and meningitis in newborns and is increasingly associated with disease in adult populations including the elderly, pregnant women, and diabetics (1). Cell surface-expressed NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript filamentous appendages known as pili have recently been identified in streptococcal pathogens that cause invasive infections in humans, including GBS (2), Group A Streptococcus (GAS) (3), and Streptococcus pneumoniae (4). Pili, also known as fimbriae, are non-flagellar polymeric organelles often involved in bacterial adherence to host cells and tissues during colonization. The analysis of multiple GBS genomes has revealed the presence of specific genetic islands that contain the necessary components for pilus formation (5). In all cases that have been described so far, the genes that encode the pilus proteins are clustered at the same genetic locus, transcribed in the same direction, and are likely part of an operon (6). The pilus operon codes for three proteins with the conserved C-terminal amino acid motif LP(X)TG for subsequent cell wall anchoring, and two genes encoding sortases required for complete pilus assembly. Recent studies have demonstrated that a single GBS pilin protein constitutes the major pilus subunit or bona fide pilus, while ancillary proteins are incorporated at the tip or base of the pilus structure (5,7,8). In addition, an immediate upstream transcriptional regulator has been shown to activate the expression of...

Regulation and Function of Pilus Island 1 in Group B Streptococcus

“…A detailed investigation of the contribution of pili to biofilm formation concluded that PI-2a pili, but not PI-1 or PI-2b pili, mediate formation of biofilms on both abiotic and biotic surfaces (27). Another study, using the background of type III GBS strain NEM316, found no role for BP-2a (PilB) in resistance to phagocytosis and intracellular killing by macrophages, in contrast to earlier work (19,23). The same study concluded that PI-2a pili contributed to virulence in a neonatal infection model but not in adult mice.…”

“…Conflicting results have been reported on the function of GBS pili in resistance to phagocytic killing (19,23). To evaluate whether the PI-1 pilus plays a role in phagocytic resistance, we first assessed GBS internalization and intracellular survival in human monocyte-derived macrophages.…”

Expression of the Type 1 Pneumococcal Pilus Is Bistable and Negatively Regulated by the Structural Component RrgA

“…However, whereas in GBS and pneumococcus the pilus appears to contribute to virulence in systemic disease (4,24,25), the opposite was found in Streptococcus pyogenes, where, unexpectedly, the presence of the pilus promotes capture of the organism in extracellular traps, thereby reducing the virulence potential of the organism (10). From an epidemiological perspective, it is also unclear whether the pilus contributes to systemic virulence in pneumococcus, as there is no difference in prevalence of pilus genes in nasopharyngeal and invasive disease isolates (5).…”

StreptococcusAdherence and Colonization

“…Immunomodulatory capabilities have also been associated with pili. Those of S. pneumoniae have been shown to promote inflammatory cytokine release (29), while S. agalactiae pili confer resistance to phagocytic killing (378). The pili of S. pyogenes promote bacterial aggregation via binding to the salivary component gp340, a process that may lead to increased bacterial clearance (145).…”