Analysis of publicly available genomes of Streptococcus pneumoniae has led to the identification of a new genomic element containing genes typical of gram-positive pilus islets (PIs). Here, we demonstrate that this genomic region, herein referred to as PI-2 (consisting of pitA , sipA , pitB , srtG1 , and srtG2 ) codes for a second functional pilus in pneumococcus. Polymerization of the PI-2 pilus requires the backbone protein PitB as well as the sortase SrtG1 and the signal peptidase-like protein SipA. Presence of PI-2 correlates with the genotype as defined by multilocus sequence typing and clonal complex (CC). The PI-2-positive CCs are associated with serotypes 1, 2, 7F, 19A, and 19F, considered to be emerging serotypes in both industrialized and developing countries. Interestingly, strains belonging to CC271 (where sequence type 271 is the predicted founder of the CC) contain both PI-1 and PI-2, as revealed by genome analyses. In these strains both pili are surface exposed and independently assembled. Furthermore, in vitro experiments provide evidence that the pilus encoded by PI-2 of S. pneumoniae is involved in adherence. Thus, pneumococci encode at least two types of pili that play a role in the initial host cell contact to the respiratory tract and are potential antigens for inclusion in a new generation of pneumococcal vaccines.

The sequence variability of protective antigens is a major challenge to the development of vaccines. For Neisseria meningitidis, the bacterial pathogen that causes meningitis, the amino acid sequence of the protective antigen factor H binding protein (fHBP) has more than 300 variations. These sequence differences can be classified into three distinct groups of antigenic variants that do not induce cross-protective immunity. Our goal was to generate a single antigen that would induce immunity against all known sequence variants of N. meningitidis. To achieve this, we rationally designed, expressed, and purified 54 different mutants of fHBP and tested them in mice for the induction of protective immunity. We identified and determined the crystal structure of a lead chimeric antigen that was able to induce high levels of cross-protective antibodies in mice against all variant strains tested. The new fHBP antigen had a conserved backbone that carried an engineered surface containing specificities for all three variant groups. We demonstrate that the structure-based design of multiple immunodominant antigenic surfaces on a single protein scaffold is possible and represents an effective way to create broadly protective vaccines.

Summary: Adherence to host cells is important in microbial colonization of a mucosal surface, and Streptococcus pneumoniae adherence was significantly enhanced by expression of an extracellular pilus composed of three subunits, RrgA, RrgB and RrgC. We sought to determine which subunit(s) confers adherence. Bacteria deficient in RrgA are significantly less adherent than wild‐type organisms, while overexpression of RrgA enhances adherence. Recombinant monomeric RrgA binds to respiratory cells, as does RrgC with less affinity, and pre‐incubation of epithelial cells with RrgA reduces adherence of wild‐type piliated pneumococci. Non‐adherent RrgA‐negative, RrgB‐ and RrgC‐positive organisms produce pili, suggesting that pilus‐mediated adherence is due to expression of RrgA, rather than the pilus backbone itself. In contrast, RrgA‐positive strains with disrupted rrgB and rrgC genes exhibit wild‐type adherence despite failure to produce pili by Western blot or immunoelectron microscopy. The density of bacteria colonizing the upper respiratory tract of mice inoculated with piliated RrgA‐negative pneumococci was significantly less compared with wild‐type; in contrast, non‐piliated pneumococci expressing non‐polymeric RrgA had similar numbers of bacteria in the nasopharynx as piliated wild‐type bacteria. These data suggest that RrgA is central in pilus‐mediated adherence and disease, even in the absence of polymeric pilus production.

Streptococcus pneumoniae is a major public health threat worldwide. The recent discovery that this pathogen possesses pili led us to investigate their protective abilities in a mouse model of intraperitoneal infection. Both active and passive immunization with recombinant pilus subunits afforded protection against lethal challenge with the S. pneumoniae serotype 4 strain TIGR4.

Pili have been identified on the cell surface of Streptococcus pneumoniae, a major cause of morbidity and mortality worldwide. In contrast to Gram-negative bacteria, little is known about the structure of native pili in Gram-positive species and their role in pathogenicity. Triple immunoelectron microscopy of the elongated structure showed that purified pili contained RrgB as the major compound, followed by clustered RrgA and individual RrgC molecules on the pilus surface. The arrangement of gold particles displayed a uniform distribution of anti-RrgB antibodies along the whole pilus, forming a backbone structure. Antibodies against RrgA were found along the filament as particulate aggregates of 2–3 units, often co-localised with single RrgC subunits. Structural analysis using cryo electron microscopy and data obtained from freeze drying/metal shadowing technique showed that pili are oligomeric appendages formed by at least two protofilaments arranged in a coiled-coil, compact superstructure of various diameters. Using extracellular matrix proteins in an enzyme-linked immunosorbent assay, ancillary RrgA was identified as the major adhesin of the pilus. Combining the structural and functional data, a model emerges where the pilus RrgB backbone serves as a carrier for surface located adhesive clusters of RrgA that facilitates the interaction with the host. Streptococcus pneumoniae (pneumococcus) is one of the most important human pathogens and a major cause of morbidity and mortality worldwide, causing respiratory tract infections, community acquired pneumonia, and invasive diseases. Although the pneumococcus is a well-studied bacterial pathogen, first described in the late 19th century, pili on its surface were discovered only recently. Pili are elongated structures extruding from the bacterial surface and were found to be important virulence factors of both Gram-positive and Gram-negative bacteria. Bacterial pili are considered to participate in bacterial adhesion to a host, a crucial step in bacterial infection. In contrast to Gram-negative pili, little is known about the structure of native Gram-positive pili. We used native purified pili of pathogenic pneumococcus TIGR4 to study its structural composition, mainly by the use of cryo EM techniques. Pili were found to be composed of protofilaments that are arranged in a coiled-coil, compact superstructure of various diameters. Adhesive properties of pilus surface located ancillary protein RrgA to selected compounds of the extracellular matrix might be part of the pilus mediated host–pathogen interplay. Analysis of native pneumococcal pili revealed structural basics of a Gram-positive pilus that could also serve as a basis for effective vaccine design.

Although the pili of Gram‐positive bacteria are putative virulence factors, little is known about their structure. Here we describe the molecular architecture of pilus‐1 of Streptococcus pneumoniae , which is a major cause of morbidity and mortality worldwide. One major (RrgB) and two minor components (RrgA and RrgC) assemble into the pilus. Results from TEM and scanning transmission EM show that the native pili are approximately 6 nm wide, flexible filaments that can be over 1 μm long. They are formed by a single string of RrgB monomers and have a polarity defined by nose‐like protrusions. These protrusions correlate to the shape of monomeric RrgB–His, which like RrgA–His and RrgC–His has an elongated, multi‐domain structure. RrgA and RrgC are only present at the opposite ends of the pilus shaft, compatible with their putative roles as adhesin and anchor to the cell wall surface, respectively. Our structural analyses provide the first direct experimental evidence that the native S. pneumoniae pilus shaft is composed exclusively of covalently linked monomeric RrgB subunits oriented head‐to‐tail.