Corynebacterium diphtheriae employs specific minor pilins to target human pharyngeal epithelial cells

Genome organization and pathogenicity of Corynebacterium diphtheriae C7(-) and PW8 strains

Corynebacterium diphtheriae is the causative agent of diphtheria. In 2003, the complete genomic nucleotide sequence of an isolate (NCTC13129) from a large outbreak in the former Soviet Union was published, in which the presence of 13 putative pathogenicity islands (PAIs) was demonstrated. In contrast, earlier work on diphtheria mainly employed the C7(-) strain for genetic analysis; therefore, current knowledge of the molecular genetics of the bacterium is limited to that strain. However, genomic information on the NCTC13129 strain has scarcely been compared to strain C7(-). Another important C. diphtheriae strain is Park-Williams no. 8 (PW8), which has been the only major strain used in toxoid vaccine production and for which genomic information also is not available. Here, we show by comparative genomic hybridization that at least 37 regions from the reference genome, including 11 of the 13 PAIs, are considered to be absent in the C7(-) genome. Despite this, the C7(-) strain still retained signs of pathogenicity, showing a degree of adhesion to Detroit 562 cells, as well as the formation of and persistence in abscesses in animal skin comparable to that of the NCTC13129 strain. In contrast, the PW8 strain, suggested to lack 14 genomic regions, including 3 PAIs, exhibited more reduced signs of pathogenicity. These results, together with great diversity in the presence of the 37 genomic regions among various C. diphtheriae strains shown by PCR analyses, suggest great heterogeneity of this pathogen, not only in genome organization, but also in pathogenicity.

The Actinomyces oris type 2 fimbrial shaft FimA mediates co-aggregation with oral streptococci, adherence to red blood cells and biofilm development

Interbacterial interactions between oral streptococci and actinomyces and their adherence to tooth surface and the associated host cells are key early events that promote development of the complex oral biofilm referred to as dental plaque. These interactions depend largely on a lectin-like activity associated with the Actinomyces oris type 2 fimbria, a surface structure assembled by sortase (SrtC2)-dependent polymerization of the shaft and tip fimbrillins, FimA and FimB respectively. To dissect the function of specific fimbrillins in various adherence processes, we have developed a convenient new technology for generating unmarked deletion mutants of A. oris. Here, we show that the fimB mutant, which produced type 2 fimbriae composed only of FimA, like the wild type co-aggregated strongly with receptor-bearing streptococci, agglutinated with sialidase-treated red blood cells, and formed monospecies biofilm. In contrast, the fimA and srtC2 mutants lacked type 2 fimbriae and were non-adherent in each of these assays. Plasmid-based expression of the deleted gene in respective mutants restored adherence to wild-type levels. These findings uncover the importance of the lectin-like activity of the polymeric FimA shaft rather than the tip. The multivalent adhesive function of FimA makes it an ideal molecule for exploring novel intervention strategies to control plaque biofilm formation.

Increase in pilus islet 2-encoded pili among Streptococcus pneumoniae isolates, Atlanta, Georgia, USA

To define the prevalence of pilus islet 2 (PI-2)-encoded pili in Streptococcus pneumoniae in a geographically defined area, we examined 590 S. pneumoniae isolates from population-based surveillance of invasive pneumococcal disease in Atlanta, Georgia, USA, 1994-2006. In 2006, PI-2 was present in 21% of all invasive isolates, including serotypes 1 (100%), 7F (89%), 11A (21%), 19A (40%), and 19F (75%). Only serotype 19F is included in the 7-valent pneumococcal conjugate vaccine that is in use worldwide. In 1999, PI-2-containing isolates were of the same serotypes but accounted for only 3.6% of all invasive isolates. The increase of PI-2 in 2006 resulted predominantly from the emergence of serotype 19A isolates of sequence type 320 and the expansion of serotype 7F isolates. The increase in PI-2-containing isolates and the finding that isolates of all identified serotypes expressed highly conserved PI-2 pili supports their potential as a vaccine candidate.

A tale of two pili: assembly and function of pili in bacteria

Bacterial pili have long been recognized as mediators of initial host-pathogen interactions important for the progression of Gram-negative bacterial diseases. An appreciation of the role of pili on virulence in Gram-positive bacteria and the unique properties of their biogenesis is a rapidly emerging area of research. In this review, we focus on recent advances in one of the longest-studied Gram-negative pilus systems, the chaperone/usher assembled pili, along with the newcomer to the field, the sortase-assembled pili of Gram-positive bacteria. In both systems, a wealth of new structural and molecular details has emerged recently. In light of this, we explore similarities between chaperone/usher and sortase-assembled pilus biogenesis and highlight paradigms unique to each, with the goal of using knowledge of each system to raise new questions and inform future studies of the other.

Complete genome sequence and lifestyle of black-pigmented Corynebacterium aurimucosum ATCC 700975 (formerly C. nigricans CN-1) isolated from a vaginal swab of a woman with spontaneous abortion

Background

Corynebacterium aurimucosum is a slightly yellowish, non-lipophilic, facultative anaerobic member of the genus Corynebacterium and predominantly isolated from human clinical specimens. Unusual black-pigmented variants of C. aurimucosum (originally named as C. nigricans) continue to be recovered from the female urogenital tract and they are associated with complications during pregnancy. C. aurimucosum ATCC 700975 (C. nigricans CN-1) was originally isolated from a vaginal swab of a 34-year-old woman who experienced a spontaneous abortion during month six of pregnancy. For a better understanding of the physiology and lifestyle of this potential urogenital pathogen, the complete genome sequence of C. aurimucosum ATCC 700975 was determined.

Results

Sequencing and assembly of the C. aurimucosum ATCC 700975 genome yielded a circular chromosome of 2,790,189 bp in size and the 29,037-bp plasmid pET44827. Specific gene sets associated with the central metabolism of C. aurimucosum apparently provide enhanced metabolic flexibility and adaptability in aerobic, anaerobic and low-pH environments, including gene clusters for the uptake and degradation of aromatic amines, L-histidine and L-tartrate as well as a gene region for the formation of selenocysteine and its incorporation into formate dehydrogenase. Plasmid pET44827 codes for a non-ribosomal peptide synthetase that plays the pivotal role in the synthesis of the characteristic black pigment of C. aurimucosum ATCC 700975.

Conclusions

The data obtained by the genome project suggest that C. aurimucosum could be both a resident of the human gut and possibly a pathogen in the female genital tract causing complications during pregnancy. Since hitherto all black-pigmented C. aurimucosum strains have been recovered from female genital source, biosynthesis of the pigment is apparently required for colonization by protecting the bacterial cells against the high hydrogen peroxide concentration in the vaginal environment. The location of the corresponding genes on plasmid pET44827 explains why black-pigmented (formerly C. nigricans) and non-pigmented C. aurimucosum strains were isolated from clinical specimens.

Corynebacterium diphtheriae invasion-associated protein (DIP1281) is involved in cell surface organization, adhesion and internalization in epithelial cells

Background

Corynebacterium diphtheriae, the causative agent of diphtheria, is well-investigated in respect to toxin production, while little is known about C. diphtheriae factors crucial for colonization of the host. In this study, we investigated the function of surface-associated protein DIP1281, previously annotated as hypothetical invasion-associated protein.

Results

Microscopic inspection of DIP1281 mutant strains revealed an increased size of the single cells in combination with an altered less club-like shape and formation of chains of cells rather than the typical V-like division forms or palisades of growing C. diphtheriae cells. Cell viability was not impaired. Immuno-fluorescence microscopy, SDS-PAGE and 2-D PAGE of surface proteins revealed clear differences of wild-type and mutant protein patterns, which were verified by atomic force microscopy. DIP1281 mutant cells were not only altered in shape and surface structure but completely lack the ability to adhere to host cells and consequently invade these.

Conclusions

Our data indicate that DIP1281 is predominantly involved in the organization of the outer surface protein layer rather than in the separation of the peptidoglycan cell wall of dividing bacteria. The adhesion- and invasion-negative phenotype of corresponding mutant strains is an effect of rearrangements of the outer surface.

Transition of Enterococcus faecium from commensal organism to nosocomial pathogen

The Gram-positive species Enterococcus faecium has long been thought of as a harmless commensal of the mammalian GI tract. In the last two decades, however, E. faecium has become an important cause of nosocomial bacteremias. These infections are often difficult to treat owing to the resistance of E. faecium to a large number of antibiotics. In this article, we review the recent transition of E. faecium from commensal to nosocomial pathogen. We focus on population biology-based studies, which suggest that several clonal populations of E. faecium are mostly responsible for causing infections. We also discuss the role of the accessory genome of E. faecium in contributing to the infectious phenotype and examine the role that surface proteins of E. faecium may have in colonization and infection.

Sortase D forms the covalent bond that links BcpB to the tip of Bacillus cereus pili

Bacillus cereus and other Gram-positive bacteria elaborate pili via a sortase D-catalyzed transpeptidation mechanism from major and minor pilin precursor substrates. After cleavage of the LPXTG sorting signal of the major pilin, BcpA, sortase D forms an amide bond between the C-terminal threonine and the amino group of lysine within the YPKN motif of another BcpA subunit. Pilus assembly terminates upon sortase A cleavage of the BcpA sorting signal, resulting in a covalent bond between BcpA and the cell wall cross-bridge. Here, we show that the IPNTG sorting signal of BcpB, the minor pilin, is cleaved by sortase D but not by sortase A. The C-terminal threonine of BcpB is amide-linked to the YPKN motif of BcpA, thereby positioning BcpB at the tip of pili. Thus, unique attributes of the sorting signals of minor pilins provide Gram-positive bacteria with a universal mechanism ordering assembly of pili.

Linkage of T3 and Cpa pilins in the Streptococcus pyogenes M3 pilus

The important human pathogen Streptococcus pyogenes (group A streptococcus, GAS) initiates infection by pilus-mediated attachment to host tissue. Thus, the pilus is an excellent target for design of anti-infective strategies. The T3 pilus of GAS is composed of multiple covalently linked subunits of the T3 protein to which the two minor pilins, Cpa and OrfB, are covalently attached. Because the proteins of GAS pili do not contain either of the motifs required for pilus polymerization in other Gram-positive bacteria, we investigated the residues involved in their linkage. We show that linkage of Cpa to T3 by the sortase family transpeptidase SrtC2 requires the VPPTG motif in the cell wall-sorting signal of Cpa. We also demonstrate that K173 of T3 is required both for T3 polymerization and for attachment of Cpa to T3. Therefore, attachment of Cpa to K173 of a T3 subunit would block further addition of T3 subunits to this end of the growing pilus. This implies that Cpa is located exclusively at the pilus tip, a location supported by immunogold electron microscopy, and suggests that, as for well-studied pili on Gram-negative bacteria, the role of the pilus is to present the adhesin external to the bacterial capsule.

Dual role for pilus in adherence to epithelial cells and biofilm formation in Streptococcus agalactiae

Streptococcus agalactiae is a common human commensal and a major life-threatening pathogen in neonates. Adherence to host epithelial cells is the first critical step of the infectious process. Pili have been observed on the surface of several gram-positive bacteria including S. agalactiae. We previously characterized the pilus-encoding operon gbs1479-1474 in strain NEM316. This pilus is composed of three structural subunit proteins: Gbs1478 (PilA), Gbs1477 (PilB), and Gbs1474 (PilC), and its assembly involves two class C sortases (SrtC3 and SrtC4). PilB, the bona fide pilin, is the major component; PilA, the pilus associated adhesin, and PilC, are both accessory proteins incorporated into the pilus backbone. We first addressed the role of the housekeeping sortase A in pilus biogenesis and showed that it is essential for the covalent anchoring of the pilus fiber to the peptidoglycan. We next aimed at understanding the role of the pilus fiber in bacterial adherence and at resolving the paradox of an adhesive but dispensable pilus. Combining immunoblotting and electron microscopy analyses, we showed that the PilB fiber is essential for efficient PilA display on the surface of the capsulated strain NEM316. We then demonstrated that pilus integrity becomes critical for adherence to respiratory epithelial cells under flow-conditions mimicking an in vivo situation and revealing the limitations of the commonly used static adherence model. Interestingly, PilA exhibits a von Willebrand adhesion domain (VWA) found in many extracellular eucaryotic proteins. We show here that the VWA domain of PilA is essential for its adhesive function, demonstrating for the first time the functionality of a prokaryotic VWA homolog. Furthermore, the auto aggregative phenotype of NEM316 observed in standing liquid culture was strongly reduced in all three individual pilus mutants. S. agalactiae strain NEM316 was able to form biofilm in microtiter plate and, strikingly, the PilA and PilB mutants were strongly impaired in biofilm formation. Surprisingly, the VWA domain involved in adherence to epithelial cells was not required for biofilm formation.

Mechanism for sortase localization and the role of sortase localization in efficient pilus assembly in Enterococcus faecalis

Pathogenic streptococci and enterococci primarily rely on the conserved secretory (Sec) pathway for the translocation and secretion of virulence factors out of the cell. Since many secreted virulence factors in gram-positive organisms are subsequently attached to the bacterial cell surface via sortase enzymes, we sought to investigate the spatial relationship between secretion and cell wall attachment in Enterococcus faecalis. We discovered that sortase A (SrtA) and sortase C (SrtC) are colocalized with SecA at single foci in the enterococcus. The SrtA-processed substrate aggregation substance accumulated in single foci when SrtA was deleted, implying a single site of secretion for these proteins. Furthermore, in the absence of the pilus-polymerizing SrtC, pilin subunits also accumulate in single foci. Proteins that localized to single foci in E. faecalis were found to share a positively charged domain flanking a transmembrane helix. Mutation or deletion of this domain in SrtC abolished both its retention at single foci and its function in efficient pilus assembly. We conclude that this positively charged domain can act as a localization retention signal for the focal compartmentalization of membrane proteins.

Cloning, expression, purification, crystallization and preliminary X-ray analysis of the pilus-associated sortase C from Streptococcus pneumoniae

The pilus-associated sortase C from Streptococcus pneumoniae (SrtC or Srt-2) acts as a polymerase for the pilus subunit proteins RrgA and RrgB. Here, the crystallization and preliminary X-ray diffraction analysis of three crystal forms of SrtC are reported. One crystal form belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 48.9, b = 96.9, c = 98.9 A, alpha = beta = gamma = 90 degrees . The other two crystal forms belong to space group P222, with unit-cell parameters a = 48.8, b = 97.2, c = 99.2 A, alpha = beta = gamma = 90 degrees and a = 48.6, b = 96.5, c = 98.8 A, alpha = beta = gamma = 90 degrees , respectively. Preliminary analysis indicates the presence of two molecules in the asymmetric unit of the crystal for all three forms.

Expression of two distinct types of pili by a hospital-acquired Enterococcus faecium isolate

Surface filamentous structures designated pili, and implicated in virulence, have been found on the surfaces of several Gram-positive pathogens. This work describes the conditional expression of two phenotypically distinct pilus-like structures, designated PilA and PilB, on the surface of a hospital-adapted Enterococcus faecium bloodstream isolate. E. faecium is an emerging Gram-positive opportunistic pathogen that can cause severe disease, particularly in immunocompromised patients. Expression of PilA- and PilB-type pili was analysed during different phases of growth in broth culture. During growth, PilA and PilB pilin subunits were expressed around the cross-wall in early-exponential-phase cells. Polymerization and migration of short PilB-type pili towards the poles occurred in cells from the exponential phase and long polymerized pili were expressed at the poles of cells grown to stationary phase. In contrast, PilA-type pili were not expressed in broth culture, but only when cells were grown on solid media. Furthermore, surface expression of the PilA- and PilB-type pili was regulated in a temperature-dependent manner, as polymerization of two distinct types of pili at the surface only occurred when cells were grown at 37 degrees C; no pili were observed on cells grown at 21 degrees C. Hospital-aquired E. faecium isolates were specifically enriched in pilin gene clusters, suggesting that conditional expression of pili may contribute to E. faecium pathogenesis.

Identification and phenotypic characterization of a second collagen adhesin, Scm, and genome-based identification and analysis of 13 other predicted MSCRAMMs, including four distinct pilus loci, in Enterococcus faecium

Attention has recently been drawn to Enterococcus faecium because of an increasing number of nosocomial infections caused by this species and its resistance to multiple antibacterial agents. However, relatively little is known about the pathogenic determinants of this organism. We have previously identified a cell-wall-anchored collagen adhesin, Acm, produced by some isolates of E. faecium, and a secreted antigen, SagA, exhibiting broad-spectrum binding to extracellular matrix proteins. Here, we analysed the draft genome of strain TX0016 for potential microbial surface components recognizing adhesive matrix molecules (MSCRAMMs). Genome-based bioinformatics identified 22 predicted cell-wall-anchored E. faecium surface proteins (Fms), of which 15 (including Acm) had characteristics typical of MSCRAMMs, including predicted folding into a modular architecture with multiple immunoglobulin-like domains. Functional characterization of one [Fms10; redesignated second collagen adhesin of E. faecium (Scm)] revealed that recombinant Scm(65) (A- and B-domains) and Scm(36) (A-domain) bound to collagen type V efficiently in a concentration-dependent manner, bound considerably less to collagen type I and fibrinogen, and differed from Acm in their binding specificities to collagen types IV and V. Results from far-UV circular dichroism measurements of recombinant Scm(36) and of Acm(37) indicated that these proteins were rich in beta-sheets, supporting our folding predictions. Whole-cell ELISA and FACS analyses unambiguously demonstrated surface expression of Scm in most E. faecium isolates. Strikingly, 11 of the 15 predicted MSCRAMMs clustered in four loci, each with a class C sortase gene; nine of these showed similarity to Enterococcus faecalis Ebp pilus subunits and also contained motifs essential for pilus assembly. Antibodies against one of the predicted major pilus proteins, Fms9 (redesignated EbpC(fm)), detected a 'ladder' pattern of high-molecular-mass protein bands in a Western blot analysis of cell surface extracts from E. faecium, suggesting that EbpC(fm) is polymerized into a pilus structure. Further analysis of the transcripts of the corresponding gene cluster indicated that fms1 (ebpA(fm)), fms5 (ebpB(fm)) and ebpC(fm) are co-transcribed, a result consistent with those for pilus-encoding gene clusters of other Gram-positive bacteria. All 15 genes occurred frequently in 30 clinically derived diverse E. faecium isolates tested. The common occurrence of MSCRAMM- and pilus-encoding genes and the presence of a second collagen-binding protein may have important implications for our understanding of this emerging pathogen.

Sortase-mediated assembly and surface topology of adhesive pneumococcal pili

The rlrA genetic islet encodes an extracellular pilus in the Gram-positive pathogen Streptococcus pneumoniae. Of the three genes for structural subunits, rrgB encodes the major pilin, while rrgA and rrgC encode ancillary pilin subunits decorating the pilus shaft and tip. Deletion of all three pilus-associated sortase genes, srtB, srtC and srtD, completely prevents pilus biogenesis. Expression of srtB alone is sufficient to covalently associate RrgB subunits to one another as well as linking the RrgA adhesin and the RrgC subunit into the polymer. The active-site cysteine residue of SrtB (Cys 177) is crucial for incorporating RrgC, even when the two other sortase genes are expressed. SrtC is redundant to SrtB in permitting RrgB polymerization, and in linking RrgA to the RrgB filament, but SrtC is insufficient to incorporate RrgC. In contrast, expression of srtD alone fails to mediate RrgB polymerization, and a srtD mutant assembles heterotrimeric pilus indistinguishable from wild type. Topological studies demonstrate that pilus antigens are localized to symmetric foci at the cell surface in the presence of all three sortases. This symmetric focal presentation is abrogated in the absence of either srtB or srtD, while deletion of srtC had no effect. In addition, strains expressing srtB alone or srtC alone also displayed disrupted antigen localization, despite polymerizing subunits. Our data suggest that both SrtB and SrtC act as pilus subunit polymerases, with SrtB processing all three pilus subunit proteins, while SrtC only RrgB and RrgA. In contrast, SrtD does not act as a pilus subunit polymerase, but instead is required for wild-type focal presentation of the pilus at the cell surface.

The molecular switch that activates the cell wall anchoring step of pilus assembly in gram-positive bacteria

Cell surface pili in gram-positive bacteria orchestrate the colonization of host tissues, evasion of immunity, and the development of biofilms. Recent work revealed that pilus assembly is a biphasic process wherein pilus polymerization is catalyzed by a pilus-specific sortase followed by cell wall anchoring of the pilus that is promoted by the housekeeping sortase. Here, we present molecular genetic and biochemical studies of a heterotrimeric pilus in Corynebacterium diphtheriae, uncovering the molecular switch that terminates pilus polymerization in favor of cell wall anchoring. The prototype pilus contains a major pilin (SpaA) forming the shaft, a tip pilin (SpaC), and another minor pilin (SpaB). Cells lacking SpaB form pilus fibers, but they are largely secreted in the medium, a phenotype also observed when cells lack the housekeeping sortase. Furthermore, the average pilus length is greatly increased in the absence of SpaB. Remarkably, a SpaB mutant that lacks the cell wall sorting signal but contains a critical lysine residue is incorporated in the pilus. However, the resulting pili fail to anchor to the cell wall. We propose that a specific minor pilin acts as the terminal subunit in pilus assembly. Cell wall anchoring ensues when the pilus polymer assembled on the pilus-specific sortase is transferred to the minor pilin presented by the housekeeping sortase via lysine-mediated transpeptidation.

Microbiological and clinical features of Corynebacterium urealyticum: urinary tract stones and genomics as the Rosetta Stone

Corynebacterium urealyticum, formerly known as coryneform CDC group D2, was first recognized to be involved in human infections 30 years ago. It is a slow-growing, lipophilic, asaccharolytic and usually multidrug-resistant organism with potent urease activity. Its cell wall peptidoglycan, menaquinone, mycolic and cellular fatty acid composition is consistent with that of the genus Corynebacterium. DNA-DNA hybridization studies and 16S rDNA sequencing analysis have been used to determine the degree of relatedness of C. urealyticum to other corynebacterial species. The genome of the type strain consists of a circular chromosome with a size of 2 369 219 bp and a mean G + C content of 64.2%, and analysis of its genome explains the bacterium's lifestyle. C. urealyticum is a common skin colonizer of hospitalized elderly individuals who are receiving broad-spectrum antibiotics. It is an opportunistic pathogen causing mainly acute cystitis, pyelonephritis, encrusted cystitis, and encrusted pyelitis. More infrequently, it causes other infections, but mainly in patients with urological diseases. Infections are more common in males than in females, and treatment requires administration of antibiotics active against the organism in vitro, mainly glycopeptides, as well as surgical intervention, the latter mostly in cases of chronic infection. Mortality directly associated with infection by this organism is not frequent, but encrusted pyelitis in kidney-recipient patients may cause graft loss. The outcome of infection by this organism is reasonably good if the microbiological diagnosis is made and patients are treated appropriately.

Novel lipoarabinomannan-like lipoglycan (CdiLAM) contributes to the adherence of Corynebacterium diphtheriae to epithelial cells

The genus Corynebacterium is part of the phylogenetic group nocardioform actinomycetes. Members of this group have a characteristic cell envelope structure composed primarily of branched long-chain lipids, termed mycolic acids, and a rich number of lipoglycans such as lipoarabinomanans (LAM) and lipomannans. In this study, we identified a novel LAM variant isolated from Corynebacterium diphtheriae named CdiLAM. The key structural features of CdiLAM are a linear alpha-1-->6-mannan with side chains containing 2-linked alpha-D-Manp and 4-linked alpha-D-Araf residues. The polysaccharide backbone is linked to a phosphatidylinositol anchor. In contrast to the LAMs of other members of actinomycetales, CdiLAM presents an unusual substitution at position 4 of alpha-1-->6-mannan backbone by alpha-D-Araf. Unlike the non-fimbrial adhesin 62-72p, CdiLAM did not function as a hemagglutinin to human red blood cells. Experimental evidences pointed to CdiLAM as an adhesin of C. diphtheriae to human respiratory epithelial cells, thereby, contributing to the pathogenesis of diphtheria.

Sortase A utilizes an ancillary protein anchor for efficient cell wall anchoring of pili in Streptococcus agalactiae

Pili are putative virulence factors and promising vaccine candidates in Streptococcus agalactiae (group B Streptococcus [GBS]) infection, a leading cause of neonatal sepsis and meningitis. The genes necessary for pilus synthesis and assembly are clustered in pilus islands (PI). Each gene encodes three structural subunits (a backbone and two ancillary proteins) bearing a C-terminal LPXTG motif and two subfamily C sortases (SrtC) involved in covalent polymerization of the subunits. GBS strains also possess the conserved "housekeeping" sortase A (SrtA), but its role in pilus assembly is unclear. To address this issue, pilus expression and cell wall anchoring were analyzed in srtA deletion mutants. Loss of SrtA did not affect pilus polymerization. However, pilus expression on the cell surface was reduced, and pili accumulated in the culture supernatant. Furthermore, cell-associated pili could be readily released by detergent treatment, indicating that SrtA is involved in covalent anchoring of pili to the cell wall. When each of the genes comprising PI-2a was systematically deleted, only the absence of ancillary subunit GBS150 or the SrtC required for incorporation of GBS150 into pili mimicked the srtA mutant phenotype. Thus, from these data a model for GBS pilus assembly can be proposed in which PI sortases are responsible for polymerization of the pilus structure, while SrtA is required to covalently attach it to the cell wall, utilizing ancillary pilus subunit GBS150 as the anchor protein.

A second pilus type in Streptococcus pneumoniae is prevalent in emerging serotypes and mediates adhesion to host cells

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.

Assembly of pili on the surface of Bacillus cereus vegetative cells

Vegetative forms of Bacillus cereus are reported to form pili, thin protein filaments that protrude up to 1 mum from the bacterial surface. Pili are assembled from two precursor proteins, BcpA and BcpB, in a manner requiring a pilus-associated sortase enzyme (SrtD). Pili are also formed on the surface of Bacillus anthracis expressing bcpA-srtD-bcpB. BcpA is distributed throughout the entire pilus, whereas BcpB appears positioned at its tip. In agreement with the hypothesis for pilus assembly in Gram-positive bacteria, BcpA encompasses the YPK pilin motif and the LPXTG sorting signal, each of which is absolutely required for the incorporation of BcpA and BcpB into pili. In contrast to BcpB, which relies on the presence of BcpA for incorporation into pili, BcpA fibre assembly occurs even in the absence of BcpB. B. anthracis sortase A (srtA), but not sortase B (srtB) or C (srtC), is required for proper anchoring of pili to the bacterial envelope, suggesting that BcpA/BcpB pili are linked to peptidoglycan cross-bridges.

The lifestyle of Corynebacterium urealyticum derived from its complete genome sequence established by pyrosequencing

Corynebacterium urealyticum is a lipid-requiring, urealytic bacterium of the human skin flora that has been recognized as causative agent of urinary tract infections. We report the analysis of the complete genome sequence of C. urealyticum DSM7109, which was initially recovered from a patient with alkaline-encrusted cystitis. The genome sequence was determined by a combination of pyrosequencing and Sanger technology. The chromosome of C. urealyticum DSM7109 has a size of 2,369,219bp and contains 2024 predicted coding sequences, of which 78% were considered as orthologous with genes in the Corynebacterium jeikeium K411 genome. Metabolic analysis of the lipid-requiring phenotype revealed the absence of a fatty acid synthase gene and the presence of a beta-oxidation pathway along with a large repertoire of auxillary genes for the degradation of exogenous fatty acids. A urease locus with the gene order ureABCEFGD may play a pivotal role in virulence of C. urealyticum by the alkalinization of human urine and the formation of struvite stones. Multidrug resistance of C. urealyticum DSM7109 is mediated by transposable elements, conferring resistances to macrolides, lincosamides, ketolides, aminoglycosides, chloramphenicol, and tetracycline. The complete genome sequence of C. urealyticum revealed a detailed picture of the lifestyle of this opportunistic human pathogen.

Pili in Gram-positive bacteria: assembly, involvement in colonization and biofilm development

Various cell-surface multisubunit protein polymers, known as pili or fimbriae, have a pivotal role in the colonization of specific host tissues by many pathogenic bacteria. In contrast to Gram-negative bacteria, Gram-positive bacteria assemble pili by a distinct mechanism involving a transpeptidase called sortase. Sortase crosslinks individual pilin monomers and ultimately joins the resulting covalent polymer to the cell-wall peptidoglycan. Here we review current knowledge of this mechanism and the roles of Gram-positive pili in the colonization of specific host tissues, modulation of host immune responses and the development of bacterial biofilms.

Housekeeping sortase facilitates the cell wall anchoring of pilus polymers in Corynebacterium diphtheriae

Many surface proteins in Gram-positive bacteria are covalently linked to the cell wall through a transpeptidation reaction catalysed by the enzyme sortase. Corynebacterium diphtheriae encodes six sortases, five of which are devoted to the assembly of three distinct types of pilus fibres--SrtA for the SpaA-type pilus, SrtB/SrtC for the SpaD-type pilus, and SrtD/SrtE for the SpaH-type pilus. We demonstrate here the function of SrtF, the so-called housekeeping sortase, in the cell wall anchoring of pili. We show that a multiple deletion mutant strain expressing only SrtA secretes a large portion of SpaA polymers into the culture medium, with concomitant decrease in the cell wall-linked pili. The same phenotype is observed with the mutant that is missing SrtF alone. By contrast, a strain that expresses only SrtF displays surface-linked pilins but no polymers. Therefore, SrtF can catalyse the cell wall anchoring of pilin monomers as well as pili, but it does not polymerize pilins. We show that SrtA and SrtF together generate wild-type levels of the SpaA-type pilus on the bacterial surface. Furthermore, by regulating the expression of SpaA in the cell, we demonstrate that the SrtF function becomes critical when the SpaA level is sufficiently high. Together, these findings provide key evidence for a two-stage model of pilus assembly: pilins are first polymerized by a pilus-specific sortase, and the resulting fibre is then attached to the cell wall by either the cognate sortase or the housekeeping sortase.

RrgA is a pilus-associated adhesin in Streptococcus pneumoniae

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.

Genomics of Actinobacteria: tracing the evolutionary history of an ancient phylum

Actinobacteria constitute one of the largest phyla among bacteria and represent gram-positive bacteria with a high G+C content in their DNA. This bacterial group includes microorganisms exhibiting a wide spectrum of morphologies, from coccoid to fragmenting hyphal forms, as well as possessing highly variable physiological and metabolic properties. Furthermore, Actinobacteria members have adopted different lifestyles, and can be pathogens (e.g., Corynebacterium, Mycobacterium, Nocardia, Tropheryma, and Propionibacterium), soil inhabitants (Streptomyces), plant commensals (Leifsonia), or gastrointestinal commensals (Bifidobacterium). The divergence of Actinobacteria from other bacteria is ancient, making it impossible to identify the phylogenetically closest bacterial group to Actinobacteria. Genome sequence analysis has revolutionized every aspect of bacterial biology by enhancing the understanding of the genetics, physiology, and evolutionary development of bacteria. Various actinobacterial genomes have been sequenced, revealing a wide genomic heterogeneity probably as a reflection of their biodiversity. This review provides an account of the recent explosion of actinobacterial genomics data and an attempt to place this in a biological and evolutionary context.

An IgG-like domain in the minor pilin GBS52 of Streptococcus agalactiae mediates lung epithelial cell adhesion

Streptococcus agalactiae is the leading cause of neonatal pneumonia, sepsis, and meningitis. The pathogen assembles heterotrimeric pilus structures on its surface; however, their function in pathogenesis is poorly understood. We report here the crystal structure of the pilin GBS52, which reveals two IgG-like fold domains, N1 and N2. Each domain is comprised of seven antiparallel beta strands, an arrangement similar to the fold observed in the Staphylococcus aureus adhesin Cna. Consistent with its role as an adhesin, deletion of gbs52 gene significantly reduces bacterial adherence to pulmonary epithelial cells. Moreover, latex beads linked to the GBS52 protein adhere to pulmonary but not to many other epithelial cells; binding to the former is specifically inhibited by antibodies against GBS52. Nonetheless, substantial binding is only observed with N2 domain-conjugated beads. This study presents the structure of a Gram-positive pilin that utilizes a distinct IgG fold variant to mediate pathogen adherence to a specific tissue.

Sortase-catalyzed assembly of distinct heteromeric fimbriae in Actinomyces naeslundii

Two types of adhesive fimbriae are expressed by Actinomyces; however, the architecture and the mechanism of assembly of these structures remain poorly understood. In this study we characterized two fimbrial gene clusters present in the genome of Actinomyces naeslundii strain MG-1. By using immunoelectron microscopy and biochemical analysis, we showed that the fimQ-fimP-srtC1-fimR gene cluster encodes a fimbrial structure (designated type 1) that contains a major subunit, FimP, forming the shaft and a minor subunit, FimQ, located primarily at the tip. Similarly, the fimB-fimA-srtC2 gene cluster encodes a distinct fimbrial structure (designated type 2) composed of a shaft protein, FimA, and a tip protein, FimB. By using allelic exchange, we constructed an in-frame deletion mutant that lacks the SrtC2 sortase. This mutant produces abundant type 1 fimbriae and expresses the monomeric FimA and FimB proteins, but it does not assemble type 2 fimbriae. Thus, SrtC2 is a fimbria-specific sortase that is essential for assembly of the type 2 fimbriae. Together, our experiments pave the way for several lines of molecular investigation that are necessary to elucidate the fimbrial assembly pathways in Actinomyces and their function in the pathogenesis of different biofilm-related oral diseases.