Synthesis and function of Actinomyces naeslundii T14V type 1 fimbriae require the expression of additional fimbria-associated genes

Short chain fatty acids induced the type 1 and type 2 fimbrillin-dependent and fimbrillin-independent initial attachment and colonization of Actinomyces oris monoculture but not coculture with streptococci

BACKGROUND

Actinomyces oris is an early colonizer and has two types of fimbriae on its cell surface, type 1 fimbriae (FimP and FimQ) and type 2 fimbriae (FimA and FimB), which contribute to the attachment and coaggregation with other bacteria and the formation of biofilm on the tooth surface, respectively. Short-chain fatty acids (SCFAs) are metabolic products of oral bacteria including A. oris and regulate pH in dental plaques. To clarify the relationship between SCFAs and fimbrillins, effects of SCFAs on the initial attachment and colonization (INAC) assay using A. oris wild type and fimbriae mutants was investigated. INAC assays using A. oris MG1 strain cells were performed with SCFAs (acetic, butyric, propionic, valeric and lactic acids) or a mixture of them on human saliva-coated 6-well plates incubated in TSB with 0.25% sucrose for 1 h. The INAC was assessed by staining live and dead cells that were visualized with a confocal microscope.

RESULTS

Among the SCFAs, acetic, butyric and propionic acids and a mixture of acetic, butyric and propionic acids induced the type 1 and type 2 fimbriae-dependent and independent INAC by live A. oris, but these cells did not interact with streptococci. The main effects might be dependent on the levels of the non-ionized acid forms of the SCFAs in acidic stress conditions. GroEL was also found to be a contributor to the FimA-independent INAC by live A. oris cells stimulated with non-ionized acid.

CONCLUSION

SCFAs affect the INAC-associated activities of the A. oris fimbrillins and non-fimbrillins during ionized and non-ionized acid formations in the form of co-culturing with other bacteria in the dental plaque but not impact the interaction of A. oris with streptococci.

New Paradigms of Pilus Assembly Mechanisms in Gram-Positive Actinobacteria

Adhesive pili in Gram-positive bacteria represent a variety of extracellular multiprotein polymers that mediate bacterial colonization of specific host tissues and associated pathogenesis. Pili are assembled in two distinct but coupled steps, an orderly crosslinking of pilin monomers and subsequent anchoring of the polymer to peptidoglycan, catalyzed by two transpeptidase enzymes - the pilus-specific sortase and the housekeeping sortase. Here, we review this biphasic assembly mechanism based on studies of two prototypical models, the heterotrimeric pili in Corynebacterium diphtheriae and the heterodimeric pili in Actinomyces oris, highlighting some newly emerged basic paradigms. The disparate mechanisms of protein ligation mediated by the pilus-specific sortase and the spatial positioning of adhesive pili on the cell surface modulated by the housekeeping sortase are among the notable highlights.

Role of SCFAs for Fimbrillin-Dependent Biofilm Formation of Actinomyces oris

Actinomyces oris expresses type 1 and 2 fimbriae on the cell surface. Type 2 fimbriae mediate co-aggregation and biofilm formation and are composed of the shaft fimbrillin FimA and the tip fimbrillin FimB. Short-chain fatty acids (SCFAs) are metabolic products of oral bacteria, but the effects of exogenous SCFAs on FimA-dependent biofilm formation are poorly understood. We performed two types of biofilm formation assays using A. oris MG1 or MG1.ΔfimA to observe the effects of SCFAs on FimA-dependent biofilm formation in 96-well and six-well microtiter plates and a flow cell system. SCFAs did not induce six- and 16-hour biofilm formation of A. oris MG1 and MG1.ΔfimA in saliva-coated 96-well and six-well microtiter plates in which metabolites produced during growth were not excluded. However, 6.25 mM butyric acid and 3.125 mM propionic acid induced FimA-dependent biofilm formation and cell death in a flow cell system in which metabolites produced during growth were excluded. Metabolites produced during growth may lead to disturbing effects of SCFAs on the biofilm formation. The pure effects of SCFAs on biofilm formation were induction of FimA-dependent biofilm formation, but the stress responses from dead cells may regulate its effects. Therefore, SCFA may play a key role in A. oris biofilm formation.

Sec-secretion and sortase-mediated anchoring of proteins in Gram-positive bacteria

Signal peptide-driven secretion of precursor proteins directs polypeptides across the plasma membrane of bacteria. Two pathways, Sec- and SRP-dependent, converge at the SecYEG translocon to thread unfolded precursor proteins across the membrane, whereas folded preproteins are routed via the Tat secretion pathway. Gram-positive bacteria lack an outer membrane and are surrounded by a rigid layer of peptidoglycan. Interactions with their environment are mediated by proteins that are retained in the cell wall, often through covalent attachment to the peptidoglycan. In this review, we describe the mechanisms for both Sec-dependent secretion and sortase-dependent assembly of proteins in the envelope of Gram-positive bacteria. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Wide distribution of virulence genes among Enterococcus faecium and Enterococcus faecalis clinical isolates

Enterococcus, a Gram-positive facultative anaerobic cocci belonging to the lactic acid bacteria of the phylum Firmicutes, is known to be able to resist a wide range of hostile conditions such as different pH levels, high concentration of NaCl (6.5%), and the extended temperatures between 5^°C and 65^°C. Despite being the third most common nosocomial pathogen, our understanding on its virulence factors is still poorly understood. The current study was aimed to determine the prevalence of different virulence genes in Enterococcus faecalis and Enterococcus faecium. For this purpose, 79 clinical isolates of Malaysian enterococci were evaluated for the presence of virulence genes. pilB, fms8, efaAfm, and sgrA genes are prevalent in all clinical isolates. In conclusion, the pathogenicity of E. faecalis and E. faecium could be associated with different virulence factors and these genes are widely distributed among the enterococcal species.

Pilus biogenesis in Lactococcus lactis: molecular characterization and role in aggregation and biofilm formation

The genome of Lactococcus lactis strain IL1403 harbors a putative pilus biogenesis cluster consisting of a sortase C gene flanked by 3 LPxTG protein encoding genes (yhgD, yhgE, and yhhB), called here pil. However, pili were not detected under standard growth conditions. Over-expression of the pil operon resulted in production and display of pili on the surface of lactococci. Functional analysis of the pilus biogenesis machinery indicated that the pilus shaft is formed by oligomers of the YhgE pilin, that the pilus cap is formed by the YhgD pilin and that YhhB is the basal pilin allowing the tethering of the pilus fibers to the cell wall. Oligomerization of pilin subunits was catalyzed by sortase C while anchoring of pili to the cell wall was mediated by sortase A. Piliated L. lactis cells exhibited an auto-aggregation phenotype in liquid cultures, which was attributed to the polymerization of major pilin, YhgE. The piliated lactococci formed thicker, more aerial biofilms compared to those produced by non-piliated bacteria. This phenotype was attributed to oligomers of YhgE. This study provides the first dissection of the pilus biogenesis machinery in a non-pathogenic Gram-positive bacterium. Analysis of natural lactococci isolates from clinical and vegetal environments showed pili production under standard growth conditions. The identification of functional pili in lactococci suggests that the changes they promote in aggregation and biofilm formation may be important for the natural lifestyle as well as for applications in which these bacteria are used.

Sortase A substrate specificity in GBS pilus 2a cell wall anchoring

Streptococcus agalactiae, also referred to as Group B Streptococcus (GBS), is one of the most common causes of life-threatening bacterial infections in infants. In recent years cell surface pili have been identified in several Gram-positive bacteria, including GBS, as important virulence factors and promising vaccine candidates. In GBS, three structurally distinct types of pili have been discovered (pilus 1, 2a and 2b), whose structural subunits are assembled in high-molecular weight polymers by specific class C sortases. In addition, the highly conserved housekeeping sortase A (SrtA), whose main role is to link surface proteins to bacterial cell wall peptidoglycan by a transpeptidation reaction, is also involved in pili cell wall anchoring in many bacteria. Through in vivo mutagenesis, we demonstrate that the LPXTG sorting signal of the minor ancillary protein (AP2) is essential for pilus 2a anchoring. We successfully produced a highly purified recombinant SrtA (SrtA(ΔN40)) able to specifically hydrolyze the sorting signal of pilus 2a minor ancillary protein (AP2-2a) and catalyze in vitro the transpeptidation reaction between peptidoglycan analogues and the LPXTG motif, using both synthetic fluorescent peptides and recombinant proteins. By contrast, SrtA(ΔN40) does not catalyze the transpeptidation reaction with substrate-peptides mimicking sorting signals of the other pilus 2a subunits (the backbone protein and the major ancillary protein). Thus, our results add further insight into the proposed model of GBS pilus 2a assembly, in which SrtA is required for pili cell wall covalent attachment, acting exclusively on the minor accessory pilin, representing the terminal subunit located at the base of the pilus.

Genetic analysis and morphological identification of pilus-like structures in members of the genus Bifidobacterium

Background

Cell surface pili in Gram positive bacteria have been reported to orchestrate the colonization of host tissues, evasion of immunity and the development of biofilms. So far, little if any information is available on the presence of pilus-like structures in human gut commensals like bifidobacteria.

Results and discussion

In this report, Atomic Force Microscopy (AFM) of various bifidobacterial strains belonging to Bifidobacterium bifidum, Bifidobacterium longum subsp. longum, Bifidobacterium dentium, Bifidobacterium adolescentis and Bifidobacterium animalis subsp. lactis revealed the existence of appendages resembling pilus-like structures. Interestingly, these microorganisms harbour two to six predicted pilus gene clusters in their genome, with each organized in an operon encompassing the major pilin subunit-encoding gene (designated fimA or fimP) together with one or two minor pilin subunit-encoding genes (designated as fimB and/or fimQ), and a gene encoding a sortase enzyme (strA). Quantitative Real Time (qRT)-PCR analysis and RT-PCR experiments revealed a polycistronic mRNA, encompassing the fimA/P and fimB/Q genes, which are differentially expressed upon cultivation of bifidobacteria on various glycans.

Dual function of a tip fimbrillin of Actinomyces in fimbrial assembly and receptor binding

Interaction of Actinomyces oris with salivary proline-rich proteins (PRPs), which serve as fimbrial receptors, involves type 1 fimbriae. Encoded by the gene locus fimQ-fimP-srtC1, the type 1 fimbria is comprised of the fimbrial shaft FimP and the tip fimbrillin FimQ. Fimbrial polymerization requires the fimbria-specific sortase SrtC1, which catalyzes covalent linkage of fimbrial subunits. Using genetics, biochemical methods, and electron microscopy, we provide evidence that the tip fimbrillin, FimQ, is involved in fimbrial assembly and interaction with PRPs. Specifically, while deletion of fimP completely abolished the type 1 fimbrial structures, surface display of monomeric FimQ was not affected by this mutation. Surprisingly, deletion of fimQ significantly reduced surface assembly of the type 1 fimbriae. This defect was rescued by recombinant FimQ ectopically expressed from a plasmid. In agreement with the role of type 1 fimbriae in binding to PRPs, aggregation of A. oris with PRP-coated beads was abrogated in cells lacking srtC1 or fimP. This aggregation defect of the ΔfimP mutant was mainly due to significant reduction of FimQ on the bacterial surface, as the aggregation was not observed in a strain lacking fimQ. Increasing expression of FimQ in the ΔfimP mutant enhanced aggregation, while overexpression of FimP in the ΔfimQ mutant did not. Furthermore, recombinant FimQ, not FimP, bound surface-associated PRPs in a dose-dependent manner. Thus, not only does FimQ function as the major adhesin of the type 1 fimbriae, it also plays an important role in fimbrial assembly.

Relevance of pili in pathogenic streptococci pathogenesis and vaccine development

A common mechanism used by bacteria to initiate adhesion to host tissues during colonization is the expression of long filamentous structures extending from their surface. These structures, known as pili or fimbriae, were initially identified in Gram-negative bacteria, and are typically formed by noncovalent interactions between pilin subunits. Pili have only recently been described in Gram-positive bacteria. In particular, in pathogenic streptococci the proteinaceous components of pili are covalently polymerized by the action of sortase enzymes similar to those involved in the covalent attachment of Gram-positive surface proteins to the peptidoglycan cell wall. With great relevance to the development of strategies to combat Gram-positive-associated infections, pilus components from pathogenic streptococci have been shown to induce protective immunity in mouse models of streptococcal disease. In addition, recent papers have created new perspectives on the role of such organelles in streptococcal pathogenesis, from the involvement in colonization and biofilm formation to translocation of tissue barriers. All this information makes the characterization of pili a hot scientific issue that we believe will lead to important future developments in understanding bacterial dynamics that lead to successful occupation of microbial niches.

Allelic exchange in Actinomyces oris with mCherry fluorescence counterselection

Described here is a method for facile generation of markerless gene deletion mutants of Actinomyces oris. Homologous integration of a nonreplicative vector carrying a gene exchange cassette into the bacterial chromosome was selected for by using mCherry fluorescence and resistance to kanamycin. Completion of allelic replacement was counterselected for by using loss of fluorescence.

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.

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.

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.

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.

Sortase as a target of anti-infective therapy

The rise in antibiotic-resistant bacteria is a major concern, in particular because it includes many different species of pathogenic microbes. These "superbugs" are further characterized by high levels of virulence and disease-associated mortality. There seems to be few new antibiotics in the drug discovery pipeline; recent work has sought to define and validate new drug targets. The assembly of surface proteins and pili in the cell wall envelope of Gram-positive bacteria is catalyzed by sortase. Sortase cleaves a conserved C-terminal sequence of these polypeptides to generate an acyl-enzyme intermediate. The acyl-enzyme is next resolved by nucleophilic attack by the amino groups within cell wall cross-bridges or pilin proteins, thereby covalently attaching the polypeptides to the cell wall or the next pilin subunit. Sortase substrates function as adhesins, internalins, blood clotting and immune evasion factors, and transporters for nutrients across the microbial cell wall envelope; without them, most pathogens cannot sustain an infection. Here we review what is known about sortase catalysis and surface protein function, how surface protein anchoring can be inhibited, and what prospects such inhibition may have for anti-infective therapy.

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

Adherence to host tissues mediated by pili is pivotal in the establishment of infection by many bacterial pathogens. Corynebacterium diphtheriae assembles on its surface three distinct pilus structures. The function and the mechanism of how various pili mediate adherence, however, have remained poorly understood. Here we show that the SpaA-type pilus is sufficient for the specific adherence of corynebacteria to human pharyngeal epithelial cells. The deletion of the spaA gene, which encodes the major pilin forming the pilus shaft, abolishes pilus assembly but not adherence to pharyngeal cells. In contrast, adherence is greatly diminished when either minor pilin SpaB or SpaC is absent. Antibodies directed against either SpaB or SpaC block bacterial adherence. Consistent with a direct role of the minor pilins, latex beads coated with SpaB or SpaC protein bind specifically to pharyngeal cells. Therefore, tissue tropism of corynebacteria for pharyngeal cells is governed by specific minor pilins. Importantly, immunoelectron microscopy and immunofluorescence studies reveal clusters of minor pilins that are anchored to cell surface in the absence of a pilus shaft. Thus, the minor pilins may also be cell wall anchored in addition to their incorporation into pilus structures that could facilitate tight binding to host cells during bacterial infection.

Amended description of the genes for synthesis of Actinomyces naeslundii T14V type 1 fimbriae and associated adhesin

The type 1 fimbriae of Actinomyces naeslundii T14V mediate adhesion of this gram-positive species to the tooth surface. The present findings show that the locus for type 1 fimbria production in this strain includes three genes, fimQ for a minor fimbrial subunit that appears to be an adhesin, fimP for the major structural subunit, and srtC1 for a type 1 fimbria-specific sortase involved in the assembly of these structures.

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.

Pili with strong attachments: Gram-positive bacteria do it differently

Bacteria attach to their appropriate environmental niche by using adhesins. To maximize their contact with the environment, adhesins are often present on the ends of long hairlike structures called pili. Recently, attention has focused on pili of Gram-positive bacteria because they may be vaccine candidates in important human pathogens. These pili differ from the well-studied pili of Gram-negative bacteria because their subunits are covalently linked, they do not require specific chaperones for assembly, and the tip protein (likely to be the adhesin) is not required to initiate formation of the pilus structure. In Gram-positive bacteria, the genes for pili occur in clusters, which may constitute mobile genetic elements. These clusters include the transpeptidase(s) of the sortase family that is/are required for polymerization of the subunit proteins. However, efficient covalent attachment of the completed pilus structure to the cell wall is accomplished, in cases where this has been studied, by the 'housekeeping' sortase, which is responsible for attachment to the peptidoglycan of most surface proteins containing cell wall sorting signals. This enzyme is encoded elsewhere on the genome. Because pili of Gram-positive bacteria have not been extensively investigated yet, we hope that this MicroReview will help to pinpoint the areas most in need of further study.

Pili in Gram-positive pathogens

Most bacterial pathogens have long filamentous structures known as pili or fimbriae extending from their surface. These structures are often involved in the initial adhesion of the bacteria to host tissues during colonization. In gram-negative bacteria, pili are typically formed by non-covalent interactions between pilin subunits. By contrast, the recently discovered pili in gram-positive pathogens are formed by covalent polymerization of adhesive pilin subunits. Evidence from studies of pili in the three principal streptococcal pathogens of humans indicates that the genes that encode the pilin subunits and the enzymes that are required for the assembly of these subunits into pili have been acquired en bloc by the horizontal transfer of a pathogenicity island.

Assembly and role of pili in group B streptococci

Streptococcus agalactiae[group B streptococcus (GBS)] is the leading cause of neonatal pneumonia, sepsis and meningitis. An in silico genome analysis indicated that GBS strain NEM316 encodes five putative sortases, including the major class A sortase enzyme and four class C sortases. The genes encoding the class C sortases are tandemly arranged in two different loci, srtC1-C2 and srtC3-C4, with a similar genetic organization and are thought to be involved in pilus biosynthesis. Each pair of sortase genes is flanked by LPXTG protein encoding genes, two upstream and one downstream, and a divergently transcribed regulatory gene located upstream from this locus. We demonstrated that strain NEM316 expresses only the srtC3-C4 locus, which encodes three surface proteins (Gbs1474, Gbs1477 and Gbs1478) that polymerize to form appendages resembling pili. Structural and functional analysis of this locus revealed that: (i) the transcriptional activator RogB is required for expression of the srtC3-C4 operon; (ii) Gbs1477, and either SrtC3 or SrtC4 are absolutely required for pilus biogenesis; and (iii) GBS NEM316 pili are composed of three surface proteins, Gbs1477, the bona fide pilin which is the major component, Gbs1474, a minor associated component, and Gbs1478, a pilus-associated adhesin. Surprisingly, pilus-like structures can be formed in the absence of the two minor components, i.e. the putative anchor Gbs1474 or the adhesin Gbs1478. Adherence assays showed that Gbs1478 confers adhesive capacity to the pilus. This study provides the first evidence that adhesive pili are also present in Gram-positive pathogens.

Sequence analyses of fimbriae subunit FimA proteins on Actinomyces naeslundii genospecies 1 and 2 and Actinomyces odontolyticus with variant carbohydrate binding specificities

Background

Actinomyces naeslundii genospecies 1 and 2 express type-2 fimbriae (FimA subunit polymers) with variant Galβ binding specificities and Actinomyces odontolyticus a sialic acid specificity to colonize different oral surfaces. However, the fimbrial nature of the sialic acid binding property and sequence information about FimA proteins from multiple strains are lacking.

Results

Here we have sequenced fimA genes from strains of A.naeslundii genospecies 1 (n = 4) and genospecies 2 (n = 4), both of which harboured variant Galβ-dependent hemagglutination (HA) types, and from A.odontolyticus PK984 with a sialic acid-dependent HA pattern. Three unique subtypes of FimA proteins with 63.8–66.4% sequence identity were present in strains of A. naeslundii genospecies 1 and 2 and A. odontolyticus. The generally high FimA sequence identity (>97.2%) within a genospecies revealed species specific sequences or segments that coincided with binding specificity. All three FimA protein variants contained a signal peptide, pilin motif, E box, proline-rich segment and an LPXTG sorting motif among other conserved segments for secretion, assembly and sorting of fimbrial proteins. The highly conserved pilin, E box and LPXTG motifs are present in fimbriae proteins from other Gram-positive bacteria. Moreover, only strains of genospecies 1 were agglutinated with type-2 fimbriae antisera derived from A. naeslundii genospecies 1 strain 12104, emphasizing that the overall folding of FimA may generate different functionalities. Western blot analyses with FimA antisera revealed monomers and oligomers of FimA in whole cell protein extracts and a purified recombinant FimA preparation, indicating a sortase-independent oligomerization of FimA.

Conclusion

The genus Actinomyces involves a diversity of unique FimA proteins with conserved pilin, E box and LPXTG motifs, depending on subspecies and associated binding specificity. In addition, a sortase independent oligomerization of FimA subunit proteins in solution was indicated.

Sortases and the art of anchoring proteins to the envelopes of gram-positive bacteria

The cell wall envelopes of gram-positive bacteria represent a surface organelle that not only functions as a cytoskeletal element but also promotes interactions between bacteria and their environment. Cell wall peptidoglycan is covalently and noncovalently decorated with teichoic acids, polysaccharides, and proteins. The sum of these molecular decorations provides bacterial envelopes with species- and strain-specific properties that are ultimately responsible for bacterial virulence, interactions with host immune systems, and the development of disease symptoms or successful outcomes of infections. Surface proteins typically carry two topogenic sequences, i.e., N-terminal signal peptides and C-terminal sorting signals. Sortases catalyze a transpeptidation reaction by first cleaving a surface protein substrate at the cell wall sorting signal. The resulting acyl enzyme intermediates between sortases and their substrates are then resolved by the nucleophilic attack of amino groups, typically provided by the cell wall cross bridges of peptidoglycan precursors. The surface protein linked to peptidoglycan is then incorporated into the envelope and displayed on the microbial surface. This review focuses on the mechanisms of surface protein anchoring to the cell wall envelope by sortases and the role that these enzymes play in bacterial physiology and pathogenesis.

Assembly of distinct pilus structures on the surface of Corynebacterium diphtheriae

Different surface organelles contribute to specific interactions of a pathogen with host tissues or infectious partners. Multiple pilus gene clusters potentially encoding different surface structures have been identified in several gram-positive bacterial genomes sequenced to date, including actinomycetales, clostridia, corynebacteria, and streptococci. Corynebacterium diphtheriae has been shown to assemble a pilus structure, with sortase SrtA essential for the assembly of a major subunit SpaA and two minor proteins, SpaB and SpaC. We report here the characterization of a second pilus consisting of SpaD, SpaE, and SpaF, of which SpaD and SpaE form the pilus shaft and SpaF may be located at the pilus tip. The structure of the SpaDEF pilus contains no SpaABC pilins as detected by immunoelectron microscopy. Neither deletion of spaA nor sortase srtA abolishes SpaDEF pilus formation. The assembly of the SpaDEF pilus requires specific sortases located within the SpaDEF pilus gene cluster. Although either sortase SrtB or SrtC is sufficient to polymerize SpaDF, the incorporation of SpaE into the SpaD pili requires sortase SrtB. In addition, an alanine in place of the lysine of the SpaD pilin motif abrogates pilus polymerization. Thus, SpaD, SpaE, and SpaF constitute a different pilus structure that is independently assembled and morphologically distinct from the SpaABC pili and possibly other pili of C. diphtheriae.

Sorting sortases: a nomenclature proposal for the various sortases of Gram-positive bacteria

Bacterial surface proteins constitute a diverse group of molecules with important functions, such as adherence, invasion, signaling and interaction with the host immune system or the environment. In Gram-positive bacteria, many surface proteins are anchored to the cell wall envelope by an enzyme named sortase, which recognizes a conserved carboxylic sorting motif. The sequence of the prototype staphylococcal SrtA has been widely used to identify homologs in bacterial genomes, revealing a profusion of sortases in almost all Gram-positive bacteria, often with more than one sortase-like protein per genome [M.J. Pallen, A.C. Lam, M. Antonio, K. Dunbar, Trends Microbiol. 9 (2001) 97-102]. In light of increasing reports on the identification and/or characterization of paralogous sortase genes, a classification of sortases now appears necessary. This report provides an analysis of sixty-one sortases from complete Gram-positive genomes, and suggests the existence of four structural groups of sortases. We propose the classification of sortases into 4 classes designated A, B, C and D. This classification should help to discriminate between sortases in the future.

Sortases and pilin elements involved in pilus assembly of Corynebacterium diphtheriae

Corynebacterium diphtheriae SpaA pili are composed of three pilin subunits, SpaA, SpaB and SpaC. SpaA, the major pilin protein, is distributed uniformly along the pilus shaft, whereas SpaB is observed at regular intervals, and SpaC seems to be positioned at the pilus tip. Pilus assembly in C. diphtheriae requires the pilin motif and the C-terminal sorting signal of SpaA, and is proposed to occur by a mechanism of ordered cross-linking, whereby pilin-specific sortase enzymes cleave precursor proteins at sorting signals and involve the side-chain amino groups of pilin motif sequences to generate covalent linkages between pilin subunits. We show here that two elements of SpaA pilin precursor, the pilin motif and the sorting signal, are together sufficient to promote the polymerization of an otherwise secreted protein by a process requiring the function of the sortase A gene (srtA). Five other sortase genes are dispensable for SpaA pilus assembly. Further, the incorporation of SpaB into SpaA pili requires a glutamic acid residue within the E box motif of SpaA, a feature that is found to be conserved in other Gram-positive pathogens that encode sortase and pilin subunit genes with sorting signals and pilin motifs. When the main fimbrial subunit of Actinomyces naeslundii type I fimbriae, FimA, is expressed in corynebacteria, C. diphtheriae strain NCTC13129 polymerized FimA to form short fibres. Although C. diphtheriae does not depend on other actinomycetal genes for FimA polymerization, this process involves the pilin motif and the sorting signal of FimA as well as corynebacterial sortase D (SrtD). Thus, pilus assembly in Gram-positive bacteria seems to occur by a universal mechanism of ordered cross-linking of precursor proteins, the multiple conserved features of which are recognized by designated sortase enzymes.

Protein sorting to the cell wall envelope of Gram-positive bacteria

The covalent anchoring of surface proteins to the cell wall envelope of Gram-positive bacteria occurs by a universal mechanism requiring sortases, extracellular transpeptidases that are positioned in the plasma membrane. Surface protein precursors are first initiated into the secretory pathway of Gram-positive bacteria via N-terminal signal peptides. C-terminal sorting signals of surface proteins, bearing an LPXTG motif or other recognition sequences, provide for sortase-mediated cleavage and acyl enzyme formation, a thioester linkage between the active site cysteine residue of sortase and the C-terminal carboxyl group of cleaved surface proteins. During cell wall anchoring, sortase acyl enzymes are resolved by the nucleophilic attack of peptidoglycan substrates, resulting in amide bond formation between the C-terminal end of surface proteins and peptidoglycan cross-bridges within the bacterial cell wall envelope. The genomes of Gram-positive bacteria encode multiple sortase genes. Recent evidence suggests that sortase enzymes catalyze protein anchoring reactions of multiple different substrate classes with different sorting signal motif sequences, protein linkage to unique cell wall anchor structures as well as protein polymerization leading to the formation of pili on the surface of Gram-positive bacteria.

Assembly of pili on the surface of Corynebacterium diphtheriae

Pili of Gram-negative pathogens are formed from pilin precursor molecules by non-covalent association within the outer membrane envelope. Gram-positive microbes employ the cell wall peptidoglycan as a surface organelle for the covalent attachment of proteins, however, an assembly pathway for pili has not yet been revealed. We show here that pili of Corynebacterium diphtheriae are composed of three pilin subunits, SpaA, SpaB and SpaC. SpaA, the major pilin protein, is distributed uniformly along the pilus shaft, whereas SpaB is observed at regular intervals and SpaC seems positioned at the pilus tip. Assembled pili are released from the bacterial surface by treatment with murein hydrolase, suggesting that the pilus fibres may be anchored to the cell wall envelope. All three pilin subunit proteins are synthesized as precursors carrying N-terminal signal peptides and C-terminal sorting signals. Some, but not all, of the six sortase genes encoded in the genome of C. diphtheriae are required for precursor processing, pilus assembly or cell wall envelope attachment. Pilus assembly is proposed to occur by a mechanism of ordered cross-linking, whereby pilin-specific sortase enzymes cleave precursor proteins at sorting signals and involve the side chain amino groups of pilin motif sequences to generate links between pilin subunits. This covalent tethering of adjacent pilin subunits appears to have evolved in many Gram-positive pathogens that encode sortase and pilin subunit genes with sorting signals and pilin motifs.

The csp operon of Streptococcus salivarius encodes two predicted cell-surface proteins, one of which, CspB, is associated with the fimbriae

A Tn917 mutant library was generated to identify genes involved in the biogenesis of Streptococcus salivarius fimbriae. A fimbria-deficient mutant was isolated by negative selection using an immunomagnetic separation technique with specific anti-fimbriae polyclonal antibodies (pAbs). The transposon was inserted in an ORF, called orf176, which encoded a protein of unknown function. The transposon prevented the transcription of orf176 as well as two genes located downstream, which are designated cspA and cspB and which form the csp operon. Sequence analyses of CspA and CspB revealed that both proteins possessed the classic cell-wall-anchoring motif (LPXTG) of Gram-positive bacterial surface proteins. Recombinant CspA (rCspA) and CspB (rCspB) proteins were generated in Escherichia coli and used to produce pAbs. Immunolocalization experiments showed that anti-rCspB, but not anti-rCspA antibodies specifically recognized S. salivarius fimbriae. Our results suggested that the csp operon encoded predicted cell-surface proteins, one of which, CspB, was associated with the fimbriae.

Different type 1 fimbrial genes and tropisms of commensal and potentially pathogenic Actinomyces spp. with different salivary acidic proline-rich protein and statherin ligand specificities

Actinomyces spp. exhibit type 1 fimbria-mediated adhesion to salivary acidic proline-rich proteins (PRPs) and statherin ligands. Actinomyces spp. with different animal and tissue origins belong to three major adhesion types as relates to ligand specificity and type 1 fimbria genes. (i) In preferential acidic-PRP binding, strains of Actinomyces naeslundii genospecies 1 and 2 from human and monkey mouths displayed at least three ligand specificities characterized by preferential acidic-PRP binding. Slot blot DNA hybridization showed seven highly conserved type 1 fimbria genes (orf1- to -6 and fimP) in genospecies 1 and 2 strains, except that orf5 and orf3 were divergent in genospecies 1. (ii) In preferential statherin binding, oral Actinomyces viscosus strains of rat and hamster origin (and strain 19246 from a human case of actinomycosis) bound statherin preferentially. DNA hybridization and characterization of the type 1 fimbria genes from strain 19246 revealed a homologous gene cluster of four open reading frames (orfA to -C and fimP). Bioinformatics suggested sortase (orfB, orf4, and part of orf5), prepilin peptidase (orfC and orf6), fimbria subunit (fimP), and usher- and autotransporter-like (orfA and orf1 to -3) functions. Those gene regions corresponding to orf3 and orf5 were divergent, those corresponding to orf2, orf1, and fimP were moderately conserved, and those corresponding to orf4 and orf6 were highly conserved. Restriction fragment length polymorphism analyses using a fimP probe separated human and monkey and rat and hamster strains into phylogenetically different groups. (iii) In statherin-specific binding, strains of A. naeslundii genospecies 1 from septic and other human infections displayed a low-avidity binding to statherin. Only the orf4 and orf6 gene regions were highly conserved. Finally, rat saliva devoid of statherin bound bacterial strains avidly irrespective of ligand specificity, and specific antisera detected either type 1, type 2, or both types of fimbria on the investigated Actinomyces strains.

Actinomyces naeslundii fimbrial protein Orf977 shows similarity to a streptococcal adhesin

The nucleotide sequence of the chromosomal DNA, upstream of Actinomyces naeslundii T14V fimbrial gene fimA, was determined. One open reading frame (orf977) encoding 977 amino acids was found, preceded by a gene homologous to elongation factor TU. Database searches revealed that Orf977 was homologous to CshA, a Streptococcus gordonii protein involved in cell adhesion. Previous studies had already determined two genes in the type 2 fimbrial gene cluster of A. naeslundii T14V: the structural subunit fimA, and orf365 with unknown function, followed by ribosomal genes. This study completes the type 2 fimbrial gene cluster sequence.

Molecular strategies for fimbrial expression and assembly

Fimbriae or pili are long, filamentous, multimeric macromolecules found on the bacterial cell surface. Bacteria express a diverse array of fimbriae or pili that are involved in bacterial adherence and invasion. Fimbriae can be categorized based on their modes of expression and assembly. Type I fimbriae and P pili are distributed peritrichously and translocated to the cell surface by a chaperone/usher pathway. Type 4 pili are located at the pole of the cell and assembled via the type II secretion system. Curli fimbriae are coiled surface structures assembled by an extracellular nucleation/precipitation pathway. Fimbriae of oral gram-negative and gram-positive bacteria have not been well-studied as compared with the fimbriae of enteric pathogens. Oral pathogens, such as Eikenella corrodens, Actinobacillus actinomycetemcomitans, and Porphyromonas gingivalis, possess fimbriae that have been implicated in bacterial adhesion and invasion. These fimbriae are potential virulence factors in oral infectious processes. A. actinomycetemcomitans and E. corrodens have Type 4-like fimbriae, whereas P. gingivalis displays a unique type of fimbriae. To date, fimbriae of the oral primary colonizers, Actinomyces naeslundii and Streptococcus parasanguis, represent the only fimbriae characterized for any gram-positive bacteria. The putative major fimbrial subunits, FimA and FimP of A. naeslundii and Fap1 of S. parasanguis, contain a signal sequence and cell-wall-sorting signal. The presence of extensive dipeptide repeats in Fap1 makes it unique among fimbrial molecules. Based on experimental data, a nucleation/precipitation pathway is proposed for fimbrial biogenesis of both S. parasanguis and A. naeslundii, although we cannot rule out an alternative covalent linkage model. The model systems described in this review served as a framework for hypotheses for how the known molecular factors of fimbriae on oral bacteria may be expressed and assembled.

An embarrassment of sortases - a richness of substrates?

A range of surface proteins is anchored to the cell walls of Gram-positive pathogens such as Staphylococcus aureus by the transpeptidase sortase. Until now, sortase-like proteins and their substrates appeared to be limited mainly to such pathogens. However, by searching for sortase homologues among complete and incomplete genome sequences, we have found them to be present in almost all Gram-positives, a single Gram-negative bacterium and an archaean. There is usually more than one sortase-like protein encoded in each Gram-positive genome, and the genes encoding the sortase-like proteins are often clustered with genes encoding their likely substrates.

Oral microbial communities: biofilms, interactions, and genetic systems

Oral microbial-plaque communities are biofilms composed of numerous genetically distinct types of bacteria that live in close juxtaposition on host surfaces. These bacteria communicate through physical interactions called coaggregation and coadhesion, as well as other physiological and metabolic interactions. Streptococci and actinomyces are the major initial colonizers of the tooth surface, and the interactions between them and their substrata help establish the early biofilm community. Fusobacteria play a central role as physical bridges that mediate coaggregation of cells and as physiological bridges that promote anaerobic microenvironments which protect coaggregating strict anaerobes in an aerobic atmosphere. New technologies for investigating bacterial populations with 16S rDNA probes have uncovered previously uncultured bacteria and have offered an approach to in situ examination of the spatial arrangement of the participant cells in oral-plaque biofilms. Flow cells with saliva-coated surfaces are particularly useful for studies of biofilm formation and observation. The predicted sequential nature of colonization of the tooth surface by members of different genera can be investigated by using these new technologies and imaging the cells in situ with confocal scanning laser microscopy. Members of at least seven genera now can be subjected to genetic studies owing to the discovery of gene-transfer systems in these genera. Identification of contact-inducible genes in streptococci offers an avenue to explore bacterial responses to their environment and leads the way toward understanding communication among inhabitants of a multispecies biofilm.

Molecular and genetic analyses of Actinomyces spp

Members of the genus Actinomyces are predominant primary colonizers of the oral cavity and play an important role in initiating plaque development. These bacteria have evolved unique mechanisms that favor colonization and persistence in this micro-environment. The expression of cell-surface fimbriae is correlated with the ability of these bacteria to adhere to specific receptors on the tooth and mucosal surfaces, and to interact with other plaque bacteria. The elaboration of sialidase is thought to enhance fimbriae-mediated adherence by unmasking the fimbrial receptors on mammalian cells. The presence of certain cell-associated or extracellular enzymes, including those involved in sucrose or urea metabolism, may provide the means for these bacteria to thrive under conditions when other growth nutrients are not available. Moreover, these enzyme activities may influence the distribution of other plaque bacteria and promote selection for Actinomyces spp. in certain ecological niches. The recent development of a genetic transfer system for Actinomyces spp. has allowed for studies the results of which demonstrate the existence of multiple genes involved in fimbriae synthesis and function, and facilitated the construction of allelic replacement mutants at each gene locus. Analyses of these mutants have revealed a direct correlation between the synthesis of assembled fimbriae and the observed adherence properties. Further genetic analysis of the various enzyme activities detected from strains of Actinomyces should allow for an assessment of the role of these components in microbial ecology, and their contribution to the overall success of Actinomyces spp. as a primary colonizer and a key player in oral health and disease.

Identification of dipeptide repeats and a cell wall sorting signal in the fimbriae-associated adhesin, Fap1, of Streptococcus parasanguis

Fap1, a fimbriae-associated protein, is involved in fimbriae assembly and adhesion of Streptococcus parasanguis FW213 (Wu et al., 1998). In this study, the sequence of the fap1 gene was resolved using a primer island transposition system. Sequence analysis indicated that fap1 was composed of 7659 nucleotides. The predicted Fap1 protein contains an unusually long signal sequence (50 amino acid residues), a cell wall sorting signal and two repeat regions. Repeat regions I and II have a similar dipeptide composition (E/V/I)S, composed of 28 and 1000 repeats respectively. The two regions combined accounted for 80% of the Fap1 coding region. The experimental amino acid composition and isoelectric point (pI) of Fap1 were similar to that predicted from the deduced Fap1 protein. Results of Northern analyses revealed that the fap1 open reading frame (ORF) was transcribed as a 7.8 kb monocistronic message. Insertional inactivation at the 3' end, downstream of the fap1 ORF, did not affect Fap1, fimbrial expression or bacterial adhesion. Insertional inactivation of fap1 immediately upstream of the repeat region II abolished expression of Fap1 and fimbriae, and was concurrent with a diminution in adhesion of FW213. Inactivation of the cell wall sorting signal of fap1 also eliminated long fimbrial formation and reduced the ability of FW213 to bind to SHA. Fap1 was no longer anchored on the cell surface. Large quantities of truncated Fap1 were found in the growth medium instead. These results suggest that the fap1 ORF alone is sufficient to support Fap1 expression and adhesion, and demonstrate that anchorage of Fap1 on the cell surface is required for long fimbriae formation. These data further document the role of long fimbriae in adhesion of S. parasanguis FW213 to SHA.

Strains of Actinomyces naeslundii and Actinomyces viscosus exhibit structurally variant fimbrial subunit proteins and bind to different peptide motifs in salivary proteins

Oral strains of Actinomyces spp. express type 1 fimbriae, which are composed of major FimP subunits, and bind preferentially to salivary acidic proline-rich proteins (APRPs) or to statherin. We have mapped genetic differences in the fimP subunit genes and the peptide recognition motifs within the host proteins associated with these differential binding specificities. The fimP genes were amplified by PCR from Actinomyces viscosus ATCC 19246, with preferential binding to statherin, and from Actinomyces naeslundii LY7, P-1-K, and B-1-K, with preferential binding to APRPs. The fimP gene from the statherin-binding strain 19246 is novel and has about 80% nucleotide and amino acid sequence identity to the highly conserved fimP genes of the APRP-binding strains (about 98 to 99% sequence identity). The novel FimP protein contains an amino-terminal signal peptide, randomly distributed single-amino-acid substitutions, and structurally different segments and ends with a cell wall-anchoring and a membrane-spanning region. When agarose beads with CNBr-linked host determinant-specific decapeptides were used, A. viscosus 19246 bound to the Thr42Phe43 terminus of statherin and A. naeslundii LY7 bound to the Pro149Gln150 termini of APRPs. Furthermore, while the APRP-binding A. naeslundii strains originate from the human mouth, A. viscosus strains isolated from the oral cavity of rat and hamster hosts showed preferential binding to statherin and contained the novel fimP gene. Thus, A. viscosus and A. naeslundii display structurally variant fimP genes whose protein products are likely to interact with different peptide motifs and to determine animal host tropism.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Actinomyces naeslundii displays variant fimP and fimA fimbrial subunit genes corresponding to different types of acidic proline-rich protein and beta-linked galactosamine binding specificity

Actinomyces naeslundii genospecies 1 and 2 bind to acidic proline-rich proteins (APRPs) and statherin via type 1 fimbriae and to beta-linked galactosamine (GalNAcbeta) structures via type 2 fimbriae. In addition, A. naeslundii displays two types of binding specificity for both APRPs-statherin and GalNAcbeta, while Actinomyces odontolyticus binds to unknown structures. To study the molecular basis for these binding specificities, DNA fragments spanning the entire or central portions of fimP (type 1) and fimA (type 2) fimbrial subunit genes were amplified by PCR from strains of genospecies 1 and 2 and hybridized with DNA from two independent collections of oral Actinomyces isolates. Isolates of genospecies 1 and 2 and A. odontolyticus, but no other Actinomyces species, were positive for hybridization with fimP and fimA full-length probes irrespective of binding to APRPs and statherin, GalNAcbeta, or unknown structures. Isolates of genospecies 1 and 2, with deviating patterns of GalNAcbeta1-3Galalpha-O-ethyl-inhibitable coaggregation with Streptococcus oralis Ss34 and MPB1, were distinguished by a fimA central probe from genospecies 1 and 2, respectively. Furthermore, isolates of genospecies 1 and 2 displaying preferential binding to APRPs over statherin were positive with a fimP central probe, while a genospecies 2 strain with the opposite binding preference was not. The sequences of fimP and fimA central gene segments were highly conserved among isolates with the same, but diversified between those with a variant, binding specificity. In conclusion, A. naeslundii exhibits variant fimP and fimA genes corresponding to diverse APRP and GalNAcbeta specificities, respectively, while A. odontolyticus has a genetically related but distinct adhesin binding specificity.

Identification of a gene involved in assembly of Actinomyces naeslundii T14V type 2 fimbriae

The nucleotide sequence of the Actinomyces naeslundii T14V type 2 fimbrial structural subunit gene, fimA, and the 3' flanking DNA region was determined. The fimA gene encoded a 535-amino-acid precursor subunit protein (FimA) which included both N-terminal leader and C-terminal cell wall sorting sequences. A second gene, designated orf365, that encoded a 365-amino-acid protein which contained a putative transmembrane segment was identified immediately 3' to fimA. Mutants in which either fimA or orf365 was replaced with a kanamycin resistance gene did not participate in type 2 fimbriae-mediated coaggregation with Streptococcus oralis 34. Type 2 fimbrial antigen was not detected in cell extracts of the fimA mutant by Western blotting with anti-A. naeslundii type 2 fimbrial antibody, but the subunit protein was detected in extracts of the orf365 mutant. The subunit protein detected in this mutant also was immunostained by an antibody raised against a synthetic peptide representing the C-terminal 20 amino acid residues of the predicted FimA. The antipeptide antibody reacted with FimA isolated from the recombinant Escherichia coli clone containing fimA but did not react with purified type 2 fimbriae in extracts of the wild-type strain. These results indicate that synthesis of type 2 fimbriae in A. naeslundii T14V may involve posttranslational cleavage of both the N-terminal and C-terminal peptides of the precursor subunit and also the expression of orf365.