Peptide Mapping of a Functionally Versatile Fimbrial Adhesin from Porphyromonas gingivalis

Bacterial biofilms in the human body: prevalence and impacts on health and disease

Bacterial biofilms can be found in most environments on our planet, and the human body is no exception. Consisting of microbial cells encased in a matrix of extracellular polymers, biofilms enable bacteria to sequester themselves in favorable niches, while also increasing their ability to resist numerous stresses and survive under hostile circumstances. In recent decades, biofilms have increasingly been recognized as a major contributor to the pathogenesis of chronic infections. However, biofilms also occur in or on certain tissues in healthy individuals, and their constituent species are not restricted to canonical pathogens. In this review, we discuss the evidence for where, when, and what types of biofilms occur in the human body, as well as the diverse ways in which they can impact host health under homeostatic and dysbiotic states.

A Distinct Type of Pilus from the Human Microbiome

Pili are proteinaceous polymers of linked pilins that protrude from the cell surface of many bacteria and often mediate adherence and virulence. We investigated a set of 20 Bacteroidia pilins from the human microbiome whose structures and mechanism of assembly were unknown. Crystal structures and biochemical data revealed a diverse protein superfamily with a common Greek-key β sandwich fold with two transthyretin-like repeats that polymerize into a pilus through a strand-exchange mechanism. The assembly mechanism of the central, structural pilins involves proteinase-assisted removal of their N-terminal β strand, creating an extended hydrophobic groove that binds the C-terminal donor strands of the incoming pilin. Accessory pilins at the tip and base have unique structural features specific to their location, allowing initiation or termination of the assembly. The Bacteroidia pilus, therefore, has a biogenesis mechanism that is distinct from other known pili and likely represents a different type of bacterial pilus.

Pathogenic microbes and community service through manipulation of innate immunity

The periodontal pathogen Porphyromonas gingivalis undermines major components of innate immunity, such as complement, Toll-like receptors (TLR), and their crosstalk pathways. At least in principle, these subversive activities could promote the adaptive fitness of the entire periodontal biofilm community. In this regard, the virulence factors responsible for complement and TLR exploitation (gingipain enzymes, atypical lipopolysaccharide molecules, and fimbriae) are released as components of readily diffusible membrane vesicles, which can thus become available to other biofilm organisms. This review summarizes important immune subversive tactics of P. gingivalis which might enable it to exert a supportive impact on the oral microbial community.

Differential virulence and innate immune interactions of Type I and II fimbrial genotypes of Porphyromonas gingivalis

INTRODUCTION

The fimA-encoded fimbriae of the periodontal pathogen Porphyromonas gingivalis display genetic diversity. Type I fimbriated P. gingivalis (Pg-I) has been most widely studied at the molecular level, whereas Pg-II is the most frequent isolate from severe periodontitis.

METHODS

To investigate virulence differences between Types I and II fimbriae, we examined strains 33277 (Pg-I) and OMZ314 (Pg-II), reciprocal swap mutants (i.e. expressing the heterologous fimbrial type), and their respective FimA-deficient derivatives. These organisms were tested in a mouse periodontitis model and in interactions with mouse macrophages, a cell type that plays important roles in chronic infections.

RESULTS

Strain 33277 induced significantly more periodontal bone loss than OMZ314 and substitution of Type II fimbriae with Type I in OMZ314 resulted in a more virulent strain than the parent organism. However, the presence of Type II fimbriae was associated with increased proinflammatory and invasive activities in macrophages.

CONCLUSION

The inverse relationship between proinflammatory potential and ability to cause experimental periodontitis may suggest that an aggressive phenotype could provoke a host response that would compromise the persistence of the pathogen.

Host adhesive activities and virulence of novel fimbrial proteins of Porphyromonas gingivalis

The fimbriae of Porphyromonas gingivalis mediate critical roles in host colonization and evasion of innate defenses and comprise polymerized fimbrilin (FimA) associated with quantitatively minor accessory proteins (FimCDE) of unknown function. We now show that P. gingivalis fimbriae lacking FimCDE fail to interact with the CXC-chemokine receptor 4 (CXCR4), and bacteria expressing FimCDE-deficient fimbriae cannot exploit CXCR4 in vivo for promoting their persistence, as the wild-type organism does. Consistent with these loss-of-function experiments, purified FimC and FimD (but not FimE) were shown to interact with CXCR4. However, significantly stronger binding was observed when a combination of all three proteins was allowed to interact with CXCR4. In addition, FimC and FimD bound to fibronectin and type 1 collagen, whereas FimE failed to interact with these matrix proteins. These data and the fact that FimE is required for the association of FimCDE with P. gingivalis fimbriae suggest that FimE may recruit FimC and FimD into a functional complex, rather than directly binding host proteins. Consistent with this notion, FimE was shown to bind both FimC and FimD. In summary, the FimCDE components cooperate and impart critical adhesive and virulence properties to P. gingivalis fimbriae.

Lipid raft-dependent uptake, signalling and intracellular fate of Porphyromonas gingivalis in mouse macrophages

Lipid rafts are cholesterol-enriched microdomains involved in cellular trafficking and implicated as portals for certain pathogens. We sought to determine whether the oral pathogen Porphyromonas gingivalis enters macrophages via lipid rafts, and if so, to examine the impact of raft entry on its intracellular fate. Using J774A.1 mouse macrophages, we found that P. gingivalis colocalizes with lipid rafts in a cholesterol-dependent way. Depletion of cellular cholesterol using methyl-beta-cyclodextrin resulted in about 50% inhibition of P. gingivalis uptake, although this effect was reversed by cholesterol reconstitution. The intracellular survival of P. gingivalis was dramatically inhibited in cholesterol-depleted cells relative to untreated or cholesterol-reconstituted cells, even when infections were adjusted to allow equilibration of the initial intracellular bacterial load. P. gingivalis thus appeared to exploit raft-mediated uptake for promoting its survival. Consistent with this, lipid raft disruption enhanced the colocalization of internalized P. gingivalis with lysosomes. In contrast, raft disruption did not affect the expression of host receptors interacting with P. gingivalis, although it significantly inhibited signal transduction. In summary, P. gingivalis uses macrophage lipid rafts as signalling and entry platforms, which determine its intracellular fate to the pathogen's own advantage.

Fimbrial proteins of porphyromonas gingivalis mediate in vivo virulence and exploit TLR2 and complement receptor 3 to persist in macrophages

Porphyromonas gingivalis is an oral/systemic pathogen implicated in chronic conditions, although the mechanism(s) whereby it resists immune defenses and persists in the host is poorly understood. The virulence of this pathogen partially depends upon expression of fimbriae comprising polymerized fimbrillin (FimA) associated with quantitatively minor proteins (FimCDE). In this study, we show that isogenic mutants lacking FimCDE are dramatically less persistent and virulent in a mouse periodontitis model and express shorter fimbriae than the wild type. Strikingly, native fimbriae allowed P. gingivalis to exploit the TLR2/complement receptor 3 pathway for intracellular entry, inhibition of IL-12p70, and persistence in macrophages. This virulence mechanism also required FimCDE; indeed, mutant strains exhibited significantly reduced ability to inhibit IL-12p70, invade, and persist intracellularly, attributable to failure to interact with complement receptor 3, although not with TLR2. These results highlight a hitherto unknown mechanism of immune evasion by P. gingivalis that is surprisingly dependent upon minor constituents of its fimbriae, and support the concept that pathogens evolved to manipulate innate immunity for promoting adaptive fitness and thus their capacity to cause disease.