Proteomics of Aggregatibacter actinomycetemcomitans Outer Membrane Vesicles

Aggregatibacter actinomycetemcomitans is an oral and systemic pathogen associated with aggressive forms of periodontitis and with endocarditis. Outer membrane vesicles (OMVs) released by this species have been demonstrated to deliver effector proteins such as cytolethal distending toxin (CDT) and leukotoxin (LtxA) into human host cells and to act as triggers of innate immunity upon carriage of NOD1- and NOD2-active pathogen-associated molecular patterns (PAMPs). To improve our understanding of the pathogenicity-associated functions that A. actinomycetemcomitans exports via OMVs, we studied the proteome of density gradient-purified OMVs from a rough-colony type clinical isolate, strain 173 (serotype e) using liquid chromatography-tandem mass spectrometry (LC-MS/MS). This analysis yielded the identification of 151 proteins, which were found in at least three out of four independent experiments. Data are available via ProteomeXchange with identifier PXD002509. Through this study, we not only confirmed the vesicle-associated release of LtxA, and the presence of proteins, which are known to act as immunoreactive antigens in the human host, but we also identified numerous additional putative virulence-related proteins in the A. actinomycetemcomitans OMV proteome. The known and putative functions of these proteins include immune evasion, drug targeting, and iron/nutrient acquisition. In summary, our findings are consistent with an OMV-associated proteome that exhibits several offensive and defensive functions, and they provide a comprehensive basis to further disclose roles of A. actinomycetemcomitans OMVs in periodontal and systemic disease.

Proteomics of protein secretion by Aggregatibacter actinomycetemcomitans

The extracellular proteome (secretome) of periodontitis-associated bacteria may constitute a major link between periodontitis and systemic diseases. To obtain an overview of the virulence potential of Aggregatibacter actinomycetemcomitans, an oral and systemic human pathogen implicated in aggressive periodontitis, we used a combined LC-MS/MS and bioinformatics approach to characterize the secretome and protein secretion pathways of the rough-colony serotype a strain D7S. LC-MS/MS revealed 179 proteins secreted during biofilm growth. Further to confirming the release of established virulence factors (e.g. cytolethal distending toxin [CDT], and leukotoxin [LtxA]), we identified additional putative virulence determinants in the secretome. These included DegQ, fHbp, LppC, Macrophage infectivity protein (MIP), NlpB, Pcp, PotD, TolB, and TolC. This finding indicates that the number of extracellular virulence-related proteins is much larger than previously demonstrated, which was also supported by in silico analysis of the strain D7S genome. Moreover, our LC-MS/MS and in silico data revealed that at least Type I, II, and V secretion are actively used to excrete proteins directly into the extracellular space, or via two-step pathways involving the Sec/Tat systems for transport across the inner membrane, and outer membrane factors, secretins and auto-transporters, respectively for delivery across the outer membrane. Taken together, our results provide a molecular basis for further elucidating the role of A. actinomycetemcomitans in periodontal and systemic diseases.

Shifts in the microbial population in relation to in situ caries progression

The shift in microbial diversity from young to mature plaque, related to caries activity on sound and restored surfaces, was studied using denaturing gradient gel electrophoresis. During a 20-week in situ study on caries progression 8 subjects wearing restored and unrestored dentin and enamel sections, biofilm was sampled after 1 and 20 weeks (young or mature plaque). A higher microbial diversity (mature plaque) was seen in caries-active compared to caries-free subjects. Rothia dentocariosa and Scardovia inopinata were absent from all caries-free sites, but appeared in 50% of the caries-active sites.

Subgingival biofilm structure

Periodontitis is an inflammatory disease of the oral cavity initiated by a microbial biofilm (or 'dental plaque'). Subgingival biofilms in periodontal pockets are not easily analyzed without the loss of structural integrity. These subgingival plaques are structured communities of microorganisms with great phylogenetic diversity embedded in a self-produced extracellular polymeric matrix. For almost three decades, knowledge of the structure of plaque located below the gingival margin has been limited to landmark studies from the 1970s that were unaware of the breadth of microbial diversity we appreciate now. Only recently has technical progress - combining histology, confocal scanning fluorescent microscopy and fluorescent in situ hybridization to localize the most abundant species from different phyla and species associated with periodontitis - provided new insights into the architecture of subgingival biofilms. This review focuses on the structure and composition of subgingival biofilms and discusses current knowledge on the nature of the extracellular matrix. We describe further structural aspects of 'subgingival' biofilms produced in vitro that are gaining considerable interest as we search for models to investigate biofilm development, resistance to antibiotics, extracellular polymeric matrix composition and function, and reciprocal host-cell-to-biofilm interactions.

Phylogenetic group- and species-specific oligonucleotide probes for single-cell detection of lactic acid bacteria in oral biofilms

BACKGROUND

The purpose of this study was to design and evaluate fluorescent in situ hybridization (FISH) probes for the single-cell detection and enumeration of lactic acid bacteria, in particular organisms belonging to the major phylogenetic groups and species of oral lactobacilli and to Abiotrophia/Granulicatella.

RESULTS

As lactobacilli are known for notorious resistance to probe penetration, probe-specific assay protocols were experimentally developed to provide maximum cell wall permeability, probe accessibility, hybridization stringency, and fluorescence intensity. The new assays were then applied in a pilot study to three biofilm samples harvested from variably demineralized bovine enamel discs that had been carried in situ for 10 days by different volunteers. Best probe penetration and fluorescent labeling of reference strains were obtained after combined lysozyme and achromopeptidase treatment followed by exposure to lipase. Hybridization stringency had to be established strictly for each probe. Thereafter all probes showed the expected specificity with reference strains and labeled the anticipated morphotypes in dental plaques. Applied to in situ grown biofilms the set of probes detected only Lactobacillus fermentum and bacteria of the Lactobacillus casei group. The most cariogenic biofilm contained two orders of magnitude higher L. fermentum cell numbers than the other biofilms. Abiotrophia/Granulicatella and streptococci from the mitis group were found in all samples at high levels, whereas Streptococcus mutans was detected in only one sample in very low numbers.

CONCLUSIONS

Application of these new group- and species-specific FISH probes to oral biofilm-forming lactic acid bacteria will allow a clearer understanding of the supragingival biome, its spatial architecture and of structure-function relationships implicated during plaque homeostasis and caries development. The probes should prove of value far beyond the field of oral microbiology, as many of them detect non-oral species and phylogenetic groups of importance in a variety of medical conditions and the food industry.

[Strength in numbers: illness-causing biofilms]

Microbial infections constitute an important reason for medical treatment, especially in dentistry. Ideas about the role played by bacteria in the development of diseases have changed steadily during the past centuries. At the present time much attention is being devoted to the role of biofilms due to their illness-causing influence with respect to stitches on hard tissue and on oral implants, knee-, hip-, and voice prostheses. In biofilms, bacteria function and communicate in organized extrapolymeric structures, attached to a surface. Biofilms provide bacteria with protection against the host's immune system and antibacterial chemicals. By gaining insights into the creation and functioning of biofilms, it will be possible to develop better antibacterial therapies.

Denaturing gradient gel electrophoresis as a diagnostic tool in periodontal microbiology

Bacteria play an important role in the initiation and progression of periodontal diseases and are part of a biofilm, which can contain over 100 different species. The aim of the present study was to show the potential of denaturing gradient gel electrophoresis (DGGE) as a tool for the detection of clinically relevant species and to compare the results of detection by DGGE with those by PCR and culturing. Hybridization of the bands from the DGGE profiles with species-specific probes was developed to confirm the band positions in the marker obtained with reference strains. The sensitivities of DGGE compared to those of cultivation for the detection of Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, Prevotella intermedia, and Tannerella forsythensis were 100, 100, 88, and 100%, respectively; and the sensitivities of DGGE compared to those of PCR were 100, 90, 88, and 96%, respectively. DGGE as a diagnostic tool could easily be extended to other species, as shown for Treponema denticola, which could be detected in 48% of the samples. Three different groups of A. actinomycetemcomitans serotypes could be distinguished by DGGE (i.e., a group comprising serotypes a, d, e, and f; a group comprising serotype b; and a group comprising serotype c). Amplicons from P. gingivalis and T. denticola migrated to the same position in the gel, and P. intermedia produced multiple bands. In the present study we show that the DGGE profiles represent clinically relevant species which can be detected by hybridization with species-specific probes. With DGGE, large numbers of samples can be analyzed for different species simultaneously, and DGGE may be a good alternative in periodontal microbial diagnostics.

Denaturing gradient gel electrophoresis analysis to study bacterial community structure in pockets of periodontitis patients

Bacteria are involved in the onset and progression of periodontitis. A promising molecular technique, denaturing gradient gel electrophoresis (DGGE), to study microbial population dynamics in the subgingival pocket is presented. Twenty-three samples were taken from the subgingival pockets of nine patients and six healthy family members. From four periodontitis patients, 12 samples were evaluated before, 1 day after and 3 months after treatment. Part of the 16S rRNA gene of all bacteria was amplified by PCR and separated by DGGE, creating banding patterns representative of the community structure. Shifts in composition and diversity of the microbial population could be determined semiquantitatively, and this showed that treatment resulted in a decrease in the diversity of the population. After 3 months a microbial population 33-47% different from the population before treatment had re-established. Intense bands representing Exiguobacterium aurantiacum were present in 13 out of 25 samples, indicating that this species may play a role in periodontal disease.