Transcriptional and translational analysis of biofilm determinants of Aggregatibacter actinomycetemcomitans in response to environmental perturbation

Transcriptomic Analysis of Aggregatibacter actinomycetemcomitans Core and Accessory Genes in Different Growth Conditions

Aggregatibacter actinomycetemcomitans genome can be divided into an accessory gene pool (found in some but not all strains) and a core gene pool (found in all strains). The functions of the accessory genes (genomic islands and non-island accessory genes) are largely unknown. We hypothesize that accessory genes confer critical functions for A. actinomycetemcomitans in vivo. This study examined the expression patterns of accessory and core genes of A. actinomycetemcomitans in distinct growth conditions. We found similar expression patterns of island and non-island accessory genes, which were generally lower than the core genes in all growth conditions. The median expression levels of genomic islands were 29%–37% of the core genes in enriched medium but elevated to as high as 63% of the core genes in nutrient-limited media. Several putative virulence genes, including the cytolethal distending toxin operon, were found to be activated in nutrient-limited conditions. In conclusion, genomic islands and non-island accessory genes exhibited distinct patterns of expression from the core genes and may play a role in the survival of A. actinomycetemcomitans in nutrient-limited environments.

A single-step transconjugation system for gene deletion in Aggregatibacter actinomycetemcomitans

Aggregatibacter (A.) actinomycetemcomitans is a periodontopathogenic bacterium causing aggressive periodontitis. Here we describe a single-step transconjugation system as novel and easily applicable protocol for site-specific genetic manipulation of A. actinomycetemcomitans. Deletion of PgaC, which is involved in the synthesis of biofilm matrix, led to a reduced biofilm formation.

Influence of Aae Autotransporter Protein on Adhesion and Biofilm Formation by Aggregatibacter actinomycetemcomitans

The periodontopathogen Aggregatibacter actinomycetemcomitans colonizes oral cavity by binding to and invading epithelial cells as well as by participating in biofilms formed on hard surfaces. Aae, an autotransporter protein, is implicated in bacterial adhesion to epithelial cells. Due to the multiple functions of bacterial autotransporter proteins, this study aimed to evaluate the role of aae in A. actinomycetemcomitans ability to adhere to both saliva-coated hydroxyapatite (SHA) and biofilm. An aae null mutant was constructed. Its hydrophobic properties as well as its ability to adhere to epithelial cells, SHA and to form biofilm were evaluated and compared with the parental strain, A. actinomycetemcomitans VT1169. The aae null mutant showed reduced hydrophobicity, as well as decreased binding to SHA and biofilm formation compared to the parental strain. These data suggest that aae mediates A. actinomycetemcomitans adhesion to epithelial cells and may be involved in biofilm formation and interaction with adsorbed salivary proteins.

Mature Biofilm Degradation by Potential Probiotics: Aggregatibacter actinomycetemcomitans versus Lactobacillus spp

The biofilm degradation of Aggregatibacter actinomycetemcomitans is essential as a complete periodontal disease therapy, and here we show the effects of potential probiotic bacteria such as Lactobacillus spp. for the biofilm of several serotypes of A. actinomycetemcomitans strains. Eight of the 13 species showed the competent biofilm degradation of ≥ 90% reduction in biofilm values in A. actinomycetemcomitans Y4 (serotype b) as well as four of the seven species for the biofilm of A. actinomycetemcomitans OMZ 534 (serotype e). In contrast, the probiotic bacteria did not have a big impact for the degradation of A. actinomycetemcomitans SUNY 75 (serotype a) biofilm. The dispersed A. actinomycetemcomitans Y4 cells through the biofilm detachment were still viable and plausible factors for the biofilm degradation were not due to the lactic acid and low pH conditions. The three enzymes, protease, lipase, and amylase may be responsible for the biofilm degradation; in particular, lipase was the most effective enzyme for the biofilm degradation of A. actinomycetemcomitans Y4 along with the protease activity which should be also important for the other serotypes. Remarkable lipase enzyme activities were detected from some of the potential probiotics and a supporting result using a lipase inhibitor presented corroborating evidence that lipase activity is one of the contributing factors for biofilm degradation outside of the protease which is also another possible factor for the biofilm of the other serotype of A. actinomycetemcomitans strains. On the other hand, the biofilm of A. actinomycetemcomitans SUNY 75 (serotype a) was not powerfully degraded by the lipase enzyme because the lipase inhibitor was slightly functional for only two of potential probiotics.

Regulation of GacA in Pseudomonas chlororaphis Strains Shows a Niche Specificity

The GacS/GacA two-component system plays a central role in the regulation of a broad range of biological functions in many bacteria. In the biocontrol organism Pseudomonas chlororaphis, the Gac system has been shown to positively control quorum sensing, biofilm formation, and phenazine production, but has an overall negative impact on motility. These studies have been performed with strains originated from the rhizosphere predominantly. To investigate the level of conservation between the GacA regulation of biocontrol-related traits in P. chlororaphis isolates from different habitats, the studies presented here focused on the endophytic isolate G5 of P. chlororaphis subsp. aurantiaca. A gacA mutant deficient in the production of N-acylhomoserine lactones (AHLs) and phenazine was isolated through transposon mutagenesis. Further phenotypic characterization revealed that in strain G5, similar to other P. chlororaphis strains, a gacA mutation caused inability to produce biocontrol factors such as phenazine, HCN and proteases responsible for antifungal activity, but overproduced siderophores. LC-MS/MS analysis revealed that AHL production was also practically abolished in this mutant. However, the wild type exhibited an extremely diverse AHL pattern which has never been identified in P. chlororaphis. In contrast to other isolates of this organism, GacA in strain G5 was shown to negatively regulate biofilm formation and oxidative stress response whilst positively regulating cell motility and biosynthesis of indole-3-acetic acid (IAA). To gain a better understanding of the overall impact of GacA in G5, a comparative proteomic analysis was performed revealing that, in addition to some of the traits like phenazine mentioned above, GacA also negatively regulated lipopolysaccharide (LPS) and trehalose biosynthesis whilst having a positive impact on energy metabolism, an effect not previously described in P. chlororaphis. Consequently, GacA regulation shows a differential strain dependency which is likely to be in line with their niche of origin.

RNA regulators of host immunity and pathogen adaptive responses in the oral cavity

The recent explosion of RNA-seq studies has resulted in a newfound appreciation for the importance of riboregulatory RNAs in the posttranscriptional control of eukaryotic and prokaryotic genetic networks. The current review will explore the role of trans-riboregulatory RNAs in various adaptive responses of host and pathogen in the oral cavity.

Posttranscriptional regulation of oral bacterial adaptive responses

Within the past 10 years, it has become increasingly evident that posttranscriptional regulation is among the most important mechanisms used by bacteria to modulate gene expression in response to environmental perturbations. Posttranscriptional mechanisms provide a much faster response and lower energy burden compared to most transcription regulatory pathways and they have the unique advantage that they can override existing transcriptional responses once the environment changes. Because of this, virulence factor gene expression is particularly suited for posttranscriptional control, and not surprisingly, an abundance of recent evidence indicates that posttranscriptional regulators are the predominant virulence regulators of human pathogens. Typically, this involves global riboregulators that primarily serve as modulators of virulence gene translation initiation and/or mRNA stability. Surprisingly little has been reported about posttranscriptional regulatory pathways in oral bacteria, but recent results suggest that oral species are equally dependent upon posttranscriptional control of their adaptive genetic responses. In this report, we discuss the major themes in RNA-based regulation of gene expression and review the available literature related to the most commonly studied oral bacterial species.

Environmental stimuli shape biofilm formation and the virulence of periodontal pathogens

Periodontitis is a common inflammatory disease affecting the tooth-supporting structures. It is initiated by bacteria growing as a biofilm at the gingival margin, and communication of the biofilms differs in health and disease. The bacterial composition of periodontitis-associated biofilms has been well documented and is under continual investigation. However, the roles of several host response and inflammation driven environmental stimuli on biofilm formation is not well understood. This review article addresses the effects of environmental factors such as pH, temperature, cytokines, hormones, and oxidative stress on periodontal biofilm formation and bacterial virulence.

ygiW and qseBC are co-expressed in Aggregatibacter actinomycetemcomitans and regulate biofilm growth

The quorum-sensing Escherichia coli regulators B and C (QseBC) two-component system were previously shown to regulate biofilm growth of the oral pathogen Aggregatibacter actinomycetemcomitans and to be essential for virulence. In this study, we use RT-PCR to show that an open reading frame, ygiW, residing upstream of qseBC and encoding a hypothetical protein is co-expressed with qseBC. In addition, using a series of lacZ transcriptional fusion constructs and 5'-rapid amplification of cDNA Ends (RACE), the promoter that drives expression of the ygiW-qseBC operon and the transcriptional start site was mapped to the 372 bp intergenic region upstream from ygiW. No internal promoters drive qseBC expression independently from ygiW. However, qseBC expression is attenuated by approximately ninefold by a putative attenuator stem-loop (ΔG = -77.0 KJ/mol) that resides in the 137 bp intergenic region between ygiW and qseB. The QseB response regulator activates expression of the ygiW-qseBC operon and transcription from the ygiW promoter is drastically reduced in ΔqseB and ΔqseBC mutants of A. actinomycetemcomitans. In addition, transcriptional activity of the ygiW promoter is significantly reduced in a mutant expressing an in-frame deletion of qseC that lacks the sensor domain of QseC, suggesting that a periplasmic signal is required for QseB activation. Finally, a non-polar in-frame deletion in ygiW had little effect on biofilm depth but caused a significant increase in surface coverage relative to wild-type. Complementation of the mutant with a plasmid-borne copy of ygiW reduced surface coverage back to wild-type levels. Interestingly, deletion of the sensor domain of QseC or of the entire qseC open reading frame resulted in significant reductions in biofilm depth, biomass and surface coverage, indicating that the sensor domain is essential for optimal biofilm formation by A. actinomycetemcomitans. Thus, although ygiW and qseBC are co-expressed, they regulate biofilm growth by distinct mechanisms.

Insights into the virulence of oral biofilms: discoveries from proteomics

This review covers developments in the study of polymicrobial communities, biofilms and selected areas of host response relevant to dental plaque and related areas of oral biology. The emphasis is on recent studies in which proteomic methods, particularly those using mass spectrometry as a readout, have played a major role in the investigation. The last 5-10 years have seen a transition of such methods from the periphery of oral biology to the mainstream, as in other areas of biomedical science. For reasons of focus and space, the authors do not discuss biomarker studies relevant to improved diagnostics for oral health, as this literature is rather substantial in its own right and deserves a separate treatment. Here, global gene regulation studies of plaque-component organisms, biofilm formation, multispecies interactions and host-microbe interactions are discussed. Several aspects of proteomics methodology that are relevant to the studies of multispecies systems are commented upon.

Exposure of Salmonella enterica Serovar typhimurium to a protective monoclonal IgA triggers exopolysaccharide production via a diguanylate cyclase-dependent pathway

Sal4 is a monoclonal polymeric IgA antibody directed against the O antigen (O-Ag) of Salmonella enterica serovar Typhimurium (S. Typhimurium), which is sufficient to protect mice against intestinal infections from S. Typhimurium. We recently reported that the exposure of S. Typhimurium to Sal4 results in the immediate loss of flagellum-based motility, in alterations to the outer membrane (OM) integrity, and in the concomitant appearance of a mucoid phenotype that is reminiscent of cells in the earliest stages of biofilm formation. We demonstrate here that prolonged (>4 h) exposure of S. Typhimurium to Sal4 at 37 °C (but not at ambient temperature [25°C]) results in measurable exopolysaccharide (EPS) accumulation and biofilm formation on both borosilicate glass surfaces and polystyrene microtiter plates. The polysaccharide produced by S. Typhimurium in response to Sal4 contains cellulose, in addition to O-Ag capsule and colanic acid. EPS production was dependent on YeaJ, a proposed inner membrane-localized diguanylate cyclase (DGC) and a known regulator of cellulose biosynthesis. An S. Typhimurium ΔyeaJ strain was unable to produce cellulose or form a biofilm in response to Sal4. Conversely, the overexpression of yeaJ in S. Typhimurium enhanced Sal4-induced biofilm formation and resulted in increased intracellular levels of cyclic dimeric guanosine monophosphate (c-di-GMP) compared to that of a wild-type control; this strongly suggests that YeaJ is indeed a functional DGC. Based on these data, we speculate that Sal4, by virtue of its ability to associate with the O-Ag and to induce OM stress, renders S. Typhimurium avirulent by triggering a c-di-GMP-dependent signaling pathway via YeaJ that leads to the suppression of bacterial motility while simultaneously stimulating EPS production.

Bacterial stress responses as determinants of antimicrobial resistance

Bacteria encounter a myriad of stresses in their natural environments, including, for pathogens, their hosts. These stresses elicit a variety of specific and highly regulated adaptive responses that not only protect bacteria from the offending stress, but also manifest changes in the cell that impact innate antimicrobial susceptibility. Thus exposure to nutrient starvation/limitation (nutrient stress), reactive oxygen and nitrogen species (oxidative/nitrosative stress), membrane damage (envelope stress), elevated temperature (heat stress) and ribosome disruption (ribosomal stress) all impact bacterial susceptibility to a variety of antimicrobials through their initiation of stress responses that positively impact recruitment of resistance determinants or promote physiological changes that compromise antimicrobial activity. As de facto determinants of antimicrobial, even multidrug, resistance, stress responses may be worthy of consideration as therapeutic targets.

Novel iron-regulated and Fur-regulated small regulatory RNAs in Aggregatibacter actinomycetemcomitans

Iron can regulate biofilm formation via non-coding small RNA (sRNA). To determine if iron-regulated sRNAs are involved in biofilm formation by the periodontopathogen Aggregatibacter actinomycetemcomitans, total RNA was isolated from bacteria cultured with iron supplementation or chelation. Transcriptional analysis demonstrated that the expression of four sRNA molecules (JA01-JA04) identified by bioinformatics was significantly upregulated in iron-limited medium compared with iron-rich medium. A DNA fragment encoding each sRNA promoter was able to titrate Escherichia coli ferric uptake regulator (Fur) from a Fur-repressible reporter fusion in an iron uptake regulator titration assay. Cell lysates containing recombinant AaFur shifted the mobility of sRNA-specific DNAs in a gel shift assay. Potential targets of these sRNAs, determined in silico, included genes involved in biofilm formation. The A. actinomycetemcomitans overexpressing JA03 sRNA maintained a rough phenotype on agar, but no longer adhered to uncoated polystyrene or glass, although biofilm determinant gene expression was only modestly decreased. In summary, these sRNAs have the ability to modulate biofilm formation, but their functional target genes remain to be confirmed.

Stress responses as determinants of antimicrobial resistance in Gram-negative bacteria

Bacteria encounter a myriad of potentially growth-compromising conditions in nature and in hosts of pathogenic bacteria. These 'stresses' typically elicit protective and/or adaptive responses that serve to enhance bacterial survivability. Because they impact upon many of the same cellular components and processes that are targeted by antimicrobials, adaptive stress responses can influence antimicrobial susceptibility. In targeting and interfering with key cellular processes, antimicrobials themselves are 'stressors' to which protective stress responses have also evolved. Cellular responses to nutrient limitation (nutrient stress), oxidative and nitrosative stress, cell envelope damage (envelope stress), antimicrobial exposure and other growth-compromising stresses, have all been linked to the development of antimicrobial resistance in Gram-negative bacteria - resulting from the stimulation of protective changes to cell physiology, activation of resistance mechanisms, promotion of resistant lifestyles (biofilms), and induction of resistance mutations.

Lactoferrin and oral diseases: current status and perspective in periodontitis

Lactoferrin (Lf), an iron-binding glycoprotein able to chelate two ferric ions per molecule, is a component of human secretions synthesized by exocrine glands and neutrophils in infection/inflammation sites. Lactoferrin in saliva represents an important defence factor against bacterial injuries including those related to Streptococcus mutans and periodontopathic bacteria through its ability to decrease bacterial growth, biofilm development, iron overload, reactive oxygen formation and inflammatory processes.A growing body of research suggests that inflammatory periodontal disease involves a failure of resolution pathways to restore tissue homeostasis. There is an important distinction between anti-inflammation and resolution; anti-inflammation is pharmacologic intervention in inflammatory pathways, whereas resolution involves biologic pathways restoring inflammatory homeostasis. An appropriate regulation of pro-inflammatory cytokine synthesis might be useful in reducing periodontal tissue destruction. Recently, the multi-functional IL-6 is emerging as an important factor able to modulate bone, iron and inflammatory homeostasis.Here, we report an overview of Lf functions as well as for the first time Lf anti-inflammatory ability against periodontitis in in vitro model and observational clinical study. In in vitro model, represented by gingival fibroblasts infected with Prevotella intermedia, Lf exerted a potent anti-inflammatory activity. In the observational clinical trial performed through bovine Lf (bLf) topically administered to volunteers suffering from periodontitis, bLf decreased cytokines, including IL-6 in crevicular fluid, edema, bleeding, pocket depth, gingival and plaque index, thus improving clinical attachment levels.Even if other clinical trials are required, these results provide strong evidence for a instead of an therapeutic potential of this multifunctional natural protein.

Development of an animal model for Aggregatibacter actinomycetemcomitans biofilm-mediated oral osteolytic infection: a preliminary study

BACKGROUND

Biofilm-induced inflammatory osteolytic oral infections, such as periodontitis and peri-implantitis, have complex etiology and pathogenesis. A significant obstacle to research has been the lack of appropriate animal models where the inflammatory response to biofilms can be investigated. The aim of this study is to develop a novel animal model to study the host response to Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans)-biofilm colonizing titanium implants.

METHODS

Titanium implants were inoculated in vitro with A. actinomycetemcomitans, establishing a biofilm for 1 to 3 days. Biofilm-inoculated and control implants were transmucosally placed into rat hard palate or alveolar ridge. Analysis included documentation of clinical inflammation, polymerase chain reaction, and culture detection of A. actinomycetemcomitans and microcomputed tomography quantitation of peri-implant bone volume.

RESULTS

Viable A. actinomycetemcomitans biofilm was successfully established on titanium implants in vitro, detected by confocal laser scanning microscopy. An inflammatory response characterized by clinical inflammation, bleeding, ulceration, hyperplasia, and necrosis was observed around biofilm-inoculated implants. A. actinomycetemcomitans was detected by polymerase chain reaction and culture analysis on 100% of biofilm-inoculated implants for up to 3 weeks and 25% for up to 6 weeks. Microcomputed tomography analysis demonstrated significantly lower bone volume (P <0.05) around biofilm-inoculated implants (29.6% ± 7.6%) compared to non-inoculated implants (50.5% ± 9.6%) after 6 weeks.

CONCLUSIONS

These results describe a novel animal model where A. actinomycetemcomitans biofilm was established in vitro on titanium implants before placement in rat oral cavity, leading to an inflammatory response, osteolysis, and tissue destruction. This model may have potential use for investigation of host responses to biofilm pathogens and antibiofilm therapy.

Setup of an in vitro test system for basic studies on biofilm behavior of mixed-species cultures with dental and periodontal pathogens

Background

Caries and periodontitis are important human diseases associated with formation of multi-species biofilms. The involved bacteria are intensively studied to understand the molecular basis of the interactions in such biofilms. This study established a basic in vitro single and mixed-species culture model for oral bacteria combining three complimentary methods. The setup allows a rapid screening for effects in the mutual species interaction. Furthermore, it is easy to handle, inexpensive, and reproducible.

Methods

Streptococcus mitis, S. salivarius and S. sanguinis, typical inhabitants of the healthy oral cavity, S. mutans as main carriogenic species, and Porphyromonas gingivalis, Fusobacterium nucleatum, Parvimonas micra, S. intermedius and Aggregatibacter actinomycetemcomitans as periodontitis-associated bacteria, were investigated for their biofilm forming ability. Different liquid growth media were evaluated. Safranin-staining allowed monitoring of biofilm formation under the chosen conditions. Viable counts and microscopy permitted investigation of biofilm behavior in mixed-species and transwell setups.

Findings

S. mitis, F. nucleatum, P. gingivalis and P. micra failed to form biofilm structures. S. mutans, S. sanguinis, S. intermedius and S. salivarius established abundant biofilm masses in CDM/sucrose. A. actinomycetemcomitans formed patchy monolayers. For in depth analysis S. mitis, S. mutans and A. actinomycetemcomitans were chosen, because i) they are representatives of the physiological-, cariogenic and periodontitis-associated bacterial flora, respectively and ii) their difference in their biofilm forming ability. Microscopic analysis confirmed the results of safranin staining. Investigation of two species combinations of S. mitis with either S. mutans or A. actinomycetemcomitans revealed bacterial interactions influencing biofilm mass, biofilm structure and cell viability.

Conclusions

This setup shows safranin staining, microscopic analysis and viable counts together are crucial for basic examination and evaluation of biofilms. Our experiment generated meaningful results, exemplified by the noted S. mitis influence, and allows a fast decision about the most important bacterial interactions which should be investigated in depth.