Survival of an Aggregatibacter actinomycetemcomitans quorum sensing luxS mutant in the mouths of Rhesus monkeys: insights into ecological adaptation

Oral microbial interactions from an ecological perspective: a narrative review

Landscape ecology is a relatively new field of study within the sub-specialty of ecology that considers time and space in addition to structure and function. Landscape ecology contends that both the configuration (spatial pattern) and the composition (organisms both at the macro and or micro level) of an ecology can change over time. The oral cavity is an ideal place to study landscape ecology because of the variety of landscapes, the dynamic nature of plaque biofilm development, and the easy access to biofilm material. This review is intended to provide some specific clinical examples of how landscape ecology can influence the understanding of oral diseases and act as a supplement to diagnosis and treatment. The purpose of this review is two-fold; (1) to illustrate how landscape ecology can be used to clarify the two most prominent microbiologically induced infections in the oral cavity, and (2) how studies of oral microbiology can be used to enhance the understanding of landscape ecology. The review will distinguish between "habitat" and "niche" in a landscape and extend the concept that a "patch", is the demarcating unit of a habitat within a landscape. The review will describe how; (1) an individual patch, defined by its shape, edges and internal components can have an influence on species within the patch, (2) spatial dynamics over time within a patch can lead to variations or diversities of species within that patch space, and (3) an unwelcoming environment can promote species extinction or departure/dispersion into a more favorable habitat. Understanding this dynamic in relationship to caries and periodontal disease is the focus of this review.

Strategies toward development of antimicrobial biomaterials for dental healthcare applications

Several approaches for elimination of oral pathogens are being explored at the present time since oral diseases remain prevalent affecting approximately 3.5 billion people worldwide. Need for antimicrobial biomaterials in dental healthcare include but is not restricted to designing resin composites and adhesives for prevention of dental caries. Constant efforts are also being made to develop antimicrobial strategies for clearance of endodontic space prior root canal treatment and for treatment of periimplantitis and periodontitis. This article discusses various conventional and nanotechnology-based strategies to achieve antimicrobial efficacy in dental biomaterials. Recent developments in the design and synthesis of antimicrobial peptides and antifouling zwitterionic polymers to effectively lessen the risks of antimicrobial drug resistance are also outlined in this review. Further, the role of contemporary strategies such as use of smart biomaterials, ionic solvent-based biomaterials and quorum quenchers incorporated biomaterials in the elimination of dental pathogens are described in detail. Lastly, we mentioned the approach of using polymers to print custom-made three-dimensional antibacterial dental products via additive manufacturing technologies. This review provides a critical perspective on the chemical, biomimetic, and engineering strategies intended for developing antimicrobial biomaterials that have the potential to substantially improve the dental health.

Expression of virulence factors under different environmental conditions in Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is a facultative anaerobic Gram-negative bacterium associated with periodontal diseases, especially aggressive periodontitis. The virulence factors of this pathogen, including adhesins, exotoxins, and endotoxin, have been extensively studied. However, little is known about their gene expression mode in the host. Herein, we investigated whether culture conditions reflecting in vivo environments, including serum and saliva, alter expression levels of virulence genes in the strain HK1651, a JP2 clone. Under aerobic conditions, addition of calf serum (CS) into a general medium induced high expression of two outer membrane proteins (omp100 and omp64). The high expression of omp100 and omp64 was also induced by an iron-limited medium. RNA-seq analysis showed that the gene expressions of several factors involved in iron acquisition were increased in the CS-containing medium. When HK1651 was grown on agar plates, genes encoding many virulence factors, including the Omps, cytolethal distending toxin, and leukotoxin, were differentially expressed. Then, we investigated their expression in five other A. actinomycetemcomitans strains grown in general and CS-containing media. The expression pattern of virulence factors varied among strains. Compared with the other five strains, HK1561 showed high expression of omp29 regardless of the CS addition, while the gene expression of leukotoxin in HK1651 was higher only in the medium without CS. HK1651 showed reduced biofilm in both CS- and saliva-containing media. Coaggregation with Fusobacterium nucleatum was remarkably enhanced using HK1651 grown in the CS-containing medium. Our results indicate that the expression of virulence factors is altered by adaptation to different conditions during infection.

LuxS/AI-2 Quorum Sensing System in Edwardsiella piscicida Promotes Biofilm Formation and Pathogenicity

LuxS/AI-2 is an important quorum sensing system which affects the growth, biofilm formation, virulence, and metabolism of bacteria. LuxS is encoded by the luxS gene, but how this gene is associated with a diverse array of physiological activities in Edwardsiella piscicida (E. piscicida) is not known. Here, we constructed an luxS gene mutant strain, the △luxS strain, to identify how LuxS/AI-2 affects pathogenicity. The results showed that LuxS was not found in the luxS gene mutant strain, and this gene deletion decreased E. piscicida growth compared to that of the wild-type strain. Meanwhile, the wild-type strain significantly increased penetration and motility in mucin compared to levels with the △luxS strain. The 50% lethal dose (LD₅₀) of the E. piscicida △luxS strain for zebrafish was significantly higher than that of the wild-type strain, which suggested that the luxS gene deletion could attenuate the strain's virulence. The AI-2 activities of EIB202 were 56-fold higher than those in the △luxS strain, suggesting that the luxS gene promotes AI-2 production. Transcriptome results demonstrated that between cells infected with the △luxS strain and those infected with the wild-type strain 46 genes were significantly differentially regulated, which included 34 upregulated genes and 12 downregulated genes. Among these genes, the largest number were closely related to cell immunity and signaling systems. In addition, the biofilm formation ability of EIB202 was significantly higher than that of the △luxS strain. The supernatant of EIB202 increased the biofilm formation ability of the △luxS strain, which suggested that the luxS gene and its product LuxS enhanced biofilm formation in E. piscicida All results indicate that the LuxS/AI-2 quorum sensing system in E. piscicida promotes its pathogenicity through increasing a diverse array of physiological activities.

Challenges and Limitations of Anti-quorum Sensing Therapies

Quorum sensing (QS) is a mechanism allowing microorganisms to sense population density and synchronously control genes expression. It has been shown that QS supervises the activity of many processes important for microbial pathogenicity, e.g., sporulation, biofilm formation, and secretion of enzymes or membrane vesicles. This contributed to the concept of anti-QS therapy [also called quorum quenching (QQ)] and the opportunity of its application in fighting against various types of pathogens. In recent years, many published articles reported promising results indicating the possibility of reducing pathogenicity of tested microorganisms and their easier eradication when co-treated with antibiotics. The aim of the present article is to point to the opposite, negative side of the QQ therapy, with particular emphasis on three fundamental properties attributed to anti-QS substances: the selectivity, virulence reduction, and lack of resistance against QQ. This point of view may highlight new directions of research, which should be taken into account in the future before the widespread introduction of QQ therapies in the treatment of people.

Differential Cell Lysis Among Periodontal Strains of JP2 and Non-JP2 Genotype of Aggregatibacter actinomycetemcomitans Serotype B Is Not Reflected in Dissimilar Expression and Production of Leukotoxin

Leukotoxic potential of Aggregatibacter actinomycetemcomitans strains has been studied by the use of several methods, and results differ depending on the methods used. The aim of the present study was to perform a comprehensive examination of the leukotoxic potential of a collection of A. actinomycetemcomitans strains by use of three quantitative methods, Western blotting, ELISA, and mRNA expression assay and compare these results with previous data obtained by a cell lysis assay. A higher leukotoxic potential among JP2 genotype strains compared to non-JP2 genotype strains of A. actinomycetemcomitans was found by Western blotting, ELISA and mRNA expression assay. Leukotoxicity as determined by cell lysis assay showed a variation among strains examined, not only depending on being part of JP2 genotype vs. non-JP2 genotype group of A. actinomycetemcomitans. The leukotoxicity of A. actinomycetemcomitans strains as determined by cell lysis assay did not correspond to the leukotoxic potential of A. actinomycetemcomitans strains as determined by three quantitative methods. A comparison of the results obtained by ELISA and mRNA expression assay showed a reasonable correlation between these two methods. It seems important to use more than one method to assess the LtxA-related virulence capacity of A. actinomycetemcomitans in order to obtain comprehensive understanding of the leukotoxic potential of A. actinomycetemcomitans strains.

Aggregatibacter actinomycetemcomitans colonization and persistence in a primate model

Aggregatibacter actinomycetemcomitans is associated with aggressive periodontitis resulting in premature tooth loss in adolescents. Tooth adherence and biofilm persistence are prerequisites for survival in the oral domain. Here, using a rhesus monkey model, 16S rRNA sequencing, and weighted network analysis, we assessed colonization of A. actinomycetemcomitans variants and ascertained microbial interactions in biofilm communities. Variants in A. actinomycetemcomitans leukotoxin (ltx) were created, labeled, inoculated, and compared with their progenitor strain for in vivo colonization. Samples of tooth-related plaque were assessed for colonization at baseline and after debridement and inoculation of labeled strains. Null, minimal, and hyper-Ltx-producing strains were created and assessed for hydroxyapatite binding and biofilm formation in vitro. Ltx-hyperproducing strains colonized with greater prevalence and at higher levels than wild type or ltx mutants (P = 0.05). Indigenous and inoculated A. actinomycetemcomitans strains that attached were associated with lactate-producing species (i.e., Leptotrichia, Abiotrophia, and Streptoccocci). A. actinomycetemcomitans was found at 0.13% of the total flora at baseline and at 0.05% 4 wk after inoculation. In vivo data were supported by in vitro results. We conclude that hyper-Ltx production affords these strains with an attachment advantage providing a foothold for competition with members of the indigenous microbiota. Increased attachment can be linked to ltx gene expression and up-regulation of adherence-associated genes. Growth of attached A. actinomycetemcomitans in vivo was enhanced by lactate availability due to consorting species. These associations provide A. actinomycetemcomitans with the constituents required for its colonization and survival in the complex and competitive oral environment.

Interference With Quorum-Sensing Signal Biosynthesis as a Promising Therapeutic Strategy Against Multidrug-Resistant Pathogens

Faced with the global health threat of increasing resistance to antibiotics, researchers are exploring interventions that target bacterial virulence factors. Quorum sensing is a particularly attractive target because several bacterial virulence factors are controlled by this mechanism. Furthermore, attacking the quorum-sensing signaling network is less likely to select for resistant strains than using conventional antibiotics. Strategies that focus on the inhibition of quorum-sensing signal production are especially attractive because the enzymes involved are expressed in bacterial cells but are not present in their mammalian counterparts. We review here various approaches that are being taken to interfere with quorum-sensing signal production via the inhibition of autoinducer-2 synthesis, PQS synthesis, peptide autoinducer synthesis, and N-acyl-homoserine lactone synthesis. We expect these approaches will lead to the discovery of new quorum-sensing inhibitors that can help to stem the tide of antibiotic resistance.