Characterization of a novel quorum sensing system in Porphyromonas gingivalis

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Somayajula, Deepti
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Oral health is closely connected with the overall systemic health. Oral biofilms are dynamic and present a very specific bacterial community which affects oral health. Oral biofilms are directly associated with the major oral infections including caries and periodontal disease. Understanding the oral microbial community is important for preventing oral disease and maintaining oral and overall health. In the first part of this study, I have explored the pathogenic load for three microorganisms Porphyromonas gingivalis, Tannerella forsythia and Aggregatibacter actinomycetemcomitans in companion cats and dogs. Results showed detectable levels of P. gingivalis and T. forsythia in both healthy and diseased cats and surprisingly the number was minimal in healthy dogs. A. actinomycetemcomitans was not detected in any healthy or diseased samples as this bacterium is found only in aggressive stages of Periodontitis. Quorum sensing (QS) system is a cell-cell signaling system that enables bacteria to interact with their neighboring cells and facilitates coordinated bacterial behavior in response to environmental changes. Gram-negative bacteria contain several quorum sensing systems in which different molecules are used as signals, including acyl-homoserine lactones (AHLs), cyclic dipeptides, autoinducers 2 and 3. The LuxI/LuxR-type quorum sensing system consists of a LuxI-like autoinducer synthase that produces AHLs as signals which are detected by a LuxR-type receptor upon exceeding a threshold concentration. The AHLs are detected through binding or activating cytoplasmic receptor proteins, which then dimerize to regulate transcription of downstream genes. P. gingivalis which demonstrated the strongest association with periodontal disease in our study of cats and dogs. P. gingivalis does not produce AHL but possesses many LuxR like homologs on its chromosome. One of these LuxR homologs is PGN_1373 which has been shown to affect the interaction between Streptococcus gordonii and P. gingivalis. Dialkyresorcinols encoded by dar gene has been previously demonstrated to bind to another luxR homolog PauR in Photorabdus asymbiotica. Porphyromonas gingivalis encodes for DarB which synthesizes Cyclohexanediones (CHDs). We hypothesized that CHD may bind to PGN_1373 and regulate virulence. Hence, I investigated the presence of a novel quorum-sensing molecule CHD synthesized by DarB and its effect on virulence factors and phenotype of Porphyromonas gingivalis. The High-Performance liquid chromatography results show that P. gingivalis secretes molecules like commercially available 1,3 cyclohexanediones. The addition of CHD extracellularly influenced expressed bacterial virulence genes at 100 µM and biofilm formation at 10 µM. The new DarB/PGN_1373 quorum sensing circuit may add to the existing pool of mechanisms regarding intra and interspecies communication in Porphyromonas gingivalis.
Porphyromonas gingivalis, Cyclohexanediones, Quorum sensing, PGN_1373