| Summary: | <p>Abstract</p> <p>Background</p> <p>Periodontitis is a bacterial infection of the periodontal tissues. The Gram-negative anaerobic bacterium <it>Porphyromonas gingivalis </it>is considered a major causative agent. One of the virulence factors of <it>P. gingivalis </it>is capsular polysaccharide (CPS). Non-encapsulated strains have been shown to be less virulent in mouse models than encapsulated strains.</p> <p>Results</p> <p>To examine the role of the CPS in host-pathogen interactions we constructed an insertional isogenic <it>P. gingivalis </it>knockout in the epimerase-coding gene <it>epsC </it>that is located at the end of the CPS biosynthesis locus. This mutant was subsequently shown to be non-encapsulated. K1 capsule biosynthesis could be restored by <it>in trans </it>expression of an intact <it>epsC </it>gene. We used the <it>epsC </it>mutant, the W83 wild type strain and the complemented mutant to challenge human gingival fibroblasts to examine the immune response by quantification of <it>IL-1β</it>, <it>IL-6 </it>and <it>IL-8 </it>transcription levels. For each of the cytokines significantly higher expression levels were found when fibroblasts were challenged with the <it>epsC </it>mutant compared to those challenged with the W83 wild type, ranging from two times higher for IL-1β to five times higher for IL-8.</p> <p>Conclusions</p> <p>These experiments provide the first evidence that <it>P. gingivalis </it>CPS acts as an interface between the pathogen and the host that may reduce the host's pro-inflammatory immune response. The higher virulence of encapsulated strains may be caused by this phenomenon which enables the bacteria to evade the immune system.</p>
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