The capacity of Streptococci milleri to influence host cellular activity

Members of the Streptocooci milleri group (SMG) are regarded as commensals of mucocutaneous surfaces. However, SMG have also been described to contribute to the pathogenesis of oralfacial infections where, as initial colonisers, they prepare the site for subsequent colonisation by obligate anaerobes. An insight into the interplay between host cells and bacteria is important for understanding pathogenic infections. Objectives: To examine the effect of soluble factors derived from a range of SMG isolates on periodontal ligament fibroblasts and endothelial cells. Methods: Eight SMG strains, representing disease and commensal isolates from oral and extra-oral sites, were cultured to mid-log phase and supernatants collected. Rat periodontal ligament fibroblasts, derived from collagenase/elastase digestion of ligament adherent to alvelolar bone, were seeded at 10⁴ cells per cm² and incubated in 0ng/ml or 20ng/ml (non-toxic concentration) of SMG supernatants. Effects on cell growth, alkaline phosphatase (ALP) activity and synthesis of matrix components collagen, decorin and biglycan were examined. Similarly, effects of 20ng/ml SMG supernatants on cell growth, cell migration and biglycan synthesis of EAhy926 cells, a human aortic endothelial cell hybridoma, were assessed. Results: All SMG supernatants increased cell growth of both periodontal fibroblasts and endothelial cells. For strains S. intermedius and S. constellatus, increased levels of fibroblast ALP activity were seen at day 5, although levels were less than control by day 13. Conversely, ALP activity decreased for fibroblasts culture in the presence of supernatant from S. anginosus. All strains increased biglycan (mRNA levels), but decreased decorin (mRNA and protein) and collagen (protein) of fibroblasts. SMG supernatants decreased cell migration and biglycan synthesis of endothelial cells. Conclusion: SMG have the capacity to alter cellular behaviour of periodontal fibroblasts and endothelial cells leading to impaired tissue repair. Of note both commensal and pathogenic strains exhibited this ability although some variation in the strains was observed.