Intracellular replication of fusobacteria requires new actin filament formation of epithelial cells

We examined survival and replication of fusobacteria inside epithelial cells. Subconfluent cultures of HaCaT keratinocytes were infected with five bacterial strains representing three Fusobacterium species: F. nucleatum, F. necrophorum, and F. mortiferum. Adhesion and invasion of the bacteria were assayed before and after antibiotic treatment that killed the adhered and extracellular bacteria. The number of live fusobacteria was examined by bacterial culturing after sonication of the epithelial cells. The role of host cell cytoskeleton functions was examined by treating the epithelial cells with cell function inhibitors. Number of viable epithelial cells was measured with the CellTiter96 kit. The tested Fusobacterium species adhered to and invaded the epithelial cells, and multiplied intracellularly for several hours. Thereafter, the intracellular number of bacteria rapidly declined. Concomitantly, viable fusobacteria were detected in the culture medium. Treatment of the infected epithelial cells with an actin formation inhibitor markedly reduced the number of living intracellular fusobacteria. Newly formed actin filaments were seen by confocal microscopy in the epithelial cells associated with the invaded bacteria. Fusobacteria infection did not reduce the number of viable epithelial cells in culture. Thus, fusobacteria are able to adhere to and invade epithelial cells, and survive under aerobic conditions. This property may enable them to survive in mucosa and participate in various disease processes of oral and pharyngeal tissues.