Abscess forming ability of streptococcus milleri group: synergistic effect with Fusobacterium nucleatum.

The abscess forming abilities of "Streptococcus milleri" strains (Streptococcus constellatus, Streptococcus anginiosus, and Streptococcus intermedius) isolated from dentoalveolar abscesses and the synergistic effect of Fusobacterium nucleatum co-inoculated with the isolates were examined on a mouse subcutaneous abscess model. Five days after inoculation, all S. milleri strains formed abscesses, which showed less pathological spread to surrounding connective tissues than those formed by Staphylococcus aureus 209P strain and were similar to those by F. nucleatum ATCC25586. When each S. milleri strain and F. nucleatum were co-inoculated, abscess sizes and each bacterial number recovered from abscesses increased in comparison to those treated by bacterial mono-inoculation of each S. milleri strain or F. nucleatum alone. The strongest synergistic effect was observed in the combination of S. constellatus and F. nucleatum. In a time course experiment with this combination, the recovery of S. constellatus subsequently decreased after the decrement of F. nucleatum, and it appeared that the association with F. nucleatum maintained the bacterial number of S. constellatus in the abscess. The cell-free supernatant of F. nucleatum had a tendency to increase the abscess size caused by S. constellatus in this model. When S. constellatus was cultured with F. nucleatum culture supernatant in vitro, growth enhancement in the early phase was observed. Furthermore, the phagocytic killing of S. constellatus by human polymorphonuclear leukocytes (PMNs) was significantly suppressed and the PMN membranes appeared to be injured by addition of the F. nucleatum culture supernatant. These results suggest that the pathogenicity of S. milleri strains in odontogenic infections may be enhanced by the co-existence of F. nucleatum.