The present investigation was carried out in order to study the histopathological effect and virulence of different black‐pigmented Bacteroides species on both soft and hard tissues in an animal model. Male Wistar rats were injected in the palate next to the first right molar with 30 μl of a bacterial suspension containing 5 × 1010 viable cells per ml. To determine the difference in virulence the histopathological effects were studied five days after injection. All strains tested caused a submucosal infiltrate at the injection site, consisting of mononuclear cells, mainly lymphocytes, monocytes and fibroblasts. These inflammatory responses were accompanied by devitalization and resorption of bone. Simultaneously active osteoblasts and bone formation were observed. Strains of Bacteroides asaccharolyticus, Bacteroides melaninogenicus, Bacteroides corporis, and Bacteroides loescheii caused a less prominent devitalization of the palatinal bone than strains of Bacteroides gingivalis and Bacteroides intermedius. Strains of the latter species caused often a nasal infiltrate and abundant bone formation at the nasal side of the palatinal bone. B. asaccharolyticus and B. melaninogenicus scarcely caused signs of a reaction at the nasal side of the palatinal bone. In a second longitudinal study the histopathological effects of B. gingivalis strain HG 66 were investigated. The inflammation was characterized by a rapid influx of polymorphonuclear cells within 2 to 4 hours. These cells gradually diminished and were replaced by a mononuclear infiltrate. One of the most characteristic features accompanying the inflammation was the disappearance of the periosteum. After two days a recovery of periosteum was observed and the osteoblasts became active in new bone formation. Devitalization from the palatinal bone started about 8 hours after injection and could be seen until 10 days after injection. No inflammatory reaction or devitalization of bone could be observed 15 days after injection.

SUMMARY. The pathogenic potential of black-pigmented Bacteroides strains was studied in an animal model in which the effect on bone tissue could be determined. Bacteria suspended in agar were injected subcutaneously in the left hind paw of a mouse. After 3-5 days, B. gingivalis strain HG 66 had caused a massive infiltration with polymorphonuclear cells, destruction of the periosteum on the metatarsals and bone resorption by osteoclasts. After 7 days only a few osteoclasts remained and reactive bone formation was observed. In a comparative study with strains of several black-pigmented Bacteroides species, differences in bone resorbing potential were seen. B. gingivalis strains caused severe inflammation which resulted in bone resorption. Strains of B. asaccharolyticus, B. endodontalis, B. intermedius, B. melaninogenicus and B. loeschei caused less inflammation and less bone resorption. Killed bacteria or agar alone caused a relatively mild inflammation and no bone resorption.