Bacterial effects on in vitro mineralization of human periodontal fibroblasts (HPF) have not yet been examined in great detail. In our study, we investigated the effects of soluble extracts of the periodontopathic bacteria Porphyromonas gingivalis, Bacteroides forsythus and, Treponema denticola on cell proliferation, mineralization, as well as on osteoblastic markers present in HPF cultured in vitro, such as alkaline phosphatase (ALP) activity and collagen content. Periodontal fibroblasts stimulated by B-glycerophosphate, ascorbic acid and dexamethasone (BAD) or by dexamethasone and ascorbic acid (DA) were compared to unstimulated cells. During the cultivation period, the stimulation of HPF by combined dexamethasone and ascorbic acid (DA) had a strong inductive effect on proliferation, ALP activity and collagen formation. The extracts obtained from the periodontal pathogens had a suppressing effect on the proliferation rate of HPF. The extracts from P. gingivalis, B. forsythus and T. denticola caused a decrease in ALP activity within 24 h of application. While extracts obtained from P. gingivalis and B. forsythus induced a reduction in collagen content in BAD- and DA-stimulated HPF cells, T. denticola extracts led to an increase in collagen. Our data suggest that specific periodontopathic bacteria may suppress tissue regeneration in vivo not only by activating host defense mechanisms but also directly via a suppression of growth and differentiation of HPF and a reduction in the extracellular collagen matrix. For the process of pocket formation, not even the direct influence of viable bacteria seems to be necessary. Additionally, long-distance effects of bacteria harboured in periodontal pockets or in root canals may be of importance.