Previous reports have shown that Treponema denticola causes rearrangement of filamentous actin (F-actin) in human gingival fibroblasts (HGF). The purpose of this investigation was to determine the effect of T. denticola on the generation of inositol phosphates (IPs) in relation to a time course for F-actin disruption in HGF. Cultured HGF were exposed to washed cells of T. denticola ATCC 35405 for 140 min. Changes in the fluorescence intensity of rhodamine-phalloidin-labeled F-actin in serial optical sections of single HGF were quantified by confocal microscopy image analysis. The percentage of cells with stress fiber disruption was also determined by fluorescence microscopy. Challenge with T. denticola caused a significant reduction in F-actin within the first hour, especially at the expense of F-actin in the ventral third of the cells, and a significant increase in the percentage of HGF with altered stress fiber patterns. Significant concentration-dependent disruption of stress fibers was also caused by HGF exposure to a Triton X-100 extract of T. denticola outer membrane (OM). IPs were measured by a radiotracer assay based on the incorporation of myo-[³H]inositol into IPs in HGF incubated with LiCl to inhibit endogenous phosphatases. HGF challenge with several strains of T. denticola and the OM extract of T. denticola ATCC 35405 resulted in a diminished accumulation of radiolabeled IPs relative to both 15 and 1% fetal bovine serum, which served as strongly positive and background control agonists, respectively. The significantly diminished IP response to T. denticola ATCC 35405 occurred within 60 min, concomitant with significant reduction of total F-actin and disruption of stress fibers. Pretreatment with the proteinase inhibitor phenylmethylsulfonyl fluoride, which had previously been found to block T. denticola's degradation of endogenous fibronectin and detachment of HGF from the extracellular matrix, had little effect on F-actin stress fiber disruption and the IP response. Therefore, in addition to its major surface chymotrypsin-like properties, T. denticola expresses cytopathogenic activities that diminish the generation of IPs during the time course associated with significant cytoskeletal disruption in fibroblasts.