Prevotella bivia has been associated with female upper genital tract infections and an increased risk of preterm delivery. In this study, the adherence and invasion capacity of P. bivia was investigated using a cervix epithelial cell line. P. bivia was furthermore analysed for its ability to evoke a proinflammatory cytokine response in epithelial cells. The invasion capacity, defined as the number of bacteria recovered from lysed HeLa cells infected with P. bivia, varied considerably among five strains, all of which were isolates from women with bacterial vaginosis. One P. bivia strain (P47) gave rise to an approximately 120-fold higher number of intracellular bacteria (7 x 10(3) bacteria per 1 x 10(5) cells) compared with the least invasive strain. Three strains expressed an intermediate or low invasiveness, showing an approximately 3- to 40-fold higher number of intracellular bacteria per 1 x 10(5) cells compared with the least invasive strain. The intracellular localization of P47 in phagosome-like vesicles was confirmed by transmission electron microscopy. All P. bivia strains adhered to HeLa cells to the same extent (range 14-22 bacteria per cell) as analysed by interference microscopy. No correlation was found between adhesion and invasion capacity of the strains. Furthermore, no fimbriae-like structures were observed on P47 detected by scanning electron microscopy or negative staining. Analysis of TNF-alpha, IL-1alpha, IL-6, IL-8, and IL-18 in P. bivia-stimulated HeLa cells showed low levels of only IL-6 and IL-8 for the most invasive P. bivia strain P47. Thus, the induction of IL-6 or IL-8 secretion appeared to be associated with invasion capacity. This work provides evidence that some P. bivia isolates can invade human cervix epithelial. Thus, a strong capacity for invasion and a weak proinflammatory cytokine-inducing capacity in P. bivia are suggested to be virulence factors in establishing a low-grade upper genital tract infection.