The pathogenic Neisseria species, N. meningitidis and N. gonorrhoeae can cause severe illnesses ranging from a life-threatening sepsis/meningitis, particularly in children, to pelvic inflammatory disease which may result in ectopic pregnancies and infertility. We investigated the molecular mechanisms that are exploited by these causative agents to establish infection using a combined genetic, biochemical and cell biological approach with the ultimate aim to contribute to the development of new therapeutic strategies and vaccine. Experimental evidence showed that different members of the variably expressed neisserial opacity outer membrane proteins (Opas and Opc) conferred cell tropism by promoting bacterial attachment to human cells through recognition of either heparan sulfate proteoglycans present at the surface of mucosal cells or CD66 cellular receptors present on professional phagocytes. This colonization was often followed by uptake of the microorganisms by the eukaryotic cells. With certain cell types, the proteoglycan-mediated uptake required additional serum/cell matrix proteins like vitronectin or fibronectin. These findings that were obtained with cultured transformed cell lines, were confirmed with primary human epithelial cell and organ cultures that were established in the lab, suggesting that they may be relevant for the natural infection. (Animal models for neisserial infections are not available.) These data imply that neisserial Opa or Opc antigens may be valuable targets of a future vaccine. Research on the function of other neisserial surface-exposed antigens, led to the identification of alternative Opa-independent pathways of bacteria entry into mammalian cells including a mechanism that appears to involve the major outer membrane porin. This finding strongly supports ongoing efforts to develop a porin based neisserial vaccine.