Bacteroides forsythus is a filamentous, non-motile, non-pigmenting, gram-negative anaerobe that has been implicated as one of the etiologic agents of periodontal disease. Few studies, however, have examined the potential virulence factors produced by this oral microorganism. Understanding a B. forsythus infection requires that the roles of its virulence factors be confirmed in the context of an in vivo host-parasite interaction. An innovative genetic technique termed in vivo expression technology (IVET) has been developed for the purpose of isolating genes expressed only during the infectious process. The utility of IVET has been proven in isolating novel virulence factors of major medical pathogens, such as Salmonella, Pseudomonas, and Staphylococci. Preliminary data presented in this application shows that IVET has been successfully used to detect genes of Porphyromonas gingivalis, a principal periodontal pathogen, that are expressed only in vivo. The specific aims of this proposed study are (1) to construct pBFIVET, the IVET system for B. forsythus, and (2) to test pBFIVET for efficacy in isolating B. forsythus genes expressed only during the infectious process. The results obtained from this study will help achieve the long-term goal of this laboratory: to examine the role of B. forsythus in periodontal infection.