Porphyromonas gingivalis, one of the gram-negative organisms associated with periodontal disease, possesses potential virulence factors, including fimbriae, proteases, and major outer membrane proteins (OMPs). In this study, P. gingivalis ATCC 33277 was cultured in a chemostat under hemin excess and presumably peptidelimiting conditions to better understand the mechanisms of expression of the virulence factors upon environmental changes. At higher growth rates, the amounts of FimA and the 75-kDa protein, forming long and short fimbriae, respectively, increased significantly, whereas gingipains decreased in amount and activity. In a nutrient-limited medium, lesser amounts of the above two fimbrial proteins were observed, whereas clear differences were not found in the amounts of gingipains. In addition, two-dimensional electrophoresis revealed that proteins in cells were generally fewer in number during nutrient-limited growth. Under aeration, a considerable reduction in gingipain activity was found, whereas several proteins associated with intact cells significantly increased. However, the expression of major OMPs, such as RagA, RagB, and the OmpA-like proteins, was almost constant under all conditions tested. These results suggest that P. gingivalis may actively control expression of several virulence factors to survive in the widely fluctuating oral environment.It has been reported that more than 80% of U.S. adults have overt gingivitis and about half of them have periodontitis (1). In periodontal diseases, teeth may be lost after inflammation spreads to the supporting periodontal tissues. Not only local events such as inflammation and trauma but also systemic and environmental factors are involved in establishment of periodontal diseases (9). The formation and maturation of plaque, especially subgingival plaque in periodontal pockets, are also involved (61). Although there are more than 600 species of bacteria in the mouth (30), specific species of gram-negative anaerobic bacteria, spirochetes, and motile rods are known to participate in formation of subgingival plaque (20,60). Porphyromonas gingivalis, a gram-negative anaerobe, is considered to contribute to periodontitis, since it is detected frequently in patients with these diseases (56, 60).Asaccharolytic P. gingivalis mainly obtains nutrients by degradation of proteins. This bacterium is known to have many potential virulence factors, including fimbriae, proteases, hemagglutinins, lipopolysaccharide, capsule polysaccharides, and major outer membrane proteins (26, 31). P. gingivalis expresses two types of fimbriae, long fimbriae composed of FimA (fimbrilin) (67), and short fimbriae composed of a 75-kDa protein (21, 51, 68). Many studies have shown that FimA has various functions, which include attachment to and invasion of gingival epithelial cells (28, 64), coaggregation with other species of bacteria (2), attachment to salivary components (4), activation of fibroblasts and macrophages (24), induction of inflammatory cytokines, and bone resorption (23)....