The oral biofilm is a multispecies community in which antagonism and mutualism coexist among friends and foes to keep an ecological balance of community members. The pioneer colonizers, such as , produce HO to inhibit the growth of competitors, like the mutans streptococci, as well as strict anaerobic middle and later colonizers of the dental biofilm. Interestingly, species, as early colonizers, physically interact (coaggregate) with A putative catalase gene () is found in most sequenced species; however, the function of this gene is unknown. In this study, we characterized the ecological function of from PK1910 by integrating it into the only transformable strain, OK5, which is negative. The strain (OK5-) became more resistant to HO Further studies demonstrated that the gene expression is induced by the addition of HO or coculture with Mixed-culture experiments further revealed that the transgenic OK5- strain not only enhanced the growth of , a strict anaerobic periodontopathogen, under microaerophilic conditions, but it also rescued from killing by A potential role of catalase in veillonellae in biofilm ecology and pathogenesis is discussed here. species, as early colonizers, can coaggregate with many bacteria, including the initial colonizer and periodontal pathogen , during various stages of oral biofilm formation. In addition to providing binding sites for many microbes, our previous study also showed that produces nutrients for the survival and growth of periodontal pathogens. These findings indicate that plays an important "bridging" role in the development of oral biofilms and the ecology of the human oral cavity. In this study, we demonstrated that the reducing activity of can rescue the growth of not only under microaerophilic conditions, but also in an environment in which is present. Thus, this study will provide a new insight for future studies on the mechanisms of human oral biofilm formation and the control of periodontal diseases.