Project Summary Infective endocarditis (IE) has high morbidity and mortality. Upwards of 30% of cases of IE are caused by oral bacterial species that enter the bloodstream and colonize heart valves. Frequent episodes of bacteremia from dental biofilm (plaque) are likely to be significant risk factors for development of IE. Our prior work demonstrates that: 1) there are only informal guidelines for prevention in 90% of people at risk for IE; 2) tooth brushing results in a high incidence of bacteremia of IE-causing species; 3) the risk of such bacteremia increases with the level of dental plaque and calculus present; and 4) patients with IE have a higher burden of dental plaque and calculus than matched non-IE controls. Although millions of people in the U.S. are at risk for IE because of pre-existing cardiac conditions, there are no prospective studies or solid evidence to show that improving oral hygiene reduces bacteremia from routine daily activities (e.g., toothbrushing). Evidence does show, however, that bacteremia is a strong surrogate marker for risk of IE. The lack of supporting data means that longstanding speculation about the importance of oral hygiene and gingival inflammation as risk factors for IE have had too little impact on practice guidelines on prevention, clinical practice, or funding for preventive care. The next step is to determine if professional scaling and oral hygiene instruction significantly reduce the incidence and duration of IE-causing bacteremia from toothbrushing. We plan to enroll 320 people at risk for IE into a clinical trial. Enrollees will be randomized to professional scaling and oral hygiene instruction versus routine oral care. We will test the steps in the hypothesized causal pathway from improved oral hygiene to decreased bacteremia from IE-causing species by: 1) determining the impact of professional scaling and oral hygiene instruction on the incidence and duration of bacteremia with IE-causing species during and following toothbrushing; 2) comparing oral hygiene and gingival health measures between randomized treatment groups and testing whether improvement in these measures is associated with reduced incidence and duration of bacteremia from toothbrushing; and 3) determining the degree to which reduction in bacteremia incidence and duration, and improvement in oral hygiene and gingival inflammation scores, are maintained following the intervention. This study will provide novel, important data to inform the healthcare community, guideline committees, and health funding agencies of the importance of improving oral hygiene and reducing gingival inflammation as primary preventive measures for all people at risk of IE.