c Polyphosphate (polyP) has gained a wide interest in the food industry due to its potential as a decontaminating agent. In this study, we examined the effect of sodium tripolyphosphate (polyP3; Na 5 P 3 O 10 ) against planktonic and biofilm cells of Prevotella intermedia, a major oral pathogen. The MIC of polyP3 against P. intermedia ATCC 49046 determined by agar dilution method was 0.075%, while 0.05% polyP3 was bactericidal against P. intermedia in time-kill analysis performed using liquid medium. A crystal violet binding assay for the assessment of biofilm formation by P. intermedia showed that sub-MICs of polyP3 significantly decreased biofilm formation. Under the scanning electron microscope, decreased numbers of P. intermedia cells forming the biofilms were observed when the bacterial cells were incubated with 0.025% or higher concentrations of polyP3. Assessment of biofilm viability with LIVE/DEAD staining and viable cell count methods showed that 0.05% or higher concentrations of polyP3 significantly decreased the viability of the preformed biofilms in a concentration-dependent manner. The zone sizes of alpha-hemolysis formed on horse blood agar produced by P. intermedia were decreased in the presence of polyP3. The expression of the genes encoding hemolysins and the genes of the hemin uptake (hmu) locus was downregulated by polyP3. Collectively, our results show that polyP is an effective antimicrobial agent against P. intermedia in biofilms as well as planktonic phase, interfering with the process of hemin acquisition by the bacterium. P revotella intermedia is a black-pigmented anaerobic Gramnegative bacterium which has long been known to be associated with oral diseases, such as chronic periodontitis (1-3), aggressive periodontitis (4-6), puberty-associated gingivitis, acute necrotizing ulcerative gingivitis (7,8), periapical periodontitis (9, 10), and noma (an acute gangrenous disease) (11, 12). Besides being involved in oral diseases, P. intermedia has also been reported to be associated with various systemic diseases, such as cystic fibrosis, chronic bronchitis (13-15), and atherosclerosis (16). Difficulty in controlling the bacterium has been attributed to resistance of P. intermedia to many antibiotics, including penicillins, cephalosporins, and tetracyclines (17, 18). Moreover, P. intermedia cells form a biofilm in which the bacterial cells become more resistant to antibiotics (19). Because biofilm can serve as a reservoir of antibiotic resistance (20), it is of clinical significance to develop alternative antimicrobial approaches for controlling antibiotic-resistant P. intermedia.Inorganic polyphosphate (polyP) is a chain of few or many hundreds of phosphate (P i ) residues linked by high-energy phosphoanhydride (21). Intracellular polyP found in bacteria is considered a virulence factor since it performs various functions, such as serving as an ATP source and substitute, a regulator of the intracellular levels of metal ions, a channel for DNA entry, and a regulator that contributes to bacter...