Ca(2)(+) release and buffering effects of synthetic hydroxyapatite following bacterial acid challenge.

BACKGROUND: Synthetic particulate hydroxyapatite (HAP; Ca5(PO4)3(OH)) is used as ingredient in oral care products but its effects on cariogenic biofilms are not clear yet. The primary mode of action of HAP may be acting as a calcium phosphate reservoir when deposited in oral biofilms and release Ca(2+) and (hydrogen) phosphate ions upon bacterial acid challenge. The aim of this in vitro study was to test this hypothesis by investigating release of Ca(2+) ions and potential buffering effects from HAP upon bacterial acid challenge in planktonic cultures and biofilms of Streptococcus mutans. METHODS: Planktonic cultures of S. mutans were grown in BHI broth with 1% sucrose or with additional 5% HAP or 5% silica for up to 48 h. Separately, biofilms of S. mutans were grown in BHI for 72 h in total. After 24 h of this biofilm culture, either BHI alone or BHI with additional 0.5% HAP or 0.5% silica was added. After 48 h, BHI with 1% sucrose was added to allow bacterial acid formation. Ca(2+) release was determined colorimetrically and pH measurements were performed using a pH electrode. For statistical analysis, non-parametrical procedures were applied (n >/= 10; Mann-Whitney U test; alpha = 0.05). RESULTS: Relevant release of Ca(2+) was only evident in planktonic cultures or biofilms with HAP but not in both other groups (p