Effect of ions in silver fluoride on cariogenic biofilm formation

Abstract

Objective: To investigate the effect of silver and fluoride ions in 2.36M (mole/L) silver fluoride (AgF) solution on a cariogenic consortium biofilm formation on root dentine surface in an artificial mouth (AM). Methods: 32 sterilized tooth blocks with one unvarnished root dentine surface were prepared from extracted sound human premolars. The tooth blocks were inoculated with 3ml bacteria suspension containing six cariogenic species and then randomly allocated into four groups to receive intervention accordingly: 1) 2.36M AgF; 2) 2.36M potassium fluoride (KF); 3) 2.36M silver nitrate (AgNO3); 4) de-ionized water (control). They were then incubated in anaerobic chamber for 12 hours and transferred into AM. 5% sucrose was delivered 3 times per day. After 10 days, the biofilm formed on them were evaluated by colony forming unit (CFU), confocal laser-scanning microscopy (CLSM), and scanning electron microscopy (SEM). Results: approximately 1.8x107 CFU was inoculated onto each unvarnished dentine surface at baseline. After intervention and 12-hour incubation, the median CFU in AgF, AgNO3, KF, and control groups were 4.0x105, 5.0x105, 1.0x108, and 8.0x107 respectively (AgF=AgNO3<KF=control, Kruskal-Wallis 1-way ANOVA, p<0.05). After 10 days in AM, the median CFU in the four groups were 60, 780, 2.6x106, and 3.3x106 respectively (AgF<AgNO3<KF=control, p<0.05). From the CLSM images, the median live-to-dead ratios of these bacteria were found to be 0.27, 0.69, 2.90, and 3.83 respectively (AgF<AgNO3<KF=control, p<0.05). SEM scanning showed scattered bacteria on tooth blocks in AgF and AgNO3 groups while a confluent layer of biofilm was found on those in KF and control groups. Conclusion: Silver ion in 2.36M AgF has significant bactericidal effect on cariogenic bacteria can thus prevent biofilm from developing on root dentine surface over 10 days. Fluoride ion per se does not have a significant bactericidal effect on cariogenic bacteria but it is synergistic to silver ion in inhibiting biofilm formation.