Different Response to Amine Fluoride by <i>Streptococcus mutans</i> and Polymicrobial Biofilms in a Novel High-Throughput Active Attachment Model

<i>Background/Aims:</i> The antimicrobial resistance of microorganisms in biofilms and the polymicrobial interactions in these biofilms that modulate resistance require novel strategies to evaluate the efficacy of caries-preventive compounds. The current study aimed to evaluate the effects of a caries-preventive agent in <i>Streptococcus mutans</i> and polymicrobial biofilms. <i>Methods:</i> We developed a novel high-throughput active attachment model. The model consisted of a custom-designed lid containing glass discs that fit on top of standard 24-well plates. Biofilms were formed using either <i>S. mutans</i> C180-2 or saliva. At the end of biofilm formation (up to 96 h) the biofilms were treated with amine fluoride (AmF) solutions. The viability of the biofilms was determined by CFU counts, and metabolic activity was measured via lactate production. <i>Results:</i> The effect of AmF on the viability of the polymicrobial biofilms was significantly less than that on the <i>S. mutans</i> biofilms, indicating a higher resistance in the complex biofilms. Both types of biofilms became more resistant to AmF with age. The higher resistance of the polymicrobial biofilms was not reflected in metabolic activity; in dose-response experiments AmF reduced lactate production in both types of biofilms to the same extent. Moreover, the age-induced increased resistance in the polymicrobial biofilms was less pronounced in terms of the inhibition of metabolic activity. <i>Conclusions:</i> This study clearly shows that when evaluating the efficacy of caries-preventive compounds it is essential to use appropriate polymicrobial biofilm models, and more importantly that efficacy needs to be judged based on the reduction of acid formation (i.e. cariogenic potential) as well as on bacterial viability.