High-velocity microsprays enhance antimicrobial activity in S. mutans biofilms

Streptococcus mutans in dental plaque biofilms play a role in caries development. The biofilm complex structure enhances the resistance to antimicrobial agents by limiting the transport of active agents inside the biofilm. We assessed the ability of high-velocity water microsprays to enhance delivery of antimicrobials into 3-days old S. mutans biofilms. Biofilms were exposed to a 90° or 30° impact, firstly using a 1-µm tracer beads solution (109 beads/mL) and secondly, a 0.2% Chlorhexidine (CHX) or 0.085% Cetylpyridinium chloride (CPC) solution. For comparison, a 30 sec diffusive transport and simulated mouthwash were also performed. Confocal microscopy was used to determine number and relative bead penetration depth (PD) into the biofilm. Instead, the antimicrobial’s depth of killing (KD) was calculated from the resultant zone of killing detected by live/dead viability staining. We firstly demonstrated that the microspray was able to deliver significantly more microbeads deeper in the biofilm compared to a simple 30-sec diffusion assay and simulated mouthwashing. Next our experiments revealed that the microspray yielded better antimicrobial penetration evidenced by a deeper killing inside the biofilm and a wider killing zone around the zone of clearance than a diffusion transport with the same antimicrobials. Interestingly the 30° impact in the distal position delivered approximately 16 times more microbeads and yielded approximately 15% more bacteria killing (for both CHX and CPC) than the 90o impact. These data suggest that high-velocity water microsprays can be used as an effective mechanism to deliver micro-particles and antimicrobials inside S. mutans biofilms. High shear stresses generated at the biofilm/burst interface might have enhanced beads and antimicrobials delivery inside the remaining biofilm by combining forced advection into the biofilm matrix and physical restructuring of the biofilm itself. Further, the impact angle has potential to be optimized both for biofilm removal and active agents’ delivery inside biofilm in those protected areas where some biofilm might remain.