Method: S. mutans were grown to form biofilm that was exposed to blue-light (wavelengths, 400-500 nm) for 1, 3, and 7 min (fluences of 37, 112, and 262 J/cm2, respectively). Then bacteria were suspended using sonication and allowed to be grown into new biofilm. Bacteria was quantified by qPCR after 24h. Structural biofilm viability and polysaccharide production were assessed using confocal laser scanning microscopy. Acid production by bacteria in biofilm (acidogenicity) was measured during 3 days and biofilm re-growth under different pH values (aciduricity) was examined.
Result: Using qPCR a reduction of ~20% in bacteria in the newly formed biofilm was shown after 7 min exposure in comparison to the 3 min exposure, with only slight deviation from control. Under confocal microscopy the reformed biofilm showed higher proportion of dead bacteria and lower proportion of polysaccharides. The acidogenicity of bacteria in the newly formed biofilm was lowered as fluences of light increased. The aciduricity of the newly formed biofilm was decreased, meaning less growth of biofilm in low pH with increasing fluences.
Conclusion: Blue light at high fluences has a delayed effects on the viability and the virulence characteristics of S. mutans bacteria grown into new biofilm. The specific mechanism of action of the blue light on the normal life cycle of the bacteria is yet to be determined.