Flow cytometric assessment of Streptococcus mutans viability after exposure to blue light-activated curcumin

• Blue light-activated curcumin is antibacterial against planktonic S. mutans.
• S. mutans biofilms are less responsive to photo-inactivation than planktonic cells.
• Subpopulations of live, dead and injured bacteria were identified by flow cytometry.
• Injured bacteria were not able to replicate and grow after cell sorting.

SummaryBackgroundStreptococcus mutans biofilms are considered as primary causative agents of dental caries. Photodynamic antimicrobial chemotherapy (PACT) has been recently proposed as a strategy for inactivating dental biofilms. This study aimed to investigate the effect of blue light-activated curcumin on S. mutans viability and to explore its potential as a new anti-caries therapeutic agent. The effect of different concentrations and incubation times of photo-activated curcumin on the survival of S. mutans in planktonic and biofilm models of growth was assessed by flow cytometry.MethodsStreptococcus mutans in planktonic suspensions or biofilms formed on hydroxyapatite disks were incubated for 5 or 10 min with curcumin prior to blue light activation. Bacteria were labeled with SYTO 9 and propidium iodide before viability was assessed by flow cytometry. Results were statistically analyzed using one-way ANOVA and Tukey multiple comparison intervals (α = 0.05).ResultsFor planktonic cultures, 0.2 μM of light-activated curcumin significantly reduced S. mutans viability (p < 0.05). For biofilm cultures, light-activated curcumin at concentration of 40–60 μM only suppressed viability by 50% (p < 0.05). Independently of the mode of growth, incubation time has no significant effect on PACT efficiency.ConclusionThis study indicates that blue light-activated curcumin can efficiently inactivate planktonic cultures of S. mutans whereas biofilms were more resistant to treatment. Flow cytometry allowed the detection of bacteria with damaged membranes that were unable to replicate and grow after cell sorting. Further studies seem warranted to optimize the efficacy of light-activated curcumin against S. mutans biofilms.