Inhibition of biofilm formation and virulence factors of cariogenic oral pathogen Streptococcus mutans by natural flavonoid phloretin.

Objectives: To evaluate the effect and mechanism of action of the flavonoid phloretin on the growth and sucrose-dependent biofilm formation of Streptococcus mutans.

Methods: Minimum inhibitory concentration, viability, and biofilm susceptibility assays were conducted to assess antimicrobial and antibiofilm effect of phloretin. Biofilm composition and structure were analysed with scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Water-soluble (WSG) and water-insoluble glucan (WIG) were determined using anthrone method. Lactic acid measurements and acid tolerance assay were performed to assess acidogenicity and aciduricity. Reverse transcription quantitative PCR (RT-qPCR) was used to measure the expression of virulence genes essential for surface attachment, biofilm formation, and quorum sensing.

Results: Phloretin inhibited S. mutans growth and viability in a dose-dependent manner. Furthermore, it reduced gtfB and gtfC gene expression, correlating with the reduction of extracellular polysaccharides (EPS)/bacteria and WIG/WSG ratio. Inhibition of comED and luxS gene expression, involved in stress tolerance, was associated with compromised acidogenicity and aciduricity of S. mutans.

Conclusions: Phloretin exhibits antibacterial properties against S. mutans, modulates acid production and tolerance, and reduces biofilm formation.

Clinical significance: Phloretin is a promising natural compound with pronounced inhibitory effect on key virulence factors of the cariogenic pathogen, S. mutans.