Photo-irradiated caffeic acid exhibits antimicrobial activity against Streptococcus mutans biofilms via hydroxyl radical formation

Taro Kanno,Peter Lingström,Midori Shirato,Keisuke Nakamura,Yoshimi Niwano,Ulf Örtengren

doi:10.1038/s41598-017-07007-z

Taro Kanno, Peter Lingström + Show 4 more

Open Access

https://doi.org/10.1038/s41598-017-07007-z

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Abstract

An antimicrobial technique based on photo-oxidation of caffeic acid (CA) has recently been developed, but its effect on biofilm-forming bacteria is unknown. The present study aimed to evaluate the effect of photo-irradiated CA against Streptococcus mutans (cariogenic bacteria) biofilm as it relates to hydroxyl radical formation. S. mutans biofilms grown on hydroxyapatite disks were immersed in CA solution (0–2 mg/mL) and irradiated with LED light at wavelengths of 365, 385, and 400 nm and at irradiances of 500, 1000, and 2000 mW/cm2 for 4 min. Biofilm viable bacterial counts were determined by colony counting. The yield of hydroxyl radicals generated by the LED irradiation of CA solution was quantified by electron spin resonance analysis. Of the conditions tested, the highest bactericidal effect, with a > 5-log reduction in viable bacterial counts, was obtained by irradiation of a 1 mg/mL CA solution with 385 nm LED and at an irradiance of 2000 mW/cm2. Hydroxyl radical formation was related to this bactericidal effect. The present study suggests that the antimicrobial technique based on the 385 nm LED irradiation of CA is effective against cariogenic biofilms and can be applied as an adjunctive chemotherapy for dental caries.

Highlights

Improved dental care and increased knowledge of oral diseases as well as associated causative factors of such diseases have brought about a dramatic decrease in the prevalence of caries[1,2,3]
The present study aimed to evaluate the influence of such treatment conditions on the bactericidal activity of photo-irradiated caffeic acid (CA) against S. mutans biofilms in relation to hydroxyl radical formation, in an attempt to optimize the conditions for the application to dental caries treatment
The viable bacterial counts for S. mutans growing in biofilms on hydroxyapatite were approximately 107 colony forming units (CFUs)/specimen (Fig. 1a)

Summary

Introduction

Improved dental care and increased knowledge of oral diseases as well as associated causative factors of such diseases have brought about a dramatic decrease in the prevalence of caries[1,2,3]. When the biofilms were treated with CA(+)L(+) at an LED wavelength of 385 nm and an irradiance of 2000 mW/cm[2], viable counts decreased in a time dependent manner (Fig. 1d). Comparing the resistance of planktonic and biofilm-forming bacteria to treatments performed using the same conditions (irradiance: 1000 mW/cm[2], concentration of CA: 1 mg/mL, treatment time: 4 min), the former resulted in lower susceptibility to CA(−)L(+), whereas the latter resulted in higher resistance to CA(+)L(+) (Fig. 3c).

Results

Conclusion