Antibacterial potential of riboflavin mediated blue diode laser photodynamic inactivation against Enterococcus faecalis: A laboratory investigation

Highlights

• Higher irradiation dosage plus a high concentration of riboflavin produced an approximately one-log effect on E. faecalis.

• Changing the irradiation doses of the blue diode laser and riboflavin concentrations in PDI process can influence antimicrobial action on E. faecalis.

• The teeth exhibited clinically acceptable color change after riboflavin treatment at concentration less than 50 μmol/L.

Abstract

Objectives

This study aimed to evaluate the inactivation potency of riboflavin-mediated blue diode laser photodynamic inactivation (PDI) against Enterococcus faecalis at planktonic and biofilm stages and also investigated its effect on the tooth color change.

Materials and methods

The antibacterial and antibiofilm activities of riboflavin mediated PDI against E. faecalis were investigated. The numbers of colony-forming units (CFUs)/mL were calculated. Teeth discoloration were evaluated using the CIE L*a*b* based color difference (ΔE).

Results

Antibacterial analysis indicated that the blue diode laser irradiation at 12, 18, 24, and 30 J/cm² alone and different concentrations of riboflavin solution (6.25 to 100 μM) reduced the number of CFU/mL of E. faecalis, but the reduction was not statistically significant (P > 0.05). Depending on the riboflavin concentration and the light dose, there can be as much as a 1-log effect on CFU/mL. In addition, E. faecalis biofilm was more affected with 30 J/cm² irradiation dosage and 100 μM riboflavin than other groups. Meanwhile, bacterial suspensions treated with 5.25% sodium hypochlorite (NaOCl) showed maximum biofilm inhibition and colony number reduction, compared with the control. The teeth exhibited clinically acceptable color change after riboflavin treatment at concentration ranging from 6.25 to 50 μM (ΔE < 3.7).

Conclusions

The riboflavin mediated PDI process is somewhat less effective than NaOCl but perhaps less toxic to tissues. It might be feasible to repeat the riboflavin + light treatment to further promote efficacy.

Introduction

The access to the root environment is morphologically difficult for antimicrobials and dental instruments, and the presence of biofilm in the lateral canals can lead to failure of root canal treatment and secondary root infection [1]. Effective elimination of microbial biofilms is the main aim of root canal treatment [2]. Enterococcus faecalis usually isolated from infected root canals. It is often found in failed endodontic treatments with its ability to survive in a nutrient-free and highly alkaline dental environment after treatment with intra-canal medication, as well as its ability to establish a stable biofilm [3,4]. Although chemomechanical procedures play an important role in the reduction of microbial load in the infected root canal, because of the complex anatomy and narrow space of root canal, some of the root canal space is left untouched [5].

Sodium hypochlorite (NaOCl) is the irrigant of choice for root canal irrigation; however, there are some concerns such as toxic effect, low penetration depth into dentinal tubules, and adverse tissue reactions [6,7]. In addition, the concentration of antibiotics and disinfectant mouthwashes can be less than standard for bacterial inhibitory purposes [8]. Hence, residual microorganisms result in treatment failure [9].

Photodynamic inactivation (PDI) is an approach that activates photosensitizers through the exposure to light at a specific wavelength in the presence of oxygen to produce free radicals and reactive oxygen species that can cause cell death [10]. Compared to other antimicrobial therapies, PDI has several advantages, including the fact that PDI is a process in which the spread of microbial resistance will be very rare. Furthermore, PDI shows a short duration of action and it is known as a cost-effective treatment method [11]. The successful outcome of PDI depends on the type and concentration of the photosensitizer, irradiation time, output power, and the dose of light emitted [12,13].

Laser irradiation has been suggested as an alternative light source, mostly due to its energy efficiency and the light quality. The use of lasers provides many benefits, such as production of high-intensity pulses, control of the power settings, and a sharply defined wavelength. The blue diode laser with a wavelength of 445 nm is a monochromatic radiation and represents a coherent light irradiation, while light-emitting diode (LED) emits a broad range of light wavelengths (385–515 nm) without homogeneous distribution [14].

The use of natural products is one of the most successful strategies for modern medicinal discoveries [15]. Riboflavin (vitamin B₂) is a nontoxic yellow pigment found naturally in some foods and used as a dietary supplement. It is recognized by the Food and Drug Administration (FDA) as generally safe [16]. Riboflavin has two major peaks in the UVA (360 nm) and blue (440 nm) spectral regions (Fig. 1) [17]. Spectrophotometry is widely used to measure color changes. This technique related to CIE L*a*b* system to track color changes. In this system, the color differences are expressed numerically, which can be related to visual perception abilities and clinical value. Furthermore, this tool minimizes light leakage at the edges’ fine structures and maximizes the collection of reflected light in different directions [18].

Several studies have evaluated the antibacterial efficacy of LED activated riboflavin against different microorganisms [19], [20], [21], [22], [23], [24]. To our knowledge, there is no reports available on the topic addressed which exhibits E. faecalis biofilm inactivation by using blue diode laser. Hence, this study compares the efficacy of various concentrations of riboflavin in combination with different irradiation doses of blue diode laser to find the best protocol for reduction of E. faecalis at planktonic and biofilm modes of growth, as well as, assess the effect of riboflavin solution on tooth color change.