Review
Antibacterial photodynamic therapy for dental caries: Evaluation of the photosensitizers used and light source properties

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Summary

Photodynamic therapy studies have shown promising results for inactivation of microorganisms related to dental caries. A large number of studies have used a variety of protocols, but few studies have analyzed photosensitizers and light source properties to obtain the best PDT dose response for dental caries. This study aims to discuss the photosensitizers and light source properties employed in PDT studies of dental caries. Three questions were formulated to discuss these aspects. The first involves the photosensitizer properties and their performance against Gram positive and Gram negative bacteria. The second discusses the use of light sources in accordance with the dye maximum absorbance to obtain optimal results. The third looks at the relevance of photosensitizer concentration, the possible formation of self-aggregates, and light source effectiveness. This review demonstrated that some groups of photosensitizers may be more effective against either Gram positive or negative bacteria, that the light source must be appropriate for dye maximum absorbance, and that some photosensitizers may have their absorbance modified with their concentration. For the best results of PDT against the main cariogenic bacteria (Streptococcus mutans), a variety of aspects should be taken into account, and among the analyzed photosensitizer, erythrosin seems to be the most appropriate since it acts against this Gram positive bacteria, has a hydrophilic tendency and even at low concentrations may have photodynamic effects. Considering erythrosin, the most appropriate light source should have a maximum emission intensity at a wavelength close to 530 nm, which may be achieved with low cost LEDs.

Introduction

Dental caries is one of the most prevalent chronic diseases in the population worldwide, affecting 60–90% of school-aged children and almost 100% of the adult population [1]. The prevalence of dental caries has been studied in many developed countries in recent years. In the USA, caries was considered the most common chronic disease of childhood, being five times more common than asthma [1], with a prevalence of 27% in preschoolers, 42% in school-aged children, and 91% of dentate adults [2].

Dental caries results from interactions over time between specific pathogenic bacteria, primarily Streptococcus mutans, which metabolize ingested carbohydrates to form acids [3], [4]. In recent decades, photodynamic therapy (PDT) has been studied as an alternative measure against the etiological factors of dental caries. PDT is a treatment that utilizes light to activate a photosensitizing agent in the presence of oxygen, resulting in the production of reactive radicals capable of inducing cell death [5].

In the literature, there is a large number of studies showing a variety of protocols for the use of PDT, but only a few of them analyze the properties of the photosensitizers and light sources used in Dentistry in order to obtain the best dose response of photodynamic therapy for dental caries. Currently, PDT is being applied mostly in the treatment of macular degeneration, pathological myopia, esophagus, lung, and skin cancer, and in the treatment of precancerous lesions in Barret esophagus patients [6]. Additionally, several studies have shown that PDT also has antimicrobial properties, in a process called “photodynamic inactivation” (PDI) or “photodynamic antimicrobial therapy” (PACT) or even “Photo-activated disinfection” [7], [8], [9], [10], [11], [12]. These antimicrobial properties have been extended and studied for the treatment of caries [13], [14], [15], [16], [17], [18], [19], [20], [21].

Several groups of photosensitizers in different illumination systems have been proposed. Even when the same photosensitizer (PS) and light source were employed, the diversity of irradiation protocols and variation of PS concentration, irradiation time, and light potencies makes comparison between the results difficult. Few studies discuss both the structural properties of PS and of the light sources to specifically achieve the optimal protocol of this therapy against dental caries. Therefore, this study aims to discuss the properties of photosensitizers and light sources employed in PDT studies for dental caries.

Section snippets

Photodynamic therapy for dental caries

The key-words photodynamic therapy; S. mutans, and dental caries were entered in Medline, Bireme, and Scielo databases, resulting in 18 articles related to PDT and dental caries (Table 1). These studies involved in vitro and in situ experiments with a variety of PDT protocols that were published between 1992 and 2010. Based on these variations, three questions were formulated in order to discuss the use of different types of photosensitizers in different concentrations and the influence of the

Which photosensitizers are more effective against specific groups of dental caries bacteria?

The main organisms recognized as associated with early caries development are the Streptococci mutans group (particularly, S. mutans and S. sobrinus) and lactobacilli species [29]. As the lesion progresses to deeper dentin, anaerobic species start to thrive and a transition takes place from predominantly facultative Gram positive bacteria to strictly anaerobic Gram positive rods and cocci, and Gram negative rods [30]. In the analyzed articles, S. mutans was the most studied bacteria, since they

Are the light sources appropriate for the different dyes?

The basic requirement for PDT light sources is that they match the activation spectrum (electronic absorption spectrum) of the photosensitizer (usually the longest wavelength peak) and generate adequate light potency at this wavelength [42]. This concept was first discussed by Isaac Newton in 1666, when he showed that the light colors red, orange, yellow, green, blue, and violet together compose white light. Later, Newton presented ‘the Newton disc’, showing that the rotation of a disc painted

How does the dye concentration influence the effectiveness of the therapy?

PDT may be classified inside the photophysical and photochemical studies. A common problem found in this kind of study is the formation of dye self-aggregates in aqueous media, as aggregation usually impairs the therapeutic response of PDT [47]. The formation of aggregates modifies the absorption spectrum and photophysical properties of the dye and affects its ability to absorb at a certain wavelength or to act as a photosensitizer [48]. In addition, in the aggregate state the PS may undergo a

Future developments

PDT will not replace classic therapy for dental caries, however, the photodynamic approach may improve, accelerate and lower the cost of treatment, in addition to acting as an extra-protective measure for dental care in the medium to long terms. Some of the advantages that may direct future insertion of this therapy include its antibacterial property, which may reduce dental structure removal during dental caries treatment, the low concentration of the dyes that result in low toxicity, high

Conclusions

The real mouth environment is totally different from the laboratorial culture or in vitro environment, which makes it difficult to provide an ideal condition for PDT studies. In spite of these limitations, in general the articles showed promising results in this field. This review article found that for optimal PDT results against cariogenic bacteria, the structural properties of the bacterium membrane, photosensitizer concentration, solubility and polarity, and light source wavelength must be

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