The combination of methylene blue and sodium dodecyl sulfate enhances the antimicrobial photodynamic therapy of Candida albicans at lower light parameters

BACKGROUND

The growth of resistant microorganisms has been a challenge for health systems. Antimicrobial Photodynamic Therapy (aPDT) has gained attention due to its effects on resistant strains. Recently, it was shown that the association of methylene blue (MB) and sodium dodecyl sulfate (SDS) is an effective strategy to increase the effect of aPDT; however, it is unknown which are the best light parameters (such as irradiance and radiant exposure, RE), to reach the most effective protocols. This work aimed to evaluate the light parameters, irradiance, and radiant exposure, in aPDT with MB when conveyed in water compared to MB associated with SDS.

METHODS

Tests were carried out to quantify the colony-forming units (CFU) of ATCC 10231 strain of Candida albicans when using MB in different media and with different light parameters: Control (water), SDS (0.25%), MB (20 mg/mL), and the MB/SDS at irradiances of 3.7; 11.2; 18.6, and 26.1 mW/cm² and varied irradiation times to reach radiant exposures of 4.4; 17.8; 26.7, and 44 J/cm².

RESULTS

The results showed that aPDT with MB/SDS had a higher antimicrobial effect than MB when conveyed in water. Furthermore, for the highest irradiance studied (26.1 mW/cm²), CFU decreases exponentially with increasing RE from 4.4 up to 44J/cm². Similarly, at a fixed RE, the higher the irradiance used, the higher the antimicrobial effect was observed, except for the lowest RE studied (4.4 J/cm²).

CONCLUSIONS

aPDT with MB/SDS had a greater antimicrobial action at the lower light parameters when compared to MB conveyed in water. The authors suggest the use of RE above 18 J/cm² and irradiance above 26mW/cm² since at the mentioned parameters the increase in its value caused a greater antimicrobial effect.

Optical fiber optofluidic laser based on surfactant solubilization of rhodamine B gain in an aqueous solution

We report a whispering gallery mode (WGM)-based fiber optofluidic laser (FOFL), in which rhodamine B (RhB) in an aqueous surfactant solution of sodium dodecylbenzene sulfonate (SDBS) is used as the laser gain medium. Here, the role of SDBS is to scatter the RhB dye molecules to effectively prevent its self-association in the aqueous solution. Therefore, the fluorescence quantum yield of the used RhB dye is improved due to the enhanced solubilization, which results in a low lasing threshold of ∼2.2 µJ/mm² when the concentration of SDBS aqueous solution reaches up to 20 mM, on par with or even better than most of the optofluidic dye lasers using RhB as the gain medium in an organic solution. We then establish a model of solubilization capacity of SDBS micelles, which successfully addresses the mechanisms of dye-surfactant interactions in the proposed FOFL system. We further apply this FOFL platform to the case of concentration sensing of the used SDBS, which exhibits a 2-order-of-magnitude improvement in sensitivity compared to the fluorescence measurement due to the signal amplification inherent to the lasing process. The proposed FOFL platform in combination with surfactant solubilization gain medium in an aqueous solution promises to enable chip-scale coherent light sources for various environmental and bio-chemical sensing applications.