The ineffectiveness of antimicrobial photodynamic therapy in the absence of preincubation of the microorganisms in the photosensitizer

Ítalo Dany Cavalcante Galo, Jéssica Assis Carvalho, Jessyca Luana Melo Costa Santos, Alexandre Braoios, Rodrigo Paschoal Prado



Considering its potential as an alternative therapy to combat multiresistant bacteria, photodynamic therapy has been improved and better studied in recent years, and determining its optimized application patterns is important.


This study aimed to evaluate the action of antimicrobial photodynamic therapy mediated by methylene blue in the absence of preincubation of infectious agents in the photosensitizer.


Standard strains of Staphylococcus aureus and Pseudomonas aeruginosa were used, which was or was not submitted to two methylene blue concentrations (0.1 μg/mL and 500 mg/mL) applied alone or in combination with a variety of red laser emission parameters (660 nm); in both cases, the streak was performed immediately after mixing between the photosensitizer and the solution containing the bacteria.


In the dishes with only methylene blue application neither antibacterial was produced, nor inhibition at the application points of the photodynamic therapy in the case of the bacterium Pseudomonas aeruginosa. However, in the cultures of Staphylococcus aureus in which laser emission was associated with the concentration of 500 mg/mL of the photosensitizer, inhibition was present at the laser application points.


The time of exposure to the photosensitizer prior to the application of phototherapy seems to be an essential factor for the optimized action of photodynamic therapy, especially in the case of Gram-negative bacteria.

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