Inactivation of Staphylococcus aureus by antimicrobial photodynamic therapy using 1,9-Dimethyl-Methylene Blue: in vitro and in vivo studies

The efficiency of antibiotics in terms of their bacterial inhibition is well known. However, studies show that its overuse, underuse, and misuse induce antimicrobial resistance, promoting the need to work with alternative methods. In this sense, antimicrobial photodynamic therapy (aPDT) is a promising selective method demonstrating excellent response. This study aimed to evaluate the antimicrobial action promoted in Staphylococcus aureus using 1,9 dimethyl methylene blue dye (DMMB) combined with red LED (λ 630 ± 20 nm, CW, 125 mW, 12 J/ cm², 192 s) in planktonic culture and rats skin wounds contaminated with staphylococcal bacteria. The experimental in vitro and in vivo groups were Control, LED, DMMB, and LED + DMMB; after aPDT, the triplicate samples for each dilution were incubated for 24 h, and the number of bacteria was determined by counting the colony-forming units, and the logarithm (CFU/mL log). Based on in vitro data obtained, the LED + DMMB group, when compared to the Control, showed a reduction in microbial load of 99.943% (p < 0.0001), with decimal reduction (RD = 3). Whereas in vivo results, the same comparing groups demonstrated a reduction in microbial load, reaching 99.994% (RD = 4). In this research, the aPDT was a unique treatment, and it is possible to repeat it to obtain higher microbial reduction, providing an alternative therapeutic that can be clinically validated to combat infections caused by S. aureus.