The increased levels of antibiotic resistance and the growing number of infections hard or impossible to treat has brought tackling antibiotic resistance as the main priority for the World Health Organization [1,2]. The antimicrobial consumption and the frequency of resistance among high-priority pathogens have been methods of monitoring the risks and the emerging resistance. In this sense, antimicrobial photodynamic therapy (aPDT) has gained attention due to its broad-spectrum activity and the absence of selection of microbial-resistant strains [3], [4], [5], [6], [7].

The antimicrobial effect of aPDT is based upon the generation of a burst of oxidative species, after the light activation of a photosensitizer. One of the most studied photosensitizers is methylene blue (MB), due to its low cost and high effectivity [5,[8], [9], [10], [11], [12], [13], [14]]. Unfortunately, the aPDT clinical studies with MB are not always related to strong antimicrobial effect as seen in vitro (yeast/biofilm inactivation), while the clinical outcomes are mild [15], [16], [17]. The media in which MB is conveyed may induce self-association (H-aggregates formation) and affect the absorption spectra (a property called metachromacy), and/or the use is associated with the presence of body fluids such as salive or exudate, affecting the photodynamic action.

Some groups have already reported the optical and photochemical effects of the combination of MB and sodium dodecyl sulfate (SDS) [18], [19], [20], [21]. Recently, this group has shown that MB-SDS presented an improved action of aPDT [22], [23], [24], [25], [26]. However, it is still not known which are the best light parameters (such as irradiance and radiant exposure - RE), to reach the most effective protocols.

In general, there is a divergence on the best parameters to be applied in aPDT with MB. For example, Queiroga et al. [27] activated MB solutions at 150 mg/L with an irradiance of 1000 mW/cm2, a RE of 60 to 180 J/cm2, and 660 nm for different Candida ssp. strains inactivation. Sousa et al. [28] while using the same concentration of MB, applied 3300 W/cm2 and 426 J/cm2 also at 660 nm, for Candida albicans ATCC 10,231. In contrast, Sabino et al. [11] worked with MB 100 μM (which is equivalent to 32 mg/L) at 100 mW/cm2 and 660 nm, finding a reduction in the number of CFU/mL of 3.0 log at 25 J/cm2 for Candida albicans ATCC 90,028. Additionally, Souza et al. evaluated a 660 nm, 92 mW/cm2 laser at different REs (15.8; 26.3 and 39.5 J/cm2) for the antimicrobial photodynamic action of MB in Candida albicans (ATCC 18,804) and noticed that the increase in RE increased the degree of the microbial reduction [29]. The literature has shown that the best parameters for an effctive antmicrobial effect of aPDT depend on some factors, such as the photosensitizing substance used [30], the medium in which PS is conveyed [[22], [23]], ligh parameters used [9], the growth phase [[31], [32]] and the organization (planktonic or biofilm) [[22], [33]].

This work aimed to evaluate the light parameters (irradiance and RE) of aPDT with MB when conveyed in water compared with MB associated with SDS.