Influence of photodynamic action on pure and mixed cultures of gram-negative bacteria: related to growth mechanisms

Gram-negative bacteria present a significant challenge to conventional treatments due to their structurally complex cell walls, setting them apart from their gram-positive counterparts. These structural distinctions, coupled with various antimicrobial resistance mechanisms, make them highly resilient. Photodynamic inactivation (PDI) has emerged as a promising technique to address this challenge, capitalizing on oxidative stress induced by the synergy of light and a photosensitizer (PS). In this study, we delved into the application of PDI and its repercussions on both Klebsiella pneumoniae and Escherichia coli bacteria, in both pure cultures and mixed populations. Our investigation encompassed an analysis of changes in growth curves when influenced by photodynamic and the response to the susceptibility to gentamicin. Methods: Various concentrations (50 mu M, 100 mu M, and 150 mu M) of synthetic curcumin solutions served as PS in treatment groups. Additionally, solvents such as dimethyl sulfoxide and 0.1% sodium dodecyl sulfate (SDS) were evaluated to enhance PS mobility and absorption. Growth curves for pure and mixed cultures were established, both pre- and post-PDI, and in the presence of 0.1% SDS. Blue light irradiation at 30 J cm-2 and 450 nm was employed. The minimum inhibitory concentration (MIC) of the antibiotic was determined with and without PDI + SDS. Results: While PDI did not achieve optimal bacterial reduction for these strains, it did introduce oxidative damage that has the potential to affect other critical aspects of the temporal progression of cultures and their responses to antibiotics. Conclusion: This study shows that even with the minimal impact of PDI in the presence of 0.1% SDS, observable alterations in bacterial growth profiles and MIC values occur and can be used in favor of treatment involving such infections.