1,9-Dimethyl-methylene Blue-Based Antimicrobial Photodynamic Inactivation: Sulfate-Reducing Bacteria Models Isolated...

Sulfate-reducing bacteria (SRB) are responsible for biogenic acidification, which negatively impacts oil quality and presents significant economic and environmental challenges for the oil industry. The improper use of biocides often leads to the emergence of biocide-resistant SRB strains, posing additional risks of contaminating aquatic environments. This study aimed to evaluate antimicrobial photodynamic inactivation (API) as a control strategy for a consortium of biocide-resistant sulfate-reducing bacteria collected from an oil extraction field in the Recôncavo Baiano Basin, Brazil, under conditions representative of oilfield microbial communities. 1,9-Dimethyl-methylene blue (DMMB) was used as a photosensitizer at concentrations of 1.0, 1.5, and 2.0 μg/mL, activated by LED light (λ 630 ± 10 nm, CWContinuous wave, 125 mW) with energy densities of 8.0, 10.0, 12.0, 14.4, and 21.6 J/cm². API using LED reduced bacterial log counts by up to 72.86% (0.57 log) after 9 min of treatment. The antimicrobial effects are consistent with a Type I API mechanism, involving the generation of reactive oxygen species (ROS) and hydroxyl radicals. The incomplete growth inhibition may be explained by complex interactions between DMMB and the lipopolysaccharide layers present in SRB of the genus Desulfovibrio. Additionally, H₂S generated by SRB acted as a quencher, limiting the activity of singlet oxygen. These findings emphasize the intricate relationship between the choice of photosensitizer, the photoactivation conditions, and the presence of quenchers. They also contribute to understanding API performance in complex anaerobic microbial consortia and highlight its potential application for SRB control in oilfield environments.