Photodynamic inactivation increases cell death rate on persistent <i>Staphylococcus aureus</i>

Bacterial persistence is characterized by a subpopulation of metabolically dormant cells that exhibit transient tolerance to antibiotics, contributing to chronic and recurrent infections, particularly in Staphylococcus aureus, a pathogen responsible for severe infections. This phenomenon is evidenced by a biphasic killing curve, where an initial rapid decline is followed by a slowed death phase. Photodynamic inactivation (PDI) represents a promising strategy for microbial eradication through the generation of reactive oxygen species (ROS). This study investigated persistence formation in two S. aureus strains and evaluated the effects of PDI using curcumin. Time-kill assays with oxacillin revealed biphasic killing curves, indicative of persistence. Heritability testing confirmed that persistence was not passed on to progeny, supporting its phenotypic nature. PDI was performed using curcumin and blue light (450 nm), resulting in a dose-dependent reduction in bacterial viability. However, populations that survived PDI exhibited tolerance-like behavior, with unchanged MIC values, suggesting that ROS generated during PDI may induce a transient dormant state. Notably, post-PDI time-kill assays conducted after metabolic recovery showed a higher rate of bacterial death, indicating enhanced antibiotic susceptibility. In contrast, methicillin-resistant strains (MRSA) showed limited persistence induction, likely due to enhanced oxidative stress defenses. These are important to the understanding of bacterial physiological states when designing therapeutic strategies. The timing of antibiotic administration relative to PDI treatment plays a crucial role in treatment efficacy, which may be either enhanced or compromised depending on bacterial adaptation and recovery dynamics.