Hypocrellin B-Mediated Photodynamic Inactivation of Gram-Positive Antibiotic-Resistant Bacteria: An In Vitro Study

The search for alternative therapeutics against antibiotic-resistant bacteria is highly desirable. A promising approach is photodynamic antimicrobial chemotherapy. This work evaluated the photodynamic inactivation (PDI) efficacy of hypocrellin B (HB) on Gram-positive antibiotic-resistant bacteria. PDI efficacy of HB on Gram-positive standard and antibiotic-resistant Staphylococcus aureus, Enterococcus faecalis, and Streptococcus pneumonia and Gram-negative Escherichia coli and Klebsiella pneumoniae was assessed. HB photoactivity on biofilms formed by the Gram-positive bacteria and its cytotoxicity on mammalian CT26 cells were also investigated. HB showed no obvious dark toxicity, but provided concentration-dependent inactivation of bacteria and mammalian cells. After irradiation with 72 J/cm² light, 100 μM of HB achieved about 7 log₁₀ reductions in bacterial survival of Gram-positive strains, but yielded only 2 log₁₀ reductions in bacterial survival of Gram-negative strains. Gram-positive bacteria were as susceptible to PDI in biofilms as in planktonic suspensions, but the efficacy was attenuated. The results suggested that HB could serve as a potential antibacterial photosensitizer against Gram-positive antibiotic-resistant bacteria.