Photodynamic inactivation of microorganisms (aPDI) is an excellent method to destroy antibiotic-resistant microbial isolates. The use of an exogenous photosensitizer or irradiation of microbial cells already equipped with endogenous photosensitizers makes aPDI a convenient tool for treating the infections whenever technical light delivery is possible. Currently, aPDI research carried out on a vast repertoire of depending on the photosensitizer used, the target microorganism, and the light delivery system shows efficacy mostly on in vitro models. The search for mechanisms underlying different responses to photodynamic inactivation of microorganisms is an essential issue in aPDI because one niche (e.g., infection site in a human body) may have bacterial subpopulations that will exhibit different susceptibility. Rapidly growing bacteria are probably more susceptible to aPDI than persister cells. Some subpopulations can produce more antioxidant enzymes or have better performance due to efficient efflux pumps. The ultimate goal was and still is to identify and characterize molecular features that drive the efficacy of antimicrobial photodynamic inactivation. To this end, we examined several genetic and biochemical characteristics, including the presence of individual genetic elements, protein activity, cell membrane content and its physical properties, the localization of the photosensitizer, with the result that some of them are important and others do not appear to play a crucial role in the process of aPDI. In the review, we would like to provide an overview of the factors studied so far in our group and others that contributed to the aPDI process at the cellular level. We want to challenge the question, is there a general pattern of molecular characterization of aPDI effectiveness? Or is it more likely that a photosensitizer-specific pattern of molecular characteristics of aPDI efficacy will occur?

Background: Streptococcus agalactiae, referred to as Group B Streptococcus (GBS), is a prominent bacterium causing life-threatening neonatal infections. Although antibiotics are efficient against GBS, growing antibiotic resistance forces the search for alternative treatments and/or prevention approaches. Antimicrobial photodynamic inactivation (aPDI) appears to be a potent alternative non-antibiotic strategy against GBS. Methods: The effect of rose bengal aPDI on various GBS serotypes, Lactobacillus species, human eukaryotic cell lines and microbial vaginal flora composition was evaluated. Results: RB-mediated aPDI was evidenced to exert high bactericidal efficacy towards S. agalactiae in vitro (>4 log₁₀ units of viability reduction for planktonic and >2 log₁₀ units for multispecies biofilm culture) and in vivo (ca. 2 log₁₀ units of viability reduction in mice vaginal GBS colonization model) in microbiological and metagenomic analyses. At the same time, RB-mediated aPDI was evidenced to be not mutagenic and safe for human vaginal cells, as well as capable of maintaining the balance and viability of vaginal microbial flora. Conclusions: aPDI can efficiently kill GBS and serve as an alternative approach against GBS vaginal colonization and/or infections.