Nosocomial infections are a major threat to modern therapeutics. The major causative agent of these infections is multidrug-resistant gram-negative bacteria, which impart high morbidity and mortality rate. This has led to an urge for the development of new antibiotics. Antimicrobial photodynamic therapy is a promising strategy to which till date no resistant strain has been reported. Since the efficacy of photodynamic therapy largely depends on the selection and administration of an appropriate photosensitizer, therefore, the realization of clinically active photosensitizers is an immediate need. Here, by using E. coli as a study model we have demonstrated the antimicrobial photodynamic potential of riboflavin. Intracellular ROS formation by DCFH-DA assay, lipid peroxidation, protein carbonylation, LDH activity was measured in treated bacterial samples. Enzymatic (SOD, CAT, GSH) antioxidants and non-enzymatic (GSH) was further evaluated. Bacterial death was confirmed by colony forming assay, optical microscopy and scanning electron microscopy. The treated bacterial cells exhibited abundant ROS generation and marked increment in the level of oxidative stress markers as well as significant reduction in LDH activity. Marked reduction in colony forming units was also observed. Optical microscopic and SEM images further confirmed the bacterial death. Thus, we can say that photoilluminated riboflavin renders the redox status of bacterial cells into a compromised state leading to significant membrane damage ultimately causing bacterial death. This study aims to add one more therapeutic dimension to photoilluminated riboflavin as it can be effectively employed in targeting bacterial biofilms occurring on hospital wares causing several serious medical conditions.

Riboflavin (RF) plays an important role in various metabolic redox reactions in the form of flavin adenine dinucleotide and flavin mononucleotide. Human serum albumin (HSA) is an important protein involved in the transportation of drugs, hormones, fatty acid and other molecules which determine the biodistribution and physiological fate of these molecules. In this study, we have investigated the interaction of riboflavin RF with HSA under simulative physiological conditions using various biophysical, calorimetric and molecular docking techniques. Results demonstrate the formation of riboflavin–HSA complex with binding constant in the order of 10⁴ M⁻¹. Fluorescence spectroscopy confirms intermediate strength having a static mode of quenching with stoichiometry of 1:1. Experimental results suggest that the binding site of riboflavin mainly resides in sub-domain IIA of HSA and that ligand interaction increases the α-helical content of HSA. These parameters were further verified by isothermal titration calorimetry ITC which confirms the thermodynamic parameters obtained by fluorescence spectroscopy. Molecular docking was employed to suggest a binding model. Based on thermodynamic, spectroscopic and computational observations it can be concluded that HSA-riboflavin complex is mainly stabilized by various non-covalent forces with binding energy of −7.2 kcal mol⁻¹.