Paradoxical potentiation of methylene blue-mediated antimicrobial photodynamic inactivation by sodium azide: Role of ambient oxygen and azide radicals

► Azide anion potentiates antibacterial photoinactivation mediated by methylene blue. ► Azide inhibits antibacterial photoinactivation mediated by PEI-ce6 conjugate. ► Azide potentiates killing by Fenton reagent but inhibits killing by H2O2/NaOCl. ► Azide allows killing by photoactivated methylene blue in the absence of oxygen. ► Spin trapping demonstrates the involvement of azidyl radicals.