Photodynamic inactivation of bacteria: Why it is not enough to excite a photosensitizer.

BACKGROUND:The development of multidrug resistance (MDR) in infectious agents is one of the most serious global problems facing humanity. Antimicrobial photodynamic therapy (APDT) shows encouraging results in the fight against MDR pathogens, including those in biofilms. METHODS:Photosensitizers (PS), monocationic methylene blue, polycationic and polyanionic derivatives of phthalocyanines, electroneutral and polycationic derivatives of bacteriochlorin were used to study photodynamic inactivation of Gram-positive and Gram-negative planktonic bacteria and biofilms under LED irradiation. Zeta potential measurements, confocal fluorescence imaging, and coarse-grained modeling were used to evaluate the interactions of PS with bacteria. PS aggregation and photobleaching were studied using absorption and fluorescence spectroscopy. RESULTS:The main approaches to ensure high efficiency of bacteria photosensitization are analyzed. CONCLUSIONS:PS must maintain a delicate balance between binding to exocellular and external structures of bacterial cells and penetration through the cell wall so as not to get stuck on the way to photooxidation-sensitive structures of the bacterial cell.