Study On Antibacterial Mechanism Of Photosensitizers Against Bacteria

Photodynamic antimicrobial chemotherapy(PACT)is one of the promising techniques to address the prevalent challenge of antibiotic resistance.In the presence of light and molecular oxygen,photosensitizer(PS)generates reactive oxygen species(ROS)such as singlet oxygen or hydroxy radical,and kills microbial cells nearby.Unlike antibiotics,photosensitizers can destroy bacteria effectively at multiple targeting sites through binding to the extracellular or intracellular localizations,thus,inactivate microorganisms without inducing the appearance of drug-resistant pathogens.Photosensitizer plays a critical role in the effectiveness of PACT.However,quantitative structure-activity relationship(QSAR)and antibacterial mechanism of PS against bacteria have not been well understood.In this study,we evaluated the antibacterial effect and mechanisms of a series of PSs carrying different number of positive charges(ZnPc(Lys)n,n=3,5,7).We found the adsorbed amount of PSs,the zeta potential of PS-bound bacteria,and the hydrophobic rate of the PS-bound bacteria were all proportional to the number of positive charges on PSs.Interestingly,ZnPc(Lys)5 possessed stronger antibacterial effect than the other two PSs,showing that the antibacterial efficacy is not proportional to the number of positive charges on PSs.We next measured the aggregation state of the PSs on bacterial surface by two different methods.The results showed the pentalysine PS(ZnPc(Lys)5)had higher fraction of monomer(62.0%)compared to the other two PSs(37.9%for tri-lysine PS and 33.9%for hepta-lysine PS).Thus,this work provides clear evidence to demonstrate an additional property of PSs,besides positive charges,in determining the antibacterial efficacy:the aggregation state of PS on bacterial surface.We also studied bacterial membrane permeability and bacterial signaling pathways affected by PACT effect.The results showed that PS damaged bacteria membrane and induced up-regulation of transcription levels of sodA,oxyR and recA genes and down-regulation of transcription level of lexA gene,showing the antibacterial mechanisms of PS against bacteria.