Photodynamic Therapy With Propolis: Antibacterial Effects on Staphylococcus aureus, Streptococcus mutans and Escherichia coli Analysed by Atomic Force Microscopy

Introduction: photodynamic therapy (pdt) is a process that uses a light source (e.g. laser), oxygen molecules and a photosensitizing agent. pdt aims to act against pathogens, including those resistant to antimicrobials. the association of pdt with natural drugs, such as propolis, has not been widely studied. methods: therefore, this study aimed to evaluate the antimicrobial effect of pdt in vitro by using propolis as a photosensitizing agent. for this purpose, the dry propolis extract was used as a photosensitizer and a low-power laser (photon laser iii model) was irradiated onto the microwells for 90 seconds. gram-positive and gram-negative bacterial strains were used in the tests at a concentration of 5 × 105 cfu/ml. initially, the antibacterial activity of the photosensitizers without laser action was determined by using a serial microdilution method before the experiment with a laser. after the incubation of the plates in a bacteriological oven, resazurin (0.1%) was added and the minimum inhibitory concentration (mic) was determined. alterations in the morphology of the bacteria were analysed by using atomic force microscopy (afm). results: bacteria were sensitive to propolis with mics ranging from 13.75 to 0.85 mg/ml, but no susceptibility was observed for methylene blue without laser application. a change was observed for mic values of propolis against staphylococcus aureus after irradiation, which decreased from 1.71 mg/ml to 0.85 mg/ml. however, this behaviour was not observed in escherichia coli, the only gram-negative strain used. in addition, afm images revealed alterations in the size of one of the bacteria tested. conclusion: the propolis is more active against gram-positive bacteria and pdt improved its activity against one of the strains tested.