Influence of strain differences on resistance of Staphylococcus aureus to photodynamic action usingmeso-substituted cationic porphyrins

Background and Objectives. Infections associated with antibiotic-resistant strains of microorganisms, including Staphylococcus aureus, pose the greatest danger in nasopharyngeal diseases and post-surgical complications. A number of studies have shown that there are interstrain differences in the sensitivity of clinically significant microorganisms to the damaging effects of antimicrobial photodynamic therapy. In this work, in order to quantify interstrain differences in the responses of bacterial cells to photodynamic exposure, we studied the effectiveness of pyridyl porphyrin compounds in combination with LED radiation against three strains of Staphylococcus aureus. Materials and Methods. The objects of the study were: methicillin-sensitive museum strain S. aureus 209 P, methicillin-sensitive clinical strain S. aureus 5, methicillin-resistant clinical strain S. aureus 11. A LED with a maximum emission spectrum at a wavelength of $\lambda$ = 405 nm and a half-width of 30 nm at a level of 0.1 of the maximum intensity, a power of 1.8 W and an integrated power density of 80 mW/cm$^2$ was used as a radiation source. In all experiments, the radiation mode was continuous. The irradiation time varied from 5 to 30 min (irradiation doses from 24 to 144 J/cm$^2$ , respectively). Watersolublemeso-substituted cationic pyridylporphyrins were used as photosensitizers: meso-tetrakis(N-(2'-hydroxyethyl)pyridinium-4-yl)porphyrin chloride (H2TOE4PyP) and its Zn(II) derivatives - zinc-meso-tetrakis [4-N-(2'-oxyethyl) pyridyl] porphyrin (Zn-TOE4PyP), zinc-meso-tetrakis [3-N-butyl pyridyl] porphyrin (Zn-TBut3PyP). To assess the level of oxidative stress and the tolerance of microorganisms to it, two different methods were used: 1) a method for determining the minimum inhibitory concentration of hydrogen peroxide, and 2) a method for determining the activity of bacterial catalase. Results and Discussion. The greatest sensitivity to the action of LED radiation was demonstrated by cells of the clinical methicillin-resistant strain S. aureus 11, activated by pyridyl porphyrins. It has been shown that when photosensitizers are used in concentrations of 0.01-0.03 mg/ml after 30 minutes of irradiation, a decrease in the number of cells of this strain occurs by 4.8 lgCFU/ml. It has been found that the activity of catalase in the cells of the methicillin-resistant strain S. aureus 11 is 17$\%$ lower compared to the activity of catalase in the cells of the standard strain S. aureus 209 P. This indirectly indicates the greater sensitivity of the strain S. aureus 11 to reactive oxygen species, formed during antimicrobial photodynamic exposure. Conclusion. It has been found that the differences in population reduction between strains range from 1.7 to 2.3 lgCFU/ml at the maximum irradiation dose, depending on the pyridylporphyrin modification used. It has been shown that the antibiotic-resistant strain S. aureus 11, which is highly sensitive to the action of ROS in the form of hydrogen peroxide and incapable of active production of catalase, is most susceptible to the complex action of LED radiation (405 nm) in combination with photosensitizers in the form of zinc- meso-tetrakis[3-N-butyl pyridyl]porphyrin (Zn-TBut3PyP).