| Abstract [eng] |
The integrity of biological membranes is essential for normal cellular function; therefore, pore-forming toxins that disrupt membrane structure and increase permeability have significant biological importance. One of these toxins is streptolysin O, a protein produced by bacteria of the Streptococcus genus and belonging to the family of cholesterol-dependent cytolysins. It selectively binds to cholesterol-containing lipid membranes, oligomerizes on their surface, and forms pores, thereby disrupting membrane integrity. The aim of this master’s thesis was to investigate the effect of streptolysin O on artificial lipid membranes and to evaluate the dependence of its activity on membrane composition and environmental pH. Tethered bilayer lipid membranes formed on gold surfaces were used as model systems due to their high mechanical and electrical stability. The electrical properties of the membranes were analyzed using electrochemical impedance spectroscopy, a sensitive and non-destructive method for monitoring changes in membrane integrity and ionic permeability. The membranes were composed of different ratios of DOPC, DOPE, and cholesterol to systematically modulate membrane surface hydrophilicity and structural properties. The effect of streptolysin O was studied under different pH conditions (pH 5.0, 6.0, 7.0, and 8.0) using a constant toxin concentration of 20 pM and an incubation time of 30 minutes. The results showed that before toxin exposure, the membranes exhibited high electrical resistance and clear capacitive behavior characteristic of intact lipid bilayers. After streptolysin O treatment, all membranes demonstrated a decrease in impedance modulus, reduced membrane resistance, and phase angle changes, indicating pore formation. DOPE-rich membranes with lower hydrophilicity were more sensitive to the toxin, whereas DOPC-rich, more hydrated membranes showed greater resistance. The strongest streptolysin O activity was observed under acidic conditions (pH 5.0), while increasing pH to 8.0 gradually reduced its effect. These findings demonstrate that electrochemical impedance spectroscopy combined with tethered bilayer lipid membranes is a suitable and sensitive system for studying streptolysin O interactions with lipid membranes. |
