Biophysical Insights Into The Antibacterial Mechanism Of Copper

Due to its outstanding antibacterial properties and not easily leading to antibiotic resistance, copper has been found great potential application in healthcare setting, food industry, agriculture and other place which need to be clean or sterilize. However, the exact action mechanisms of copper to bacteria is rather complex and has not fully understood so far.In the thesis, kinds of biophysical tools, including synchrotron Fourier transform infrared(FTIR) spectroscopy, atomic force microscopy(AFM)-based nanomechanics measurements and artificial phospholipid bilayer were used to investigate the effect of CuCl2 on the model system bacteria Escherichia coli(E. coli). PeakForce quantitative nano-mechanics mode(PF-QNM) AFM showed that copper ions(Cu2+) have greatly changed the topography and mechanical properties of bacterial cells. After exposed to CuCl2 along with time, the surface morphology of E. coli became uneven and exhibited protrusions, while its modulus and stiffness properties continuously decreased. Interestingly, the adhesion force between cells surfaces and the AFM tip increased first and then decreased. These results indicated chemical compositions of bacterial cells might alter following the copper treatment. To test the hypothesis, FTIR was applied to detect the Cu2+-induced biochemical changes in E. coli. The FTIR spectral results revealed that Cu2+ treatment could affect the membrane lipids, nucleic acids, peptidoglycans and proteins in E. coli. Importantly, for short-time treated cells, the effects on phospholipids composition were clearly shown while no significant alterations of proteins, nucleic acids and peptidoglycans were found. In order to further explore the interaction copper ions with membrane lipids, high resolution AFM observation were carried out on the artificial membrane constructed by E. coli extract lipids. AFM images showed that high concentration(20 mM) of copper ions can induce phase separation in lipid bilayer, but low concentration(under 4mM) of Cu2+ as well as other bivalent metal ions(Ca2+ and Zn2+) can’t cause this phenomenon. In addition, PO43- can not only inhibit and relieve the domain formation induced by Cu2+ but alleviate the antibacterial effect of copper, suggesting that copper ions might interact with the phosphate group of lipid and the Cu2+-induced domain formation may be associated with its antibacterial effect.In conclusion, the action of copper ions on bacterial cells is multifactorial rather than aims at one target. In particularly, copper ions affect the survival of E. coli by target the membrane lipid and induce phase separation of the lipid bilayer.