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Magnetite nanoparticles and their assembly comprise a new area of development for new technologies. The magnetic particles can interact and assemble in chains or networks. Magnetotactic bacteria are one of the most interesting microorganisms, in which the assembly of nanoparticles occurs. These microorganisms are a heterogeneous group of gram negative prokaryotes, which all show the production of special magnetic organelles called magnetosomes, consisting of a magnetic nanoparticle, either magnetite (Fe3O4) or greigite (Fe3S4), embedded in a membrane. The chain is assembled along an actin-like scaffold made of MamK protein, which makes the magnetosomes to arrange in mechanically stable chains. The chains work as a compass needle in order to allow cells to orient and swim along the magnetic field of the Earth.
The formation of magnetosomes is known to be controlled at the molecular level. The physico–chemical conditions of the surrounding environment also influence biomineralization. The work presented in this manuscript aims to understand how such external conditions, in particular the extracellular oxidation reduction potential (ORP) influence magnetite formation in the strain Magnetospirillum magneticum AMB-1. A controlled cultivation of the microorganism was developed in a bioreactor and the formation of magnetosomes was characterized.
Different techniques have been applied in order to characterize the amount of iron taken up by the bacteria and in consequence the size of magnetosomes produced at different ORP conditions. By comparison of iron uptake, morphology of bacteria, size and amount of magnetosomes per cell at different ORP, the formation of magnetosomes was inhibited at ORP 0 mV, whereas reduced conditions, ORP – 500 mV facilitate biomineralization process.
Self-assembly of magnetosomes occurring in magnetotactic bacteria became an inspiration to learn from nature and to construct nanoparticles assemblies by using the bacteriophage M13 as a template. The M13 bacteriophage is an 800 nm long filament with encapsulated single-stranded DNA that has been recently used as a scaffold for nanoparticle assembly. I constructed two types of assemblies based on bacteriophages and magnetic nanoparticles. A chain – like assembly was first formed where magnetite nanoparticles are attached along the phage filament. A sperm – like construct was also built with a magnetic head and a tail formed by phage filament.
The controlled assembly of magnetite nanoparticles on the phage template was possible due to two different mechanism of nanoparticle assembly. The first one was based on the electrostatic interactions between positively charged polyethylenimine coated magnetite nanoparticles and negatively charged phages. The second phage –nanoparticle assembly was achieved by bioengineered recognition sites. A mCherry protein is displayed on the phage and is was used as a linker to a red binding nanobody (RBP) that is fused to the one of the proteins surrounding the magnetite crystal of a magnetosome.
Both assemblies were actuated in water by an external magnetic field showing their swimming behavior and potentially enabling further usage of such structures for medical applications. The speed of the phage - nanoparticles assemblies are relatively slow when compared to those of microswimmers previously published. However, only the largest phage-magnetite assemblies could be imaged and it is therefore still unclear how fast these structures can be in their smaller version.
Magnetotactic bacteria possess an intracellular structure called the magnetosome chain. Magnetosome chains contain nano−particles of iron crystals enclosed by a membrane and aligned on a cytoskeletal filament. Due to the presence of the magnetosome chains, magnetotactic bacteria are able to orient and swim along the magnetic field lines. A detailed study of structural properties of magnetosome chains in magnetotactic bacteria has primary scientific interests. It can provide more insight into the formation of the cytoskeleton in bacteria. In this thesis, we develop a new framework to study the structural properties of magnetosome chains in magnetotactic bacteria.
First, we address the bending stiffness of magnetosome chains resulting from two main contributions: the magnetic interactions of magnetosome particles and the bending stiffness of the cytoskeletal filament to which the magnetosomes are anchored. Our analysis indicates that the linear configuration of magnetosome particles without the stabilisation to the cytoskeleton may close to ring like structures, with no net magnetic moment, which thus can not perform as a compass in cellular navigation. As a result we think that one of the roles of the filament is to stabilize the linear configuration against ring closure.
We then investigate the equilibrium configurations of magnetosome particles including linear chain and closed−ring structures. We notably observe that for the formation of a stable linear structure on the cytoskeletal filament, presence of a binding energy is needed. In the presence of external stimuli the stability of the magnetosome chain is due to the internal dipole−dipole interactions, the stiffness and the binding energy of the protein structure connecting the magnetosome particles to the filament. Our observations, during and after the treatment of the magnetosome chain with the external magnetic field substantiates the stabilisation of magnetosome chains to the cytoskeletal filament by proteinous linkers and the dynamic feature of these structures.
Finally, we employ our model to study the FMR spectra of magnetosome chains in a single cell of magnetotactic bacteria. We explore the effect of magnetocrystalline anisotropy in three-fold symmetry observed in FMR spectra and the peculiarity of different spectra arisen from different mutants of these bacteria.