Formation and positioning of the magnetosome chain in Magnetospirillum Magneticum AMB-1

Abstract

Magnetotactic bacteria are a group of prokaryotes that share the ability to align with external magnetic fields, due to the presence within their cytoplasm of one or several chains of nanometer-sized magnetic crystals called the magnetosomes. The orientation of the chain within the cell is critical for magnetotaxis, which allows these bacteria to swim along the geomagnetic field lines. To do so, the magnetic moment and thus the chain need to lie parallel to the swimming direction which, for elongated bacteria such as AMB-1, is roughly parallel to the long axis of the cell. In most studies, the alignment between the magnetic moment and the cell axis is taken for granted, however no precise measurement has been performed to confirm this. In this thesis, experiments performed to test this assumption are presented, and the results show that for most studied bacteria the alignment is not perfect. The effect on the orientation distributions is discussed and accounted for in the analysis performed to measure the magnetic moment of individual bacteria.

A second project presented in this thesis is focused on the biomineralization process in AMB-1. Magnetotactic bacteria synthesize crystals characterized by a well-controlled morphology and a high chemical purity, which makes them interesting for biomedical applications. To study how these crystals are produced, we used scanning trans- mission X-ray microscopy, and preliminary results show that this tool is suitable for studying this complex process. The methods developed and improved during this MSc to perform these experiments are presented, and the first results show an evolution in the spectroscopy of the magnetosomes as they grow.