In this thesis, the collective motion of Magnetospirillum magneticum strain AMB-1 is studied. Bacteria have developed various types of mechanism in order to search for the optimum environment, which are called taxis. There are two types of taxis, aerotaxis and magnetotaxis, that are involved in AMB-1. It is commonly believed that the vertical component of the Earth magnetic field helps to align the bacteria and confine them in one dimension, whereas the oxygen gradient determines their swimming velocity. In this way, magnetotaxis is conceived as a passive response, while aerotaxis is active and they work independently. Previous works done by our group may suggest an alternative picture. In this thesis, two experiments were carried out to gain a better understanding of the taxis of AMB-1. In the first experiment, bacterial suspension of density around 10⁸ cells cm⁻³ was injected into a chamber of dimensions 5×5×0.025 mm³. The motion of the bacteria was captured by a video camera mounted in the microscope under darkfield condition. Tracking of the quasi-2D motions under different applied magnetic field generated by two pairs of Helmholtz coils was done. In this way, we could determine the effect of magnetic field on basic physical properties of the bacteria, such as the diffusive motion, the rate of change of the motion direction. In the second experiment, a one dimensional oxygen gradient was achieved inside a microfluidic chamber. An aerotactic band of highly concentrated bacteria was observed under darkfield condition when bacterial suspension of density around 10⁹ cells cm⁻³. It is found that the time for the formation of the band varies under different applied magnetic fields, which suggests that the magnetotaxis in AMB-1 may not be completely passive. / 這篇論文報告關於趨磁螺菌AMB-1在集體運動上的研究。為了在野外尋找最好的生存環境，細菌發展出不同的趨向性機制。AMB-1擁有趨磁性和趨氧性兩個機制。在普遍的認知裡，地球磁場的垂直分量會把牠們的運動局限為一維，氧氣濃度梯度則影響牠們游泳的速度。趨磁性一直被認為是被動反應，而趨氧是主動的，兩者獨立運作。我課題組最近的實驗結果提出了存在另一種機制的可能性。為更了解AMB-1的趨向性機制，我們進行了兩個實驗。 / 在首個實驗中，濃度約為每立方厘米10⁸個的細菌溶液被注入到5×5×0.025立方毫米的空間。我們用攝像機把顯微鏡暗場觀測下的細菌運動記錄下來，在兩對亥姆霍茲線圈產生的不同磁場下，追蹤細菌的準二維運動。這樣，我們可以測定細菌的基本物理性質在不同磁場下的影響，例如其擴散運動、運動方向的轉換頻率等等。在第二個實驗中，我們在一個微流控空間裡建立了一維的氧氣濃度梯度。當細菌懸液濃度達到每立方厘米10⁹個時，在暗場條件下可以一條由高濃度細菌組成的趨氧性帶。我們發現帶形成所需的時間隨不同的外加磁場而改變，顯示AMB-1的趨磁性可能不是一個完全被動的反應。 / Lau, Tsz Ki = 趨磁螺菌AMB-1的集體運動觀測 / 劉子麒. / Thesis M.Phil. Chinese University of Hong Kong 2013. / Includes bibliographical references (leaves 53-58). / Abstracts also in Chinese. / Title from PDF title page (viewed on 12, October, 2016). / Lau, Tsz Ki = Qu ci luo jun AMB-1 de ji ti yun dong guan ce / Liu Ziqi. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.

The facultative denitrifying alphaproteobacterium Magnetospirillum sp. strain 15-1 had been isolated from the hypoxic rhizosphere of a constructed wetland model fed with toluene. This bacterium can catabolize toluene anaerobically but not aerobically. Here, we used strain 15-1 to investigate regulation of expression of the highly oxygen-sensitive glycyl radical enzyme benzylsuccinate synthase, which catalyzes the first step in anaerobic toluene degradation. In cells growing aerobically with benzoate, the addition of toluene resulted in a ~20-fold increased transcription of bssA, encoding for the catalytically active subunit of the enzyme. Under anoxic conditions, bssA mRNA copy numbers were up to 129-fold higher in cells growing with toluene as compared to cells growing with benzoate. Proteomics showed that abundance of benzylsuccinate synthase increased in cells growing anaerobically with toluene. In contrast, peptides of this enzyme were never detected in oxic conditions. These findings show that synthesis of benzylsuccinate synthase was under stringent post-transcriptional control in the presence of oxygen, which is a novel level of regulation for glycyl radical enzymes.

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