Isolation And Characteristics Of Magnetotactic Bacterium MG-1

Magnetotactic bacteria represent a group of prokaryote which could passively align and actively swim along geomagnetic field line. Magnetotactic bacteria synthesize nano-sized ferromagnetism magnetosomes aligned in lines, while diverse from each other in morphology, physiology, and phylogenetics.It is quite difficult to cultivate magnetotactic bacteria, for their strict demand of oxygen concentration gradient and chemistry conditions. There are only several magnetotactic bacteria strains have been cultivated so far, and all of them are obligate microaerobe or obligate anaerobe or facultative of both. Because of this problem, most magnetotactic bacteria have not established efficient genetic system, and it is not clear about the functional genes, proteins and mechanism involved in the synthesis of magnetosomes; the technique of isolating and purifying magnetosomes still requires further perfection; and the application of magnetosomes requires further researches.The purpose of this research is to isolate and cultivate characteristic strain which could be easily cultivated and could synthesize a relatively large number of magnetosomes; to optimize the conditions for synthesize magnetosomes and the technique for isolating and purifying magnetosomes.Many kinds of magnetotactic bacteria were enriched from paleosol samples with relatively high susceptibility for the first time. After collecting magnetotactic bacteria, isolating culture medium and colony by the home-made apparatus , two strains were isolated in pure culture, which were denominated as MG-1 and MG-2 respectively. Both of them were magnetic-sensitive bacteria which contain 2-8 of intracellular electron dense particles. Analysis of 16SrDNA sequence indicated that MG-1 strain was related to Acinetobacter within theα-Proteobacteria, while MG-2 strain was related to Bacillus within theγ-Proteobacteria. Inspection under transmission electron microscope(TEM) revealed the presence of eight electron dense particles in the MG-1 cells, which were circular-like and about 80nm in diameter.The energy dispersive X-ray spectrum of electron-dense-particle region of MG-1 cells indicated that it consisted of 10.23% Fe, which was about 24 times as the electron-dense-particle-free region.MG-1 cells could migrate along the magnetic field to its S pole in semisolid medium, which showed its magnetotaxis.The dry culture of MG-1 cell which grew on agar plate could stick to magnet.Magneto-thermal gravity analysis (M-TGA) showed that MG-1 cells contain ferromagnetic crystals, whose Curie temperature (Tc) were about 95.82℃.Added dry culture of MG-1 cells to the loess (low susceptibility) sample at the proportion of 1:7, the result showed that the susceptibility of loess sample was evidently enhanced, and the average amplification reached 16.13%.Inoculated and cultivated MG-1 strain into paleosol sample, the magnetic susceptibility of paleosol sample was increased as the increase of inoculum size of MG-1. The result predicated that biogenetic magnetosomes was an indispensable factor among the mechanisms for the susceptibility enhance of paleosol during the far-flung geologic time.The magnetic minerals of the paleosol sample whose susceptibility had been enhanced by cultivation of MG-1 were magnetically separated. Then the particle-size distribution of magnetic minerals was analysed by JL-1155 Laser Particle Size Analyzer. The result showed that the average grain size of magnetic minerals in the experiment sample was reduced obviously, for the SD and PSD sized magnetic particles were increased sharply wherease the MD sized magnetic particles were reduced sharply. So it could be speculated that the mechanism which from the view of grain-size dependence for the enhanced susceptibility of paleosol by MG-1 has two aspects: one is that MG-1 could mineralize magnetosomes which increased the amount of SSD magnetic minerals whose susceptibility were relatively high; the other is that the growth of MG-1 and its magnetosomes synthesis required largely uptake of ion, it was speculated that MG-1 could generate extracellular enzyme or metabolite which could degrade the magnetic minerals, and this would lead to the decrease of size of MD, PSD, and SSD magnetic minerals to particles with less magnetic domain which had higher susceptibility.All the results above had proven that strain MG-1 taken on typical characteristic of magnetotactic bacteria (synthesize nano-sized electron-dense magnetosomes which aligned in lines in cells; the cells have magnetotaxis and magnetism), and MG-1 is a novel strain of magnetotactic bacteria.The medium composition and culture condition for the growth of cells and synthesis of magnetosomes were optimized through orthogonal experiments.Combined the home-made vertical apparatus for magnetic separation of magnetosomes with French pressure and high speed centrifugation, we successfully purified relatively pure magnetosomes with their membrane intact.The achievements of this research provide significant groundwork for the further work. We are aimed to reveal genes and protein involved in the synthesis of magnetosomes, strive to construct genetic engineering bacteria so that they could be easily cultivated and synthesize magnetosomes effectively; carry on the research on application of magnetosomes, especially on the linkage with bioactive substances (enzyme, antibody, medicines et al.) so as to be used in biodetection, targeted therapy of tumors and so on.