Isolation, Identification And Characteristics Of A Magnetotactic Bacterium WM-1 From Freshwater Sediments

Magnetotactic bacteria are capable of forming intracellular magnetosomes, which in most magnetotactic bacteria are nanometer-sized, membrane-bound, and ferromagnetic single-domain (SD) particles. The magnetosomes, generally arranged in a chain, as a biomagnetic compass-needle, enable orientation of the bacterium in the geomagnetic field. The magnetosomes produced by magnetotactic bacteria have good biocompatibility without cytotoxicity, can be used for material, bioengineering, clinic medicine, waster treatment, physics, and electrochemistry. Magnetotactic bacteria have become a foreland in microbiology and attract the interest of multiple disciplinary researchers including microbiology, physics, chemistry, geology, and materials science. However, the studies have been limited with few fastidious bacteria which considerably hamper the progress in the understanding of the biogenesis and physiological function of the magnetosomes.Sediment was collected from the site of North Lake, located in qingshan of Wuhan, China. The sample was enriched with enrichment medium for one month, and magnetotactic bacteria were collected through a modified collecting apparatus. Subsequently, a magnetotactic bacterium strain WM-1 was isolated in pure culture. Strain WM-1 was helical in shape, and 0.2-0.4 by 3.0-4.0μm. There were fifteen electron dense particles in strsin WM-1 by TEM, which were linear arrangement. The energy dispersive X-ray spectrum of the electron dense particles indicated that the magnetosomes consisted of iron oxide. There was 1.2% iron in the dried cell if WM-1 by EDX. The data of selected area electron diffraction of magnetosomes were identical to the data of the PDF card (19-0629), indicated that the magnetosomes formed by strain WM-1were magnetite (Fe3O4). The statistical analysis of the magnetosomes showed the average length of magnetosomes in WM-1 was 54±12.3 nm, the average was 43±10.9 nm, the average number of magnetosomes in each WM-1 was 8±3.4.Strain WM-1 was Gram-negative, able to produce magnetosomes when growing in microaerobic condition, and could not produce magnetic partcles in aerobic condition. Some biophsiological and biochemical characteristics among strains WM-1, WD-1, and MS-1 showed much dissimilarity. Analysis of 16S rDNA sequences indicated that strain WM-1 was within the Alpha-Proteobacteria. The affinity between WM-1 and genus Magnetospirillum was much close. The G+C content of WM-1 was 65.7% mol% indicated the WM-1 fell into genus Magnetospirillum according to Bergey's Manual of Systematic Bacteriology (2004). The homology between WM-1 and Magnetospirillum AMB-1 was 97.6%. Stachebrandt had raised the viewpoint that the homology of 16S rRNA between two bacteria was under 97.5% indicated it was a new species, otherwise there was farther study on them. The RAPD test was firstly used to analyze the two Magnetospirillum strains with high 16S rDNA homology. The orthogonal experiments were used to optimize the RAPD reaction for better repeatability. The results of RAPD showed that the two strains had much diversities, the similarity was about 45% between the two strains. These results indicated that the strain WM-1 was a new species of genus Magnetpsorillum.The iron content of cell of WM-1 was analyzed by atomic absorption spectrometry was 10.6 mg/g, dry wt, which was 50 times higher than that of representative heterotrophic bacteria cultured in a medium of similar total iron content. Uptake of ferric iron was found to obey simple Michaelis-Menten kinetics equationwith a Km of 2.5μM Fe(Ⅲ) and a Vmax of 0.83min-1 through the matching method of Lineweaver-Burk plot.The single crystal pattern of electron diffraction corresponded to the [011]zone, and the face of ccrystal was｛111｝,｛222｝,｛220｝,｛200｝and｛400｝. The strain WM-1 was characterized by means of TEM, X-ray diffraction, PPMS and M?ssbauer Spectroscopy. Furthermore, the Moskowitz test was able to authenticate the linear magnetosomes whether they came from magnetotactic bacteria.The Hc of the cells WM-1 was twice as Hc of magnetosomes.The optimum culture conditions of magnetotactic bacteria WM-1 consisted of 0.68 gL-1 KH2PO4,0.035 gL-1ascorbate,10.0 mlL-1 Wolfe,s vitamin solution,5.0 ml L-1Wolfe,s mineral solution,2.0 ml L-1 0.01 M ferric quinate, 4% O2,mtartrate:msuccinate=1:2,100 mgL-1NaNO3,pH=7.0. The cultue temperature was 28℃.