We have studied isolated magnetic bacteria using light and electron microscopy, and classified those which contain bullet‐shaped magnetic particles into four distinct morphological types, designated BS‐1 to BS‐4. Two of the types, BS‐1 and BS‐2, are new, and have unusual arrangements of magnetic particles. The rod‐shaped bacterium BS‐1 has a dense network of several thousand magnetic particles, whereas BS‐2, a coccus, contains magnetic particles arranged radially in bands.
We have investigated the evolutionary relationships between two facultatively anaerobic Magnetospirillum strains (AMB-1 and MGT-1) and fastidious, obligately microaerophilic species, such as Magnetospirillum magnetotacticum, using a molecular phylogenetic approach. Genomic DNA from strains MGT-1 and AMB-1 was used as a template for amplification of the genes coding for 16S rRNA (16S rDNA) by the polymerase chain reaction. Amplified DNA fragments were sequenced (1,424 bp) and compared with sequences for M. magnetotacticum MS-1 and Magnetospirillum gryphiswaldense MSR-1. Phylogenetic analysis of the aligned 16S rDNA sequences indicated that the two new magnetic spirilla, AMB-1 and MGT-1, lie within the a subdivision (a-1) of the eubacterial group Proteobacteria and are closely related to Rhodospirillum fulvum and to several endosymbiotic bacteria. Strains AMB-1, MGT-1, and MS-1 formed a cluster, termed group I, in which they were more closely related to each other than to group II, which contained M. gryphiswaldense MSR-1. Group I strains were also physiologically distinct from strain MSR-1. Sequence alignment studies allowed elucidation of genus-specific regions of the 16S rDNA, and oligonucleotide primers complementary to two of these regions were used to develop a specific polymerase chain reaction assay for detection of magnetic spirilla in natural samples.Magnetic bacteria orient and swim along magnetic field lines and are widely distributed in marine and freshwater habitats (2,6,22). However, these bacteria are fastidious and difficult to grow in the laboratory. Few pure cultures of magnetic bacteria have been obtained, and studies have remained limited mostly to isolates of the genus Magnetospirillum (32). Magnetic sensitivity is due to the presence of magnetosomes, particles of magnetite (Fe304), which are synthesized within membrane vesicles and aligned in chains inside the cell (1). The function of these magnetosomes remains unclear, although it has been suggested that they may act as navigational compasses directing microaerophilic bacteria downward, along a geomagnetic field into microaerobic sediments (7,15,35).We have recently obtained in pure culture two new Magnetospirillum strains (AMB-1 and MGT-1) (24,25). In contrast to Magnetospirillum magnetotacticum and Magnetospirillum gryphiswaldense, which are obligate microaerophilic chemoorganotrophs (9, 32), the facultatively anaerobic strains AMB-1 and MGT-1 can grow well either aerobically (25) or anaerobically by dissimilatory nitrate reduction (24). In addition, Magnetospirillum sp. strain AMB-1 can easily form colonies on agar plates and can be grown readily in the laboratory to high cell densities. Because of these properties, we were able to use conjugative gene transfer techniques (34), commonly used for photosynthetic bacteria (10-12, 33) and other gram-negative bacteria, with Magnetospirillum sp. strain AMB-1 (23). This bacterium is therefore being used as a model system to investigate the molecular genetics of biogenic magnetite synthesis.Gen...
Most of the 16S ribosomal RNA gene of a sulfate-reducing magnetic bacterium, RS-1, was sequenced, and phylogenetic analysis was carried out. The results suggest that RS-1 is a member of the delta-Proteobacteria, and it appears to represent a new genus. RS-1 is the first bacterium reported outside the alpha-Proteobacteria that contains magnetite inclusions. RS-1 therefore disrupts the correlation between the alpha-Proteobacteria and possession of magnetite inclusions, and that between the delta-Proteobacteria and possession of greigite inclusions. The existence of RS-1 also suggests that intracellular magnetite biomineralization is of multiple evolutionary origins.
PCR primers specific to the 16S ribosomal DNA (rDNA) of magnetic cocci were designed and used to amplify DNA from magnetically isolated magnetic cocci. The PCR products were subcloned by ligation into plasmid vector pCRII, and five clones containing approximately 270-bp fragments of amplified DNA were sequenced. The specific primers were also used to detect magnetic coccus 16S rDNA in environmental samples. Magnetic coccus 16S rDNA was amplified from the water column above sediment kept in an anoxic environment in the laboratory, but little was amplified from a water column kept in an oxic environment. These results suggest that magnetic cocci in the water column in an anoxic environment had migrated there from the sediment as a response to the microoxic or anoxic conditions, rather than having been present previously in a nonmagnetic form and having become magnetic due to these conditions. The specific primers were also used to detect magnetic cocci in aquatic sediment. DNA was extracted from sediment by direct lysis and purified for use as a PCR template by electrophoresis on an agarose-polyvinylpyrrolidone gel. 16S rDNA was then amplified and subcloned, and two clones were sequenced. The clones were screened for chimeric DNA by comparing sections of each with the GenBank database.
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