Growth and magnetite formation in Magnetospirillum gryphiswaldense MSR-1 were found close to the maximum at an extracellular iron concentration of 15-20 microM. Ferrous iron was incorporated by a slow, diffusion-like process. Several iron chelators including various microbial siderophores were unable to promote transport of iron into the cells. In contrast, spent culture fluids stimulated the uptake of ferric iron in iron-depleted cells at a high rate, whereas fresh medium and transport buffer were unable to promote iron uptake. However, no siderophore-like compound could be detected in spent culture fluids by the Chrome Azurol S assay. Ferric iron uptake followed Michaelis-Menten kinetics with a Km of 3 microM and a Vmax of 0.86 nmol min-1 (mg dry weight)-1, suggesting a comparatively low-affinity, but high-velocity transport system. Iron incorporation was sensitive to 2,4-dinitrophenol and carbonylcyanide-m-chlorophenylhydrazone, indicating an energy-dependent transport process.

Iron uptake and magnetite (Fe3O4) crystal formation could be studied in the microaerophilic magnetic bacteriumMagnetospirillum gryphiswaldense by using a radioactive tracer method for iron transport and a differential light-scattering technique for magnetism. Magnetite formation occurred only in a narrow range of low oxygen concentration, i.e., 2 to 7 μM O2 at 30°C. Magnetic cells stored up to 2% iron as magnetite crystals in intracytoplasmic vesicles. This extraordinary uptake of iron was coupled tightly to the biomineralization of up to 60 magnetite crystals with diameters of 42 to 45 nm.

The motor that powers the rotation of the bacterial flagellum reaches through both membranes into the cytoplasm of Gram-negative bacteria. The flagellum is connected by a flexible link (hook) to the motor axis, which passes through the center of a structure called the basal disk. The basal disk functions with the L-P ring complex as a bushing, enabling the rotation of the motor in the cell wall. The protein subunits of the basal disk of Wolinella succinogenes form an Archimedian spiral. The polymerization of subunits from a nucleation point at the motor in the form of a spiral allows constant growth of the basal disk. The disk is thought to provide a reinforcement at the flagellar insertion at the cell pole and to disperse forces that are generated by the momentum of the flagellar rotation.

The basal body of Wolinella succinogenes consists of a central rod, a set of two rings (L and P rings), a basal disk front 70 to 200 nm in diameter, and a terminal knob. In negatively stained preparations of flagellar hook-basal body complexes, some disks remain fixed perpendicularly to the grid and show that such a disk is located on the distal side of the P ring. The basal disks have been isolated with and without the P ring; in both cases there is a hole in the center of the disk. The diameter of the disk is smaller in the presence of the P ring. The L-P ring complex is therefore assumed to be a bushing for the rod. Thin sections of whole bacteria and spheroplasts reveal that the disk is attached to the inner surface of the outer membrane. At the insertions of the flagellar hook-basal body-basal disk complexes, depressions are visible in negatively stained preparations of whole bacteria and spheroplasts. A new ringlike structure is connected to an elongation of the basal body into the cytoplasm in both preparations. Its diameter (60 nm) is larger than that of the M ring. A heavily stained compartment can be seen in between the new ringlike structure and the basal disk, which may be formed by the energy transducing units.For a long time the entity called the intact flagellum (6-8) was thought to be the essential rotary device of the flagellar motor. In Escherichia coli and Salmonella typhimurium, this assembly consists of the flagellar filament, the hook, and the basal body. The basal body appears as a set of four rings coaxial with the rod. Two rings are very likely to be correlated to membranes: the L ring to the outer membrane and the M ring to the inner membrane. The P ring is in the periplasmic space and is assumed to interact with the peptidoglycan layer. The S ring is clearly visible in negatively stained electron micrographs in both bacteria, but from genetic, biochemical, and physiological studies no indications for its existence emerged until now (10, 11). In an extensive search for flagellar and motility mutants with the paralyzed or switch-defective phenotype, none mapped to the gene of the M ring (fliF) (10); all mapped to five genes encoding proteins that are not present on isolated basal bodies. Three of them (fiG, fliM, fliN) encode flagellar components that participate in both energy transduction and switching (15), whereas the other two (motA, motB) encode proteins that appear to function in energy transduction only (5,18,21,22). It is therefore unlikely that the M ring is the active rotor of the bacterial flagellar apparatus. Furthermore, it was proposed about 13 years ago by Coulton and Murray (3, 4) that essential structures of the flagellar apparatus were lost during the procedures for isolation of intact flagella. They found circlets of about 15 particles around the rod in the cytoplasmic membrane and outer membraneassociated large disks, called concentric membrane rings, in Aquaspirillum serpens.From Wolinella succinogenes, an anaerobic, gram-negative bacterium with one polar flagellum,...

Magnetic bacteria are found in various morphologies as cocci, vibrios, spirilli and rods in aquatic mud layers. Magnetite (Fe3O4) is stored in phospholipid vesicles as bullet-shaped. hexagonal or cubooctahydral crystals. S i z e and form of these crystals are species-specific and precisely controlled. The microaemphilic MagnetospirilIum gryphiswaldense forms in a near to linear chain up to 60 cubooctahedral, single domain magnetite crystals of 42-45nm diameter, which generates a magnetic dipole. Six proteins were detected in SDS-gels of the special phospholipid vesicles, which envelope the magnetite crystals These proteins are probably involved in iron transport (Fen orland Fem), in nucleation catalysis, in redox orland pH colhoL Iron uptake and dynamics of magnetite crystal formation were estimated simultaneously by change of light scattering of the cells within homogeneous magnetic fields. A very &cient, energy-dependent uptake of Fern in presence of spent, irondeficient growth medium was found. Iron uptake was tightly coupled to magnetite crystal formation and magnetisation. When FeIU was added to iron-starved cells under inducing conditions, Fern was immediately transpotted into the cells and superparamagnetic crystals of less than 20nm were formed first within 30 min.