We report a study of the deformability of a bacterial wall with an atomic force microscope (AFM). A theoretical expression is derived for the force exerted by the wall on the cantilever as a function of the depths of indentation generated by the AFM tip. Evidence is provided that this reaction force is a measure for the turgor pressure of the bacterium. The method was applied to magnetotactic bacteria of the species Magnetospirillum gryphiswaldense. Force curves were generated on the substrate and on the bacteria while scanning laterally. With the mechanical properties so gained we obtained the spring constant of the bacterium as a whole. Making use of our theoretical results we determined the turgor pressure to be in the range of 85 to 150 kPa.

With evolution, Nature has ingeniously succeeded in giving rise to an impressive variety of inorganic structures. Every organism that synthesizes biogenic minerals does so in a form that is unique to that species. This biomineralization is apparently biologically controlled. It is thus expected that both the synthesis and the form of every specific biogenic mineral is genetically determined and controlled. An investigation of the mechanism of biomineralization has only become possible with the development of modern methods in molecular biology. Unicellular organisms such as magnetic bacteria, calcareous algae, and diatoms, all of which are amongst the simplest forms of life, are particularly suited to be investigated by these methods. Crystals and composites of proteins and amorphous inorganic polymers are formed as complex structures within these organisms; these structures are not known in conventional inorganic chemistry.

A simple optical method was developed for assaying cellular magnetism in culture samples of magnetic spirilla. Cells are aligned parallel to the field lines in a magnetic field, resulting in a change in light scattering. The ratio of scattering intensities at different angles of magnetic field relative to the light beam CC,,,,) is used to characterize the average magnetic orientation of the cells. Cmag was found to be well correlated with the average number of particles in different magnetic cell populations. Thus, estimations of magnetosome content can be made using magnetically induced differential light scattering. The method provides a fast and sensitive tool for monitoring the magnetite formation in growing cultures of Magnetospiriflum gryphiswaldense.

Gram-negative magnetic bacteria of the species Magnetospirillum gryphiswaldense were investigated by atomic force microscopy (AFM) in buffer solution. The highly positively charged silane trimethoxysilyl-propyl-diethylenetriamine was used to coat cover glass surfaces for adsorption of the bacteria. The resulting bacterial surface was flat, and in most cases it was not possible to resolve any structures. Force curves were obtained for the substrate and for the bacteria while scanning laterally, in a technique called "force mapping". To obtain a quantitative measure for the elasticity of the cell wall the contribution of the internal osmotic pressure had to be estimated. There was no detectable change in the observed elastic response when the osmolarity of the surrounding medium was changed; this showed that the elastic response was due to the cell wall. It was thus possible to determine the effective compressibility of the cell wall, which was about 42 mN/m. Magnetic bacteria are apparently ubiquitous, and are found in various morphologies in aquatic mud layers. Magnetite (Fe 3 O 4 ) is stored in special, intracytoplasmic phospholipid vesicles in single magnetic domains. The size and form of these crystals are species specific, and are precisely controlled; for a review see [1]. These magnetosomes are mainly arranged in one short chain forming a magnetic dipole [2]. They therefore orient themselves passively along the field lines of the earth's magnetic field. In a current hypothesis it is assumed that they are driven by their flagellar motor to preferred microaerophilic environments following aerotaxis [3].Currently only two species are available by type in culture collections: Magnetospirillum magnetotacticum and Magnetospirillum gryphiswaldense [4]. M. gryphiswaldense of the strain MSR-1 are Gram-negative, helical cells, 0.2-0.7 µm in diameter and 1-20 µm in length; younger cells are 3-4 µm in length. They carry 0-40 magnetosomes in one linear chain, and possess a monotrichous flagellum on each pole. As * To whom correspondence should be addressed Gram-negative bacteria they are enveloped by a plasma membrane and a single sheet of mureine (peptidoglycan sheet). A thick layer of mostly anionic lipopoly-saccharides and lipoproteins incorporated into a lipid membrane is covalently attached to the murein sacculus [5][6][7]. The plasma membrane, the peptidoglycan sheet and the outer membrane are about 20 nm in thickness. The most important duty of this cell wall is to resist the internal pressure of the bacterium caused by the different osmolarities of the internal cell medium and the external liquid (fresh water). The plasma membrane maintains the internal milieu actively: thus it is too soft for this task. A rigid outer membrane or cell wall is therefore needed to hinder osmotic lysis. The chemistry and structure of the cell wall are well investigated; for a review see [8]. However, little is known about the mechanical properties. This is because there were no instruments available to measure the mechanical proper...

A simple optical method was developed for assaying cellular magnetism in culture samples of magnetic spirilla. Cells are aligned parallel to the field lines in a magnetic field, resulting in a change in light scattering. The ratio of scattering intensities at different angles of magnetic field relative to the light beam (Cmag) is used to characterize the average magnetic orientation of the cells. Cmag was found to be well correlated with the average number of particles in different magnetic cell populations. Thus, estimations of magnetosome content can be made using magnetically induced differential light scattering. The method provides a fast and sensitive tool for monitoring the magnetite formation in growing cultures of Magnetospirillum gryphiswaldense.