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.

SummaryAtomic force microscopy (AFM) in aqueous solution was used to investigate native nacre of the marine snail Haliotis laevigata on the microscopic scale and the interaction of purified nacre proteins with calcium carbonate crystals on the nanoscopic scale. These investigations were controlled by scanning electron microscopy (SEM), light microscopy (LM) and biochemical methods. For investigations with AFM and SEM, nacre was cleaved parallel to the aragonite tablets in this biogenic polymer/mineral composite. Multilamellar organic sheets consisting of a core of chitin with layers of proteins attached on both sides lay between the aragonite layers consisting of confluent aragonite tablets. Cleavage appeared to occur between the aragonite tablet layer and the protein layer. AFM images revealed a honeycomb-like structure to the organic material with a diameter of the 'honeycombs' equalling that of the aragonite tablets. The walls of the structures consisted of filaments, which were suggested to be collagen. The flat regions of the honeycomb-like structures exhibited a hole with a diameter of more than 100 nm. When incubated in saturated calcium carbonate solution, aragonite needles with perfect vertical orientation grew on the proteinacous surface. After treatment with proteinase K, no growth of orientated aragonite needles was detected. Direct AFM measurements on dissolving and growing calcite crystals revealed a surface structure with straight steps the number of which decreased with crystal growth. When the purified nacre protein perlucin was added to the growth solution (a super-saturated calcium carbonate solution) new layers were nucleated and the number of steps increased. Anion exchange chromatography of the water-soluble proteins revealed a mixture of about 10 different proteins. When this mixture was dialysed against saturated calcium carbonate solution and sodium chloride, calcium carbonate crystals precipitated together with perlucin leaving the other proteins in the supernatant. Thus perlucin was shown to be a protein able to nucleate calcium carbonate layers on calcite surfaces, and in the presence of sodium chloride, is incorporated as an intracrystalline protein into calcium carbonate crystals.

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...

Instrumental: 1 H (300 MHz) and 13 C (60 MHz) NMR spectra were recorded with a Bruker ARX 300 spectrometer. Tetramethylsilane (TMS) was used as internal standard. IR experiments were carried out on a Bruker IFS 55 spectrometer. Gel permeation

Instrumental: 1 H (300 MHz) and 13 C (60 MHz) NMR spectra were recorded with a Bruker ARX 300 spectrometer. Tetramethylsilane (TMS) was used as internal standard. IR experiments were carried out on a Bruker IFS 55 spectrometer. Gel permeation