Abstract
Magnetospirillum gryphiswaldense is a helix-shaped magnetotactic bacterium that synthesizes iron-oxide nanocrystals, which allow navigation along the geomagnetic field. The bacterium has already been thoroughly investigated at the molecular and cellular levels. However, the fundamental physical property enabling it to perform magnetotaxis, its magnetic moment, remains to be elucidated at the single cell level. We present a method based on magnetic tweezers; in combination with Stokesian dynamics and Boundary Integral Method calculations, this method allows the simultaneous measurement of the magnetic moments of multiple single bacteria. The method is demonstrated by quantifying the distribution of the individual magnetic moments of several hundred cells of M. gryphiswaldense. In contrast to other techniques for measuring the average magnetic moment of bacterial populations, our method accounts for the size and the helical shape of each individual cell. In addition, we determined the distribution of the saturation magnetic moments of the bacteria from electron microscopy data. Our results are in agreement with the known relative magnetization behavior of the bacteria. Our method can be combined with single cell imaging techniques and thus can address novel questions about the functions of components of the molecular magnetosome biosynthesis machinery and their correlation with the resulting magnetic moment.
Highlights
The magnetic field of the earth plays a role in the orientation and navigation of a wide variety of organisms including bacteria, algae, bees, pigeons and mice[1]
There are multiple open questions concerning the large discrepancies between the magnetic moments that were reported so far for these and other magnetotactic bacteria: Is it - in the case of M. gryphiswaldense - necessary to take the helical shape of the cells into account or is it sufficient to approximate them with a simplified geometry such as a cylinder or an ellipsoid? Is it necessary to take the dimension of each individual cell into account or is it sufficient to use the average dimensions of a bacterial ensemble? Does the use of dead cells that do not induce non-thermal noise lead to more consistent results if different methods are compared?
We analyzed the dimensions of 111 bacteria by transmission electron microscopy (TEM), and we found that the spread of the diameter values was very small
Summary
The magnetic field of the earth plays a role in the orientation and navigation of a wide variety of organisms including bacteria, algae, bees, pigeons and mice[1]. It was assumed that the cells were identical in size and their helical shape was simplified as a cylindrical or ellipsoidal geometry These studies yielded average magnetic moments that differ by more than one order of magnitude[10, 22, 23] despite the fact that the measurements were performed with external magnetic fields below 2 mT, which should lead to comparable magnetic moments that differ only by approximately 2%21. The authors showed that the use of different methods led to magnetic moments that varied by almost one order of magnitude They found that methods relying on viscous relaxation of the cell orientation gave results that were comparable to magnetosome measurements, whereas methods relying on statistical mechanics assumptions gave systematically lower values. Since living cells were used in the study of Nadkarni et al, the authors suggested that the non-thermal noise induced by the living cells is a potential source of error in measurements of the magnetic moment of bacteria
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