A Comparison of Methods to Measure the Magnetic Moment of Magnetotactic Bacteria through Analysis of Their Trajectories in External Magnetic Fields

Abstract

Magnetotactic bacteria possess organelles called magnetosomes that confer a magnetic moment on the cells, resulting in their partial alignment with external magnetic fields. Here we show that analysis of the trajectories of cells exposed to an external magnetic field can be used to measure the average magnetic dipole moment of a cell population in at least five different ways. We apply this analysis to movies of Magnetospirillum magneticum AMB-1 cells, and compare the values of the magnetic moment obtained in this way to that obtained by direct measurements of magnetosome dimension from electron micrographs. We find that methods relying on the viscous relaxation of the cell orientation give results comparable to that obtained by magnetosome measurements, whereas methods relying on statistical mechanics assumptions give systematically lower values of the magnetic moment. Since the observed distribution of magnetic moments in the population is not sufficient to explain this discrepancy, our results suggest that non-thermal random noise is present in the system, implying that a magnetotactic bacterial population should not be considered as similar to a paramagnetic material.

Destabilization of a flow focused suspension of magnetotactic bacteria

“…1(a)]. This experiment has long been considered as a way to estimate the magnetic momentum of bacteria, assuming that only the thermal bath contributes to orientational noise [19,20]. However, a recent study suggests that other sources can be involved in the cell disorientation phenomena [21].…”

“…Considering a magnetic momentum M = 1 ± 0.2 × 10 −16 A m 2 [20,21], V flow = 50 ± 10 μm/s, B = 1.4 mT, and w = 25 ± 5 μm, we obtain a hydrodynamic radius for bacteria r = 1.1 ± 0.2 μm, which perfectly agrees with the size of the core bacteria, obtained with scanning electronic microscopy, around 1 μm [14]. Overall, the magnetotactic bacteria with their magnetic driving of physical nature, matching the Langevin paramagnetism, provide a model system of driven active microsystems, whose focusing interaction with nonuniform flow can be fully quantitatively captured.…”

Magnetotactic bacteria

“…The alignment of the bacteria and their trajectories with the magnetic field has been analyzed in several studies, which find that alignment is indeed described by Eq. (5), but only if a higher effective temperature is used [35,36]. The effective temperature values vary between studies and depend on the magnetic moment that is assumed (or inferred from electron microscopy images).…”

Estimating the magnetic moment of microscopic magnetic sources from their magnetic field distribution in a layer of nitrogen-vacancy (NV) centres in diamond

“…Since then images have been obtained of magnetic particles as small as 20 nm [7] and of magnetosomes inside magneto-tactic bacteria [8]. However, these measurements indicated a magnetic moment 10 times smaller than measured by methods based on the motion of these bacteria in magnetic fields [10]. Therefore, it is of great interest to replicate such measurements with particles of known magnetic moment to see if further insights could be gained into the technique.…”

“…Nevertheless, the measurements are not always easy to interpret, as it is difficult to control all parameters on a microscopic scale. For example, magnetic moments measured for magnetosomes inside magnetotactic bacteria differ by almost an order of magnitude depending on the method used [10,11]. In order to understand this promising technique better, it is helpful to apply it to particles of known magnetic moment, which will lead to optimised and more accurate measurement protocols.…”