Experimental Determination of the Four Principal Drag Coefficients of Magnetospirillum magneticum AMB-1 cells

Abstract

Magnetotactic bacteria (MTB) possess organelles called magnetosomes which contain magnetite (Fe_3O_4) or greigite (Fe_3S_4) nanocrystals. These particles generate a magnetic moment allowing the use of external magnetic fields to control the cell orientation. MTB use this magnetic moment to reach environments with optimal oxygen concentration, a process called magnetotaxis. There are many possible technological applications for MTB, for example, they have been used as nanorobots to push beads and they can be used to remove heavy metals and radionuclides from waste water. In order to fully understand the motion of these micron-size organisms, which takes place at very low Reynolds number where friction dominates over inertia, we set out to measure their drag coefficients. As a starting point, we used a well-studied species of MTB with a corkscrew shape, Magnetospirillum magneticum AMB-1. Simulations were done to find the best external magnetic field strength at which to observe their diffusion. We then imaged non-motile cells placed in these preferred uniform magnetic fields and used automated image analysis to determine the position and orientation of the cells in each frame. This allowed calculating orientation correlation functions and mean-squared displacements, from which rotational and translational diffusion coefficients were obtained for each individual cell. We observed that the four principal drag coefficients of these cells greatly vary as a function of cell length as predicted for cylindrical or elliptical objects with comparable radius. However, we also detecting a coupling between the rotation around and translation along the long axis of the cell only observed for chiral objects. We were able for the first time to experimentally fully characterize the friction matrix for a micron-size elongated chiral object.

Continuing our work on MTB, to study live cells for long periods of time, we looked to confine them in PDMS nanowells, but found that MTB were not growing well in this environment. We then turned to a device, which incorporated a PDMS microchannel to provide continuous nutrients and a gel membrane to enable cellular growth into a 2D monolayer. Hopefully, this experimental setup combined with time-lapse microscopy can in the future be used to observe cell growth and cell division, and further to determine whether the magnetosome of the mother is passed on equally between daughter cells.