Magnetotactic bacteria are a group of motile prokaryotes that synthesize chains of lipid-bound, magnetic nanoparticles called magnetosomes. This study exploits their innate magnetism to investigate previously unexplored facets of bacterial hydrodynamics at surfaces. Through use of weak, uniform, external magnetic fields and local, micromagnetic surface patterns, the relative strength of hydrodynamic, magnetic, and flagellar force components is tuned through magnetic control of the bacteria's orientation. The resulting swimming behaviors provide a means to experimentally determine hydrodynamic parameters and offer a high degree of control over large numbers of living microscopic entities. The implications of this controlled motion for studies of bacterial motility near surfaces and for micro- and nanotechnology are discussed.

Spontaneous self-organization (clustering) in magnetically oriented bacteria arises from attractive pairwise hydrodynamics, which are directly determined through experiment and corroborated by a simple analytical model. Lossless compression algorithms are used to identify the onset of many-body self-organization as a function of experimental tuning parameters. Cluster growth is governed by the interplay between hydrodynamic attraction and magnetic dipole repulsion, leading to logarithmic time dependence of the cluster size. The dynamics of these complex far-from-equilibrium structures are relevant to broader phenomena in condensed matter, statistical mechanics and biology.

Abstract

Self-assembly is an important process in biological systems and also a promising avenue toward dynamic and responsive micro- and nano-technologies. This study discusses the non-equilibrium self-assembly of inherently magnetic bacteria oriented perpendicular to a solid surface. An interplay between hydrodynamic and magnetic interactions leads to stable three-dimensional clusters in the long-time regime, which may be programmatically assembled, disassembled, and translated across a surface. The implications of the findings for the rational design of non-equilibrium self-assembly in general are discussed.

Magnetotactic bacteria (MTB) thrive in aquatic sediments all over the world, but their complete role(s) in their geobiological habit as well as their significance in the fossil record remains unresolved. We were able to collect, enrich, and purify MTB from Mickey Hot Springs, a unique arsenic-rich hot spring system located in the Alvord Desert of southeastern Oregon. Populations of MTB were present in water and surface sediment samples in a geothermal spring at temperature 47 °C, pH of 8.0, and 0.96 mg/L arsenic. Using 16S rDNA analysis, the organism was found to belong to the phylum Nitrospirae. Cells had rod to vibrioid morphology, a single flagellum and single magnetosome chain as determined by scanning transmission electron microscopy (STEM). The magnetosomes contained bullet-shaped crystals of magnetite (Fe3O4) approximately 84 nm long and 39 nm wide. This is the first instance of MTB collected from an arsenic-rich, moderately thermophilic environment. The discovery of MTB at this site extends the limits of habitats for these bacteria and provides a proxy for the search for magnetofossils in the rock record.