Search

Magnetotactic bacteria (MTB) biomineralize intracellular magnetite (Fe3O4 ) crystals surrounded by a magnetosome membrane (MM). The MM contains membrane-specific proteins that control Fe3O4 mineralization in MTB. Previous studies have demonstrated that Mms13 is a critical protein within the MM. Mms13 can be isolated from the MM fraction of Magnetospirillum magneticum AMB-1 and a Mms13 homolog, MamC, has been shown to control the size and shape of magnetite nanocrystals synthesized in-vitro. The objective of this study was to use several independent methods to definitively determine the localization of native Mms13 in M. magneticum AMB-1. Using Mms13-immunogold labeling and transmission electron microscopy (TEM), we found that Mms13 is localized to the magnetosome chain of M. magneticum AMB-1 cells. Mms13 was detected in direct contact with magnetite crystals or within the MM. Immunofluorescence detection of Mms13 in M. magneticum AMB-1 cells by confocal laser scanning microscopy (CLSM) showed Mms13 localization along the length of the magnetosome chain. Proteins contained within the MM were resolved by SDS-PAGE for Western blot analysis and LC-MS/MS (liquid chromatography with tandem mass spectrometry) protein sequencing. Using Anti-Mms13 antibody, a protein band with a molecular mass of ~14 kDa was detected in the MM fraction only. This polypeptide was digested with trypsin, sequenced by LC-MS/MS and identified as magnetosome protein Mms13. Peptides corresponding to the protein’s putative MM domain and catalytic domain were both identified by LC-MS/MS. Our results (Immunogold TEM, Immunofluorescence CLSM, Western blot, LC-MS/MS), combined with results from previous studies, demonstrate that Mms13 and homolog proteins MamC and Mam12, are localized to the magnetosome chain in MTB belonging to the class Alphaproteobacteria. Because of their shared localization in the MM and highly conserved amino acid sequences, it is likely that MamC, Mam12, and Mms13 share similar roles in the biomineralization of Fe3O4 nanocrystals., National Science Foundation, grant number EAR-2038207, EAR-1423939, Ministerio de Economía y Competitividad, SPAIN and Fondo Europeo de Desarrollo Regional, FEDER grant numbers CGL2010-18274 and CGL2013-46612

One of the most common swimming strategies employed by microorganisms is based on the use of rotating helical filaments, called flagella, that are powered by molecular motors. Determining the physical properties of this propulsive system is crucial to understanding the behavior of these organisms. Furthermore, the ability to dynamically monitor the activity of the flagellar motor is a valuable indicator of the overall energetics of the cell. In this work, inherently magnetic bacteria confined in micromagnetic CoFe traps are used to directly and noninvasively determine the flagellar thrust force and swimming speed of motile cells. The technique permits determination of the ratio of propulsive force/swimming speed (the hydrodynamic resistance) and the power output of the flagellar motor for individual cells over extended time periods. Cells subjected to ultraviolet radiation are observed to experience exponential decays in power output as a function of exposure time. By noninvasively measuring thrust, velocity, and power output over time at a single-cell level, this technique can serve as the foundation for fundamental studies of bacterial hydrodynamics and also provides a novel, to our knowledge, tether-free probe of single-cell energetics over time.

Magnetotactic bacteria mineralize nanometer-size crystals of magnetite (Fe3O4) through a series of protein-mediated reactions that occur inside of organelles called magnetosomes. Mms6 is a transmembrane protein thought to play a key role in magnetite mineralization. We used both electron and fluorescent microscopy to examine the subcellular location of Mms6 protein within single cells of Magnetospirillum magneticum AMB-1 using Mms6-specific antibodies. We also purified magnetosomes from M. magneticum to determine if Mms6 was physically attached to magnetite crystals. Our results show that Mms6 proteins are present during crystal growth, and Mms6 is found in direct contact with the magnetite crystals or within the lipid/protein membrane surrounding the magnetite crystals. Mms6 was not detected at other subcellular locations within the bacteria or isolated fractions. Because Mms6 was found to completely surround the magnetosomes rather than being localized to one specific area of the magnetosome, it appears that this protein could act on the entire magnetite crystal during the biomineralization process. This supports a model in which Mms6 functions to regulate Fe3O4 crystal morphology. This knowledge is important for future in vitro experiments utilizing Mms6 to synthesize tailored nanomagnets with specific physical or magnetic properties.

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.

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.

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.