Lina M. Gonzalez

Sudden motility reversal indicates sensing of magnetic field gradients in Magnetospirillum magneticum AMB-1 strain

Many motile unicellular organisms have evolved specialized behaviors for detecting and responding to environmental cues such as chemical gradients (chemotaxis) and oxygen gradients (aerotaxis). Magnetotaxis is found in magnetotactic bacteria and it is defined as the passive alignment of these cells to the geomagnetic field along with active swimming. Herein we show that Magnetospirillum magneticum (AMB-1) show a unique set of responses that indicates they sense and respond not only to the direction of magnetic fields by aligning and swimming, but also to changes in the magnetic field or magnetic field gradients. We present data showing that AMB-1 cells exhibit sudden motility reversals when we impose them to local magnetic field gradients. Our system employs permalloy (Ni80Fe20) islands to curve and diverge the magnetic field lines emanating from our custom-designed Helmholtz coils in the vicinity of the islands (creating a drop in the field across the islands). The three distinct movements we have observed as they approach the permalloy islands are: unidirectional, single reverse and double reverse. Our findings indicate that these reverse movements occur in response to magnetic field gradients. In addition, using a permanent magnet we found further evidence that supports this claim. Motile AMB-1 cells swim away from the north and south poles of a permanent magnet when the magnet is positioned less than ∼30 mm from the droplet of cells. All together, these results indicate previously unknown response capabilities arising from the magnetic sensing systems of AMB-1 cells. These responses could enable them to cope with magnetic disturbances that could in turn potentially inhibit their efficient search for nutrients.

Controlling Magnetotactic Bacteria through an Integrated Nanofabricated Metallic Island and Optical Microscope Approach

Herein, we demonstrate the control of magnetotactic bacteria through the application of magnetic field gradients with real-time visualization. We accomplish this control by integrating a pair of macroscale Helmholtz coils and lithographically fabricated nanoscale islands composed of permalloy (Ni80Fe20). This system enabled us to guide and steer amphitrichous Magnetospirillum magneticum strain AMB-1 to specific location via magnetic islands. The geometries of the islands allowed us to have control over the specific magnetic field gradients on the bacteria. We estimate that magnetotactic bacteria located less than 1 μm from the edge of a diamond shaped island experience a maximum force of approximately 34 pN, which engages the bacteria without trapping them. Our system could be useful for a variety of applications including magnetic fabrication, self-assembly, and probing the sensing apparatus of magnetotactic bacteria.

Sensing of Local, Highly Concentrated Magnetic Field Gradients in Magnetotactic Bacteria Induces Motility Reversal

Many motile unicellular organisms have evolved specialized behaviors for detecting and responding to chemical gradients (chemotaxis) or oxygen (aerotaxis), while magnetotactic bacteria sense magnetic fields to align their direction of movement. Herein we show that Magnetospirillum magneticum (AMB-1) have the ability to sense and respond not only to the direction of magnetic fields of naturally occurring magnitude, but also to local, highly concentrated magnetic field gradients that do not occur in their natural environment. We imposed these gradients through our system integrating Helmholtz coils and permalloy (Ni80Fe20) microstructures. The AMB-1 exhibit three distinct behaviors as they approached gradients near the microstructures—unidirectional, single direction reversal, and double direction reversal. These results indicate previously unknown capabilities of the magnetic sensing systems of AMB-1.Copyright

Engineering Magnetic Nanomaterial Production in Magnetotactic Bacteria Through Gene Regulation

Magnetotactic bacteria endogenously synthesize intracellular magnetic nanoparticles (magnetosomes); however, little is known regarding the genetic regulatory networks that control magnetosome production. In this paper, we explore the genetic response of Magnetospirillum magneticum strain AMB-1 to an applied electromagnetic field as a means to identify genes activated by magnetic stimulation. The expression of magnetosome island, flagellar and cytoskeletal genes was found to be differentially altered by magnetic stimulation at short and long times points. These results indicate previously uncharacterized endogenous gene network modules that could be exploited to engineer magnetic bacteria as magnetic nanomaterial producing-machines through gene regulation.Copyright

Controlling Swimming Magnetotactic Bacteria Through Microfabrication Toward a Non-Destructive Biological Sensor

The development of non-destructive sensors is an important direction to pursue to advance approaches, which are less intrusive in their monitoring. One area where significant advances can be made in this field is through using biological inspiration. In this work, we demonstrate the ability of swimming magnetic bacteria, Magnetospirillum magneticum strain AMB-1, to respond to microfabricated magnetic field concentrators. These magnetic bacteria are envisioned as a potential non-destructive sensor approach for detecting defects in submerged metal structures such as ship hulls and bridges. To accomplish this, a pair of Helmholtz coils was built to magnetize microfabricated permalloy (NiFe) structures of different shapes that mimic defects in metals. The NiFe structures were fabricated through photolithography and depositing a thin film of NiFe on a master mold. The substrate was a thin piece of glass (<170μm) that permitted us to observe the interaction of the magnetic bacteria with the NiFe structures that were positioned at the center of the Helmholtz coils. The response of the bacteria around the structures was quantified through analyzing the images of the bacteria in terms of orientation. The bacteria change orientation as they approach the fabricated structures with concentrated fields due to their geometries. In addition, we used magnetic particles to examine the fields and estimated the local magnetic fields exerted on the particles through calculations involving the forces on the beads. We believe that this work is important in a variety of areas from bacterial mechanics to synthetic biology to aqueous non-destructive sensors.Copyright