Biology and Physics of Magnetotactic Bacteria

*Fernanda Abreu and Daniel Acosta-Avalos*

## **Abstract**

Magnetotactic bacteria are able to align their swimming direction to the geomagnetic field lines because they possess a magnetic moment. These bacteria biomineralize magnetic nanoparticles, magnetite or greigite, inside a membrane. The membrane + nanoparticle set is known as magnetosome and intracellular magnetosomes are disposed in a linear chain. Cytoskeleton-like filaments are resposible for the mechanical stability of this chain. The genes responsible for the magnetosome membrane and for the cytoskeleton proteins have been largely studied: the mam genes. The magnetosome chain also confers to the bacterial body a magnetic moment that can be measured through different physical techniques. Because of their response to magnetic field inversions, magnetotactic bacteria are good models to study bacterial motion. Theoretical and experimental studies show that magnetotactic bacteria swim following a trajectory similar to cylindrical helix. Magnetotactic microorganisms have been observed avoiding regions with UV or violet-blue light of high intensity. If the intensity is lower, magnetotactic microorganims show photokinesis, increasing their velocity in the presence of red light and decreasing their velocity in the presence of green light, both relative to the velocity with blue light.

**Keywords:** magnetotactic bacteria, magnetosomes, magnetic moment, mam genes, magnetotaxis

### **1. Introduction**

Bacteria are one of the simplest organisms found in nature. They are distinguished from eukaryotes and superior organisms because their genetic material is not contained in a nucleus but is free in the cytoplasm. However, despite their relative simplicity, bacteria inhabit Earth for longer than many other organisms and constitute the most abundant type of cell on our planet [1]. Magnetotactic bacteria (MTB) are microorganisms that biomineralize magnetic nanoparticles inside their cytoplasm. These magnetic nanoparticles are involved by a lipidic membrane, and each "membrane + magnetic nanoparticle" set is known as magnetosome [2]. Magnetosomes are arranged in linear chains in the cytoplasm, conferring a magnetic moment to the bacterial body, being able to interact with the geomagnetic field to orient its navigational direction to the geomagnetic field lines. This response is known as magnetotaxis, resulting from the magnetic torque among the geomagnetic field and the magnetic moment of the magnetosome chain, and for that reason, MTB are described as "living compasses." MTB were discovered

independently by Salvatore Bellini in 1963 and Richard Blakemore in 1975 [3]. The first to observe MTB was Salvatore Bellini, an Italian physician from Pavia, Italy. In 1958, physicians from Pavia were asked to analyze the quality of water for human consume. Bellini was part of the team that studied the water quality, and he observed in water samples some bacteria that consistently accumulate in one side of water drops. After trying different stimuli, he discovered that they were affected by magnets. He called that bacteria as magnetosensitive. The first published observation of MTB was done in 1975 by Richard Blakemore in Massachusetts, USA [4]. His discovery was accidental, because his goal was to isolate *Spirochaeta plicatilis* from marine marsh muds. During his observations, he noted microorganisms migrating to one end of the drop of the mud on the microscope slide, and discover that the presence of magnets altered their swimming direction. He called magnetotaxis as the tactic response to magnetic fields.

called South-seeking. In the Northern Hemisphere, the opposite situation occurs

The study of MTB movement in an oxygen gradient showed that MTB change flagellar rotation and, consequently, the direction of movement depending on the oxygen concentration, migrating to the optimal conditions [8]. These observations were used to infer MTB dislocation along the vertical chemical gradient in the environment, which is based on the vertical component of the magnetic field and depends on the cellular state regarding oxygen. In the upper layers of the chemically stratified environment, where oxygen is abundant and higher than the optimal for MTB growth, the bacterium is on the oxidized state and rotate flagella to migrate downward, where the environment is more reduced. While in reducing conditions, in which oxygen is not abundant and concentrations are lower than that required for MTB growth, bacteria rotate the flagella in the opposite direction to reach upper layers of the gradient with optimal oxygen concentrations. Therefore, the presence of specific cell structures in MTB, as the magnetosome chain, flagella, and storage inclusions, represent adaptive advantages for dislocation across the chemical gradient to explore resources in the environment. For example, during the day, the oxygen gradient changes among the stratified layers of the environment and microorganisms dislocate across these layers to reach regions with suitable conditions for survival and growth. Because MTB are microaerophilic and/or capable of anaerobic respiration, which means that they are sensitive to high oxygen concentrations, an efficient mechanism to orient and migrate in the environment

Only a few species of MTB have been isolated in axenic cultures [9], and fewer type strains are available in cell line repositories. Many uncultured MTB species have been characterized from environmental samples, because it is possible to purify these cells based on their response to an applied magnetic field using a magnet [10]. **Figure 2** shows examples of cultured and uncultured species of MTB and their characteristics according to their phylogenetic affiliation. Note that MTB

magnetosomes are only synthesized by MTB belonging to Deltaproteobacteria class. *Magnetospirillum* species, which include *M. gryphiswaldense* strain MSR-1, *M. magneticum* strain AMB-1, and *M. magnetotacticum* strain MS-1, among other strains, are spirilla with flagella at each pole of the cell and represent the most characterized MTB. Information about the biomineralization process and

magnetosome organization within the cell is mainly based on species belonging to this genus [11]. Cryoelectron tomography studies using *M. magneticum* strain AMB-1 have shown that the magnetosome vesicle is a result of the cytoplasmic membrane invagination, which occurs before the synthesis of the magnetite nanocrystal [11, 12]; forming the magnetosome membrane vesicle in which proteins related to the biomineralization will be anchored. These proteins that are involved in all steps of the magnetosome formation are anchored to this invaginated portion of the membrane and will participate in the process by recruiting other proteins that integrate the process, for example, iron transport, crystal nucleation and growth,

According to studies performed in *M. gryphiswaldense* strain MSR-1, mature magnetite magnetosomes are found already organized in a chain within the cytoplasm of the cell 15 min after formation has started [14]. All *Magnetospirillum* species produce a single chain of cuboctahedral magnetite magnetosomes that are 40–45 nm in size [10]. Other MTB species are capable of synthesizing magnetite magnetosomes with cuboctahedral, prismatic, or anisotropic shapes [2] and/or greigite magnetosomes, which are usually classified as irregular. Usually, a magnetotactic bacterium species is capable of producing magnetosomes with one

are spread among different phyla in Bacteria domain and that greigite

size and shape control, and organization of the magnetosomes [11, 13].

and MTB presenting this behavior are called North-seeking.

*Biology and Physics of Magnetotactic Bacteria DOI: http://dx.doi.org/10.5772/intechopen.79965*

guarantees species survival.

**5**
