Magnetotactic Bacteria: From Magnetic Navigation to Biomedical Applications
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Hamed Zahraee,1 Zahra Khoshbin,2 Mahboobeh Nakhaei Moghaddam,3,*
1. Department of Biology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
2. Department of Medicinal Chemistry, School of Pharmacy, Mashhad University of Medical Sciences, Mashhad, Iran
3. Department of Biology, Mashhad Branch, Islamic Azad University, Mashhad, Iran
- Introduction: The physicist William Gilbert was the first scientist about 420 years ago that described the earth as a giant magnet affecting magnetic objects on its surface. He designed the experiments that showed the positional changes of the direction magnetic needle under the gradient of the earth's magnetic field along the horizontal and vertical axis. Hence, he proved the difference in the gradient of the earth's magnetic field. Despite many endeavors to find a sense of cognitive magnetism in vertebrates, scientific experiments in this field have been fruitless. It is obligatory to deeply understand the molecular mechanisms of the phenomenon in some model organisms, such as bacteria, that can introduce them as a magnetic unit of life. Magnetotactic bacteria are microaerophilic or anaerobic bacteria that can detect magnetic field lines and migrate to the sediments and anaerobic areas having the optimal conditions for their growth by using their flagellum. The phenomenon is mostly known as "magnetotaxis". This characteristic indicates the existence of a different organelle or cell structure than other bacteria, called magnetosomes.
Magnetosome is an intracellular organelle synthesized by the magnetotactic bacteria that enables them to detect the magnetic field lines of the earth so that they are still able to orient in the magnetic field even after death. This cellular substructure consists of an organic component (phospholipid bilayer membrane) and an inorganic element (mineral magnetic nanoparticles), the inner space of which is usually occupied by a mineral iron crystal of iron oxide (Fe3O4), iron sulfide (Fe3S4), or pyrite (FeS2). Fe3O4and Fe3S4, also called magnetite and greigite, respectively, have elevated magnetic properties but FeS2 lacks magnetic properties, known as a diamagnetic compound. Although these crystals are morphologically diverse, both types of nanoparticles are composed of a lipid-bilayer membrane containing unique proteins. The size of the magnetosomes is in the range of 35-120 nm. According to the emphasis of nanoscience on efficiency of nanoparticles with dimensions less than 100 nm, these nanoparticles have a high potential for utilization in various industries. Most of the magnetosomes are chained or scattered around the longitudinal axis of the cell.
The number of magnetosomes varies in the different types of magnetotactic bacteria. For instance, the average number of nanoparticles per cell is about 17.6 in MS-1, as the magnetite type nanoparticle. Besides, the distance between the surface of the nanoparticles and the magnetosome membrane is 1-6 nm and the magnetosomes are spaced about 3-18 nm. In ‘Candidatus Magnetoglobus multicellularis’, there is an average of about 80 magnetosomes (up to 180 nm) with greigite nanoparticles per cell.
Mineral of most magnetosomes is magnetite, however, some magnetotactic bacteria contain greigite nanoparticles, and also, there is a known type of bacteria that synthesizes both nanoparticles. The structure and composition of elements, shape, and size of magnetosomes can be studied using high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDXA), scanning X-ray fluorescence microscopy (SXFM), and nanoscale X-ray absorption near-edge structure (nano-XANES). The mineralization process of magnetosomes is a highly regulated process in which their morphology, composition, and size are directed and controlled at the gene expression levels. The mineral processes for the construction of the nanoparticles in magnetosomes are genetically controlled by a group of genes in magnetotactic bacteria. More than 30 genes are involved in the synthesis of magnetosomes. For example, in the Magnetospirillum gryphiswaldense MSR-1 (MSR), magnetosome genes are organized into five polycistronic operons, including mms6, mamXY, mamAB, mamGFDC, and feoAB1.
The production methods of magnetosomes include biosynthesis and laboratory synthesis by genetic engineering techniques. Biosynthesized nanoparticles possess greater advantages over synthetic magnetic ones, including biocompatibility, purity, and excellent magnetic properties. Magnetosomes have engrossed a highly consideration in magnetic resonance imaging (MRI), hyperthermia, drug delivery, cancer theranostics, gene therapy, biomedicine, tumor detection, biosensors, and enzyme immobilization.
- Methods: Review paper
- Results: Review paper
- Conclusion: Discovery of the Earth's magnetic field, importance of finding the origin of magnetic properties in living organisms, and also, the growth and development of nanotechnology and its applications in various industries have highlighted magnetotactic bacteria among the others. Magnetotactic bacteria have a unique subcellular structure called magnetosome consisting of a magnetic nanoparticle of magnetite (Fe3O4) or greigite (Fe3S4) mineral with the dimensions of 30-120 nm and a lipid-bilayer phospholipid coating, which replace them with synthetic magnetic nanoparticles. Magnetosomes have been highly considered as the intelligent and safe carrier for targeted drug delivery, purification of macromolecules, biosensors, and hyperthermia, due to having high purity magnetic nanoparticles and lipid-bilayer membrane with proprietary proteins.
- Keywords: Magnetotactic bacteria, Magnetosomes, Magnetic field, Magnetic nanoparticles, Biomineralization
