Magnetite nanoparticles in organisms: From bacteria to the human brain

Magnetite (Fe3O4) nanoparticles are ubiquitous in the environment. They are geologically
formed in igneous and metamorphic environments and are emitted from anthropogenic
processes such as coal combustion and tra ic. Magnetite particles are also
biomineralized by microbes in soil and sediments and have even been found in tissues
including the human brain. The most studied magnetite-producing organisms are
magnetotactic bacteria (MTB), which use magnetite for an alignment with the
geomagnetic field that guides them to their preferred habitat. While the biomineralization
processes in MTB are well understood, the presence of magnetite in the brain remains
enigmatic. Particles may be produced in the brain through controlled biomineralization,
such as in MTB, but could also originate from imbalanced iron metabolism, or reach the
brain from the outside environment as air pollution. Our magnetic measurements of
postmortem human brain stems and the surrounding meninges suggest that magnetite
nanoparticles undergo dynamic processes to reach and enter the brain after circulating
in the bloodstream. This is concerning due to the particles' high redox activity and surface
charge, which has the potential to significantly impact health.