Biogenic Nanomagnetic Carriers Derived from Magnetotactic Bacteria: Magnetic Parameters of Magnetosomes Inside Magnetospirillum spp.

Magnetic parameters of magnetosomes inside the bacteria of MSR-1, LBB-42, AMB-1, SP-1, BB-1, and SO-1 strains of the genus Magnetospirillum fixed by 5% formalin in the nutrient medium were estimated by measurements of the nonlinear longitudinal response to a weak ac magnetic field (NLR-M2) at room temperature. For the BB-1, MSR-1, and AMB-1 strains, the measurements of the electron magnetic resonance (EMR) spectra with the special X-band spectrometer for wide-line registration were also carried out. To trace the evolution of the magnetic state of the magnetosomes during the long-term storage, freshly prepared samples (new) and samples after a year of storage at 4 degrees C (old) were studied. The assessment of the state of the bacteria ensemble in the medium after the long-term storage was carried out for one typical strain (BB-1) using atomic force microscopy. The stable single-domain state of magnetic centers in the magnetosomes indicating their proximity to a superparamagnetic (SPM) regime was found at the scan frequency 0.02 Hz of the steady magnetic field. This allowed a semi-quantitative analysis of M2 data to be carried out with the formalism based on the numerical solution of the kinetic Fokker-Planck equation for SPM particles. Processing the NLR-M2 data demonstrated the presence of two kinds of magnetosomes in both the new and old samples: (i) those with the large magnetic moment (the heavy, monodisperse mode) and (ii) those with the comparatively small magnetic moment (the light, highly dispersed mode). The EMR spectra were formed mostly by the heavy fraction for both samples. The presence of two peaks in the spectra evidenced the presence of conventional uniaxial magnetic anisotropy in the magnetosomes. The appearance of one or two additional peaks in the spectra in the old fraction of some strains implied their instability at the long-term storage, even when fixed by formalin and sealed in the nitrogen atmosphere.

Automated summary

The magnetic parameters of magnetosomes inside bacteria of different strains were studied. The samples were stored for different lengths of time and analyzed using various spectroscopic techniques. The results showed the presence of magnetosomes with different magnetic properties and variations in the spectral peaks between new and old samples.