Alteration of the magnetic properties of Aquaspirillum magnetotacticum by a pulse magnetization technique.

DÍAZ RICCI, JUAN C.; WOODFORD, BARBARA J.; KIRSCHWINK, JOSEPH L.; HOFFMANN, MICHAEL R.

APPLIED AND ENVIRONMENTAL MICROBIOLOGY

AMER SOC MICROBIOLOGY

Lugar: Washington; Año: 1991 vol. 57 p. 3248 - 3254

0099-2240

The presence of anarrow shape and size distribution for magnetite crystals within magnetotacticorganisms suggests strongly that there are species-specific mechanisms thatcontrol the process of biomineralization. In order to explore the extent ofthis control, cultures of Aquaspirillummagnetotacticum in the exponential growth phase were exposed to increasingmagnetic pulses with the aim of separating cell populations on the basis oftheir magnetic coercitivies. Isothermal remanent magnetization and anhystereticremanent magnetization studies were performed with freeze-dried magnetic cellsafter the remagnetization treatment. Subpopulations of A. magnetotacticum that showed an increase in the coercivity correlatedwith the intensity of the magnetic pulses were isolated. After successivesubcultures of the remaining north-seeking cells, a maximum bulk coercivity (Hbmax) of 40 mTwas obtained after treatment with a 55 mT pulse. Although we obtained A. magnetotacticum variants playinghigher coercivities than the wild-type strain, changes in crystal size or shapeof the magnetite crystals were below reliable detection limits withtransmission electro microscopy. Attempts to shift the coercivity toward highervalues caused it to decrease, a change which was accompanied by themagnetostatic interactions of the magnetosome chains as well as an increase inthe cell population displaying an abnormal distribution of the magnetosomechains. Ultrastructural analyses of cells and magnetosomes revealed theappearance of cystlike bodies which occasionally contained magnetosomes. Theincrease in cystlike cells and abnormal magnetosome chains when higher magneticpulses were used suggested that magnetosomes were collapsing because ofstronger interparticle magnetostatic forces.