THE OBSERVATION OF LARGE MAGNETITE (FE3O4) CRYSTALS FROM MAGNETOTACTIC BACTERIA BY ELECTRON AND ATOMIC-FORCE MICROSCOPY

Magnetite crystals inside coccoid magnetotactic bacteria found in lago ons near Rio deJaneiro city were examined by electron microscopy (EM) and atomic force microscopy (AFM). For AFM, ultrathin sections of bact eria embedded in Epon resin were etched with an ethanolic NaOH solutio n and observed both in the height and in the force modes. Comparative electron microscope images were useful for identifying crystalline rel iefs in the etched sections. Different situations representing particu lar arrangements of crystal chains were observed by AFM. The majority of the bacteria examined presented unusually large magnetite crystals which remained strongly attached in linear chains even after the labor atory procedures for their isolation. This behaviour is different from all other biogenic magnetite crystals isolated so far. It is suggeste d that this attachment is due to the strong field between individual c rystals as well as to the contact areas, which are the largest observe d until now. The correct identification of a particular topography by AFM as a crystal relief may be critical when crystals are not aligned in chains; in these cases the linear dimensions and the presence of we ll-defined edges and faces are important features to be taken into acc ount. Characterization of the crystal faces is important for the study of magnetotactic micro-organisms since the crystalline habits seem to be species-specific. Observation of etched sections proved to be a he lpful approach for crystal relief observation, especially when small a mounts of bacteria were available.