Deep-sea bacteria harboring bacterial endosymbionts in a cytoplasm?: 3D electron microscopy by serial ultrathin sectioning of freeze-substituted specimen

Abstract

In 2012, we discovered a unique microorganism (Parakaryon myojinensis) that has intermediate cellular structures between prokaryotes and eukaryotes from the deep sea off the coast of Japan. Observations of ultrathin sections of deep-sea specimens with electron microscopy often revealed bacteria that contained intracellular bacteria. Here, we carried out a three-dimensional analysis of one bacterium that contained several bacteria within its cytoplasm by serial ultrathin sectioning electron microscopy of freeze-substituted specimen. We found that the host bacterium was not intact and the cell wall was broken; hence, the bacteria found inside of the host were not endosymbionts, but happen to be associated independently within cytoplasm of dead bacteria. This study emphasizes the importance of 3D analysis for understanding the interactions of microorganisms.