An electron microscopic study of microorganisms: from influenza virus to deep-sea microorganisms

Three topics from electron microscopic studies of microorganisms car-ried out in my laboratory in recent ten years are described. 1) Influenza A virus was observed in water by an ice-embedding method using phase contrast electron microscopy developed in Japan. Virions appeared as spherical or elongated particles consisting of spikes, an envelope, and a core with high contrast. 2) A new term the “structome” was introduced and defined as “the quantitative and three-dimensional structural information of a whole cell at electron microscopic level.” We performed structome analyses of Saccharomyces cerevisiae by using freeze-substitution and serial ultrathin sectioning electron microscopy. We found that there were one to four mitochondria and about 195,000 ribosomes in a cell. 3) In the deep-sea off the coast of Japan, we discovered a unique microorganism appearing to have cellular features intermediate between prokaryotes and eukaryotes. The organism, named as the Myojin parakaryote, was two orders of magnitude larger than a typical bacterium and had a large “nucleoid”, surrounded by a single layered “nucleoid membrane”, and bacteria-like “endosymbionts”, but it lacked mitochondria. This organism exemplifies a potential evolutionary path between prokaryotes and eukaryotes, and the presence of the organism supports the endosymbiotic theory for the origin of mitochondria and the karyogenetic hypothesis for the origin of the nucleus. These studies show that the electron microscopy is a powerful tool for studying a wide range of problems of microorganisms. The ultrastructure of frozen-hydrated influenza A virus was examined by Zernike phase contrast electron microscopy. Using this new technique, the virions were clearly observed with high contrast and appeared as spherical or elongated particles consisting of peripheral spikes, an envelope, and a core. Not only lipid bilayers but also individual glycoprotein spikes on viral envelopes were clearly resolved. About 450 glycoprotein spikes were present in an average-sized spherical virion. Eight ribonucleoprotein complexes, that is, a central one surrounded by seven others, were distinguished in one viral particle. Thus, Zernike phase contrast electron microscopy is a powerful tool for resolving the ultrastructure of viruses in natural and hydrated state, because it enables high-contrast images of ice-embedded particles. in state was 1) , . cell at electron microscopic level." In the present study, we performed structome analysis of Saccharomyces cerevisiae , one of the most widely researched biological materials, by using freeze-substitution and serial ultrathin sectioning electron microscopy. Our analysis revealed that there were one to four mitochondria and about 195,000 ribosomes in a cell and 13-28 endoplasmic reticula/Golgi apparatus, which do not form networks in the cytoplasm in G1 phase. The nucleus occupied 10.1 % of the cell volume, the cell wall occupied 17.7 %, the vacuole occupied 4.0 %, the cytoplasm occupied 66.2 % and the mitochondria occupied only 1.6 % in G1 phase. These would a to consider the function of cells. There are only two kinds of organisms on the Earth: prokaryotes and eukaryotes. Eukaryotes are thought to have developed from prokaryotic predecessors; however the large differences in their cellular structures results in equally large questions of how the process might have occurred. In 2012, in the deep-sea off the coast of Japan, we discovered a unique microorganism appearing to have cellular features intermediate between prokaryotes and eukaryotes. The organism, named as the Myojin parakaryote, was two orders of magnitude larger than a typical bacterium and had a large “nucleoid”, consisting of naked DNA fibers, surrounded by a single layered “nucleoid membrane”, and bacteria-like “endosymbionts”, but it lacked mitochondria. This organism exemplifies a potential evolutionary path between prokaryotes and eukaryotes, and the presence of this organism supports the endosymbiotic theory for the origin of mitochondria and the karyogenetic hypothesis for the origin of the nucleus. In this chapter, we describe how the Myojin parakaryote was discovered, the features of this organism, the significance of the discovery, and perspectives on future research.