Prokaryote Inclusions: Descriptions and Discoveries

Abstract

Spherical inclusions were observed in a “colorless egg-shaped alga” in 1786, but were not identified as sulfur until 1875. The discovery of additional inclusions required improvements to the light microscope. Thus, cyanophycin, polyphosphate, lipid, and glycogen inclusions were not observed until late in the nineteenth century and insecticidal protein crystals in 1915. Polyhydroxybutyrate was identified as a lipid inclusion in 1926, but the confirmed identity of the others as well as the discovery of new inclusions required the introduction of the electron microscope as well as other technological advances. Glycogen, polyphosphate, and insecticidal inclusions were confirmed in 1944, 1947, and 1953, respectively. Cyanophycin was not identified until 1971. Although lipid bodies were seen in 1893, triacylglycerols were not identified as accumulated materials in prokaryotes until 1976, and wax ester accumulation, discovered in 1960, was not confirmed as an inclusion until 1985. Recombinant protein inclusions, requiring the development of gene cloning technology, were not identified until 1982. The acidicalcisome, a polyphosphate accumulation in eukaryotes, was demonstrated to be present in a prokaryote in 2003. Characterization of the known inclusions, their structure, production and utilization, as well as the discovery of additional inclusions continue today.