Typically, spheroids vary in size from 1 to 2um but spheroids as large as 10um can also be observed from a culture. Ocassionally, membrane bound sac-like structures up to 8um in diameter, with up to 8 cells, termed rotund bodies, are observed. It can be speculated that rotund bodies are formed either by the event of numerous cell divisions of a single cell within spheroids or are formed by fusion of numerous cells. The spheroids and rotund bodies lyse immediately in the presence of SDS though the cell shape remains intact. However, granulation of the cells occurs and whether cell viability is retained after SDS remains to be determined.
Cells stain gram-negative.
Electron microscopic examination of thin sections of Fervidobacterium reveals a compex multilayered cell envelope ultrastructure typical of gram negative bacteria. The cell envelope consists of an outer layer which is in contact with the environment and an inner layer which lies adjacent to the cell membrane. The inner and outer layers are joined together except in the areas where the protruding spheroids are formed. The outer layer forms the spheroid whereas the inner layer remains attached to the cell membrane. Electron microscopic examination of negative stained preparation of cells attached to butvar-carbon coated nickle electron microscope grids after incubation in hot springs, also reveal the presence of spheroids, indicating that they are not cultural artifacts.