Cysts are produced as resting stages by some gram-negative bacteria found in soils. Azotobacter species are one example. The myxobacteria also produce cysts termed microcysts or microspores. Cysts of both of these Proteobacterial groups are relatively large, ultimately larger than 0.25 m, because they are formed from a vegetative cell by the addition of extra layers outside the cell wall. These specialized cells are produced in the aerial mycelium as a resistant dispersal reproductive cell.
Many bacteria form very thin filaments.
The spirochetes are one group that contains species whose cell diameters may be 0.1 m. Small Subcellular Structures Small structures have the potential of producing small fossils, although this author is not aware that any of them have been reported as fossils. Candidate structures from contemporary bacteria include prosthecae and stalks that are extensions of the cell and that are smaller than the diameter of the cell. In addition, gas vesicles are very small proteinaceous structures formed by some Bacteria and Archaea. These structures are normally associated with the much larger organism that produces them.