TEM images of L-forms, spores and cells. TEM was used to obtain images of L-forms, spores and cells to compare their morphology and structure. The L-form population lacks a cell wall resulting in spherical or pleomorphic cell morphology (Figure 3A and 3B). The cell membrane (M) is visible, and in many cases, a dark protrusion (D) of unknown function is observed (3B). Images of cells clearly show the cell wall (CW), and C. thermocellum’s normal rod morphology (Figure 3C and 3D). Coccoid-looking cells in Figure 3C are indicative of cells that were cross-sectioned across their diameter, but the cell wall structure is still easily recognized. The spore coat (SC) is also easily recognized as a several dense layers (Figure 3D).