Characterisation by staining: Endospore staining
Bacteria in genera such as Bacillus and Clostridium produce quite a resistant structure capable of surviving for long periods in an unfavorable environment and then giving rise to a new bacterial cell. This structure is called an endospore since it develops within the bacterial cell. Endospores are spherical to elliptical in shape and may be either smaller or larger than the parent bacterial cell. Endospore position within the cell is characteristic and may be central, subterminal, or terminal.
Endospores do not stain easily, but, once stained, they strongly resist decolorization. This property is the basis of the Schaeffer-Fulton (Alice B. Schaeffer and MacDonald Fulton were microbiologists at Middlebury College, Vermont, in the 1930s) or Wirtz-Conklin method (Robert Wirtz and Marie E. Conklin were bacteriologists in the early 1900s) of staining endospores. The endospores are stained with malachite green. Heat is used to provide stain penetration. The rest of the cell is then decolorized and counterstained a light red with safranin.