Purpose: The endospore stain is a differential stain which selectively stains bacterial endospores.
How it works: Bacterial endospores are metabolically inactive, highly resistant structures produced by some bacteria as a defensive strategy against unfavorable environmental conditions. The bacteria can remain in this suspended state until conditions become favorable and they can germinate and return to their vegetative state. The primary stain applied is malachite green, which stains both vegetative cells and endospores. Heat is applied to help the primary stain penetrate the endospore. The cells are then decolorized with water, which removes the malachite green from the vegetative cell but not the endospore. Safranin is then applied to counterstain any cells which have been decolorized. At the end of the staining process, vegetative cells will be pink, and endospores will be dark green. (Note: Endospore stains are performed on smears that have been heat-fixed.)
Overview of endospore-staining process:
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