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Bacterial Endospores

Bacterial endospores are highly resistant to hostile physical and chemical conditions. Only a few genera of bacteria such as Bacillus and Clostridium are capable of forming endospores. These are a dormant form of the bacterium that allows it to survive sub-optimal environmental conditions. Because these spores are resistant to heat, radiation, disinfectants, and desiccation, they are difficult to eliminate from medical and pharmaceutical materials and are a frequent cause of contamination. At the start of this video, spores can be seen as the bright, refractile objects seen under phase contrast microscopy. The second part of the video show green spores differentiated from pink vegetative cells by a spore staining procedure:

Spore stainSpores have a tough outer covering made of keratin and are highly resistant to heat and chemicals. The keratin also resists staining, so specialized procedures are necessary to stain endospores:

- Malachite green stain is forced into the spore by heating the cells.
- Vegetative cells are then decolorized with water and stained pink with safranin counterstain.

Endospores may be located in the middle of the cells (central), at the end (terminal), or between the end and the middle of the cells (subterminal). The endospores themselves may be round or oval.


You can also see endospores as bright refractile dots inside the cells in the video of Bacillus halodenitrificans, but they can be seen more clearly using darkfield microscopy:


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