Endospores are composed of a central spore cell, which is surrounded by various protective
layers.  The outermost layer is the exosporium, which is a thin covering made of protein.
Below this is the spore coat which is made up of highly cross-linked keratin and layers of
spore-specific proteins. The cortex consists of loosely cross-linked peptidoglycan.  The
innermost spore cell contains the components of the vegetative bacterial cell (the cell wall,
cytoplasmic membrane, cytoplasm, nucleoid, etc.).  These layers are demonstrated by the
following illustration of an endospore.

Components of an endospore.  Picture from microbiology lab manual.

The water content of endospores is only about 10-30% of the water content of
vegetative cells; therefore, endospores are capable of surviving  at levels of
dehydration that would kill vegetative cells.  The low water content also provides the
endospore with chemical resistance (to chemicals such as hydrogen peroxide) and it
causes the remaining enzymes of the spore cell to become inactive.

One chemical produced by endospores that is thought to lend to their high
resistance is dipicolinic acid.  This chemical has been found in the spore cell of
all endospores examined.  Dipicolinic acid interacts with calcium ions to form
calcium dipicolinate, which is the main substance believed to lend endospores their
resistance and represents about 10% of the dry weight of an endospore.  The spore
cell also contains small acid-soluble spore proteins (SASPs).  These function to protect
DNA from UV radiation, dessication and dry heat, and they also serve as a carbon
and energy source during the germination process (conversion back to a
vegetative cell).

Another component of endospores that contributes to their resistant to chemical
agents is the strong spore coat, which is composed of highly cross-linked keratin.
Identification of particular organisms can be aided by the presence, location and
size of endospores.  Endospores can be located centrally, terminally or
subterminally within a cell.  Sometimes the endospore is much larger in diameter
than the cell, which causes the cell to appear swollen at the location of the endospore.

  
Centrally (left), terminally (center), and subterminally (right) located endospores.
Pictures from Brock Biology of Microorganisms (lecture textbook)

• Introduction • Discovery of Endospores • Endospore Structure • Endospore Formation •
• Staining Endospores • Miscellaneous Facts • References •  Home