Endospores and Endospore Staining
••• Endospore Formation •••
Endospores are formed by vegetative cells in a process
called sporulation.
Sporulation is initiated when conditions for growth
of the vegetative cells become
harsh (for example, when water or an essential nutrient
is limiting). Sporulation is
a complex process involving as many as 200 genes.
These genes are activated by
an environmental trigger, causing sporulation of the
cell. The steps leading to
endospore formation result in the creation of a dry,
metabolically inert and extremely
resistant endospore from a moist, metabolically active
vegetative cell. Extensive
sporulation studies have been done on Bacillus
subtilis, and this microbe can
perform the entire sporulation process in about eight
hours.
Endospores can remain dormant for many years, but,
when conditions allow, they
can convert back to vegetative cells fairly rapidly.
This process is called germination,
and it involves three steps: activation, germination
and outgrowth. Activation
can be accomplished by heating freshly formed endospores
at a high temperature.
Activated spores can then be conditioned to germinate
by placing them in the
presence of specific nutrients. During germination,
the spore becomes less resistant.
This stage includes loss of calcium dipicolinate and
degradation of small acid-soluble
spore proteins. The final step is outgrowth,
which involves swelling due to water
uptake and the synthesis of new DNA, RNA and proteins.
The cell grows out
of the broken spore coat and eventually resumes normal
cell function. The
vegetative cells continue to grow and divide until
harsh environmental conditions
once again trigger the sporulation process.
The basic sporulation and germination cycle is illustrated by the picture below.
