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1. THE BACTERIAL ENDOSPORE
A number of gram-positive bacteria can form a special resistant, dormant structure called an endospore. Endospores
develop within vegetative bacterial cells of several genera: Bacillus and Clostridium (rods), Sporosarcina (cocci), and
others. These structures are extraordinarily resistant to environmental stresses such as heat, ultraviolet radiation,
gamma radiation,chemical disinfectants,anddesiccation.Because of theirresistance andthe factthat several speciesof
endospore-formingbacteriaare dangerouspathogens,endosporesare of great practical importance in food, industrial,
and medical microbiology. This is because it is essential to be able to sterilize solutions and solid objects. Endospores
often survive boiling for an hour or more; therefore autoclaves must be used to sterilize many materials. In the
environment, endospores aid in survival when moisture or nutrients are scarce
Endosporescanbe examinedwithbothlightandelectronmicroscopes. Because endospores are impermeable to most
stains,they often are seen as colorless areas in bacteria treated with methylene blue and other simple stains; special
endospore stains are used to make them clearly visible. Endospore position in the mother cell (sporangium)
Endospore structure iscomplex andcomposedof the followinglayeri.e 1. Spore often is surrounded by a thin, delicate
coveringcalledthe exosporium2.A spore coat liesbeneaththe exosporium,is composed of several protein layers, and
may be fairlythick.Itis impermeable tomanytoxicmoleculesandis responsible forthe spore’sresistance to chemicals.
The coat also isthoughtto containenzymes that are involved in germination 3. The cortex, which may occupy as much
as half the spore volume,restsbeneaththe spore coat.Itismade of a peptidoglycanthatislesscross-linkedthan that in
vegetative cells4.The spore cell wall (orcore wall) is inside the cortex and surrounds the protoplast or spore core. The
core has normal cell structures such as ribosomes and a nucleoid, but is metabolically inactive
The endospore is so resistant to heat and other lethal agents. As much as 15% of the spore’s dry weight consists of
dipicolinicacidcomplexedwithcalciumions,which is located in the core. It has long been thought that dipicolinic acid
was directlyinvolvedinheatresistance,butheat-resistantmutantslacking dipicolinic acid have been isolated. Calcium
does aid in resistance to wet heat, oxidizing agents, and sometimes dry heat. It may be that calcium-dipicolinate
stabilizesthe spore’snucleicacids.Inaddition,specializedsmall,acid-solubleDNA-bindingproteins(SASPs),are foundin
the endospore.Theysaturate spore DNA andprotectitfrom heat,radiation,dessication,andchemicals. Dehydration of
the protoplast appears to be very important in heat resistance. The cortex may osmotically remove water from the
protoplast, thereby protecting it from both heat and radiation damage. The spore coat also seems to protect against
enzymesandchemicalssuchashydrogenperoxide.Finally, spores contain some DNA repair enzymes. DNA is repaired
once the spore germinatesandthe cell becomesactive again.Insummary,endospore heatresistance probablyis due to
several factors: calcium-dipicolinate and acid-soluble protein stabilization of DNA, protoplast dehydration, the spore
coat, DNA repair, the greater stability of cell proteins in bacteria adapted to growth at high temperatures, and others.