Endospore

An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria in the phylum Bacillota. The name "endospore" is suggestive of a spore or seed-like form (endo means 'within'), but it is not a true spore (i.e., not an offspring). It is a stripped-down, dormant form to which the bacterium can reduce itself. Endospore formation is usually triggered by a lack of nutrients, and usually occurs in Gram-positive bacteria. In endospore formation, the bacterium divides within its cell wall, and one side then engulfs the other. Endospores enable bacteria to lie dormant for extended periods, even centuries. There are many reports of spores remaining viable over 10,000 years, and revival of spores millions of years old has been claimed. There is one report of viable spores of Bacillus marismortui in salt crystals approximately 25 million years old. When the environment becomes more favorable, the endospore can reactivate itself into a vegetative state. Most types of bacteria cannot change to the endospore form. Examples of bacterial species that can form endospores include Bacillus cereus, Bacillus anthracis, Bacillus thuringiensis, Clostridium botulinum, and Clostridium tetani. Endospore formation does not occur within the Archaea or Eukaryota.

The endospore consists of the bacterium's DNA, ribosomes and large amounts of dipicolinic acid. Dipicolinic acid is a spore-specific chemical that appears to help in the ability for endospores to maintain dormancy. This chemical accounts for up to 10% of the spore's dry weight.

Endospores can survive without nutrients. They are resistant to ultraviolet radiation, desiccation, high temperature, extreme freezing and chemical disinfectants. Thermo-resistant endospores were first hypothesized by Ferdinand Cohn after studying Bacillus subtilis growth on cheese after boiling the cheese. His notion of spores being the reproductive mechanism for the growth was a large blow to the previous suggestions of spontaneous generation. Astrophysicist Steinn Sigurdsson said "There are viable bacterial spores that have been found that are 40 million years old on Earth—and we know they're very hardened to radiation." Common antibacterial agents that work by destroying vegetative cell walls do not affect endospores. Endospores are commonly found in soil and water, where they may survive for long periods of time. A variety of different microorganisms form "spores" or "cysts", but the endospores of low G+C gram-positive bacteria are by far the most resistant to harsh conditions.

Some classes of bacteria can turn into exospores, also known as microbial cysts, instead of endospores. Exospores and endospores are two kinds of "hibernating" or dormant stages seen in some classes of microorganisms.

The bacterial life cycle does not necessarily include sporulation. Sporulation is usually triggered by adverse environmental conditions, so as to help the survival of the bacterium. Endospores exhibit no signs of life and can thus be described as cryptobiotic. Endospores retain viability indefinitely and they can germinate into vegetative cells under the appropriate conditions. Endospores have survived thousands of years until environmental stimuli trigger germination. They have been characterized as the most durable cells produced in nature.

Bacteria produce a single endospore internally. The spore is sometimes surrounded by a thin covering known as the exosporium, which overlies the spore coat. The spore coat, which acts like a sieve that excludes large toxic molecules like lysozyme, is resistant to many toxic molecules and may also contain enzymes that are involved in germination. In Bacillus subtilus endospores, the spore coat is estimated to contain more than 70 coat proteins, which are organized into an inner and an outer coat layer. The X-ray diffraction pattern of purified B. subtilis endospores indicates the presence of a component with a regular periodic structure, which Kadota and Iijima speculated might be formed from a keratin-like protein. However, after further studies this group concluded that the structure of the spore coat protein was different from keratin. When the B. subtilis genome was sequenced, no ortholog of human keratin was detected. The cortex lies beneath the spore coat and consists of peptidoglycan. The core wall lies beneath the cortex and surrounds the protoplast or core of the endospore. The core contains the spore chromosomal DNA which is encased in chromatin-like proteins known as SASPs (small acid-soluble spore proteins), that protect the spore DNA from UV radiation and heat. The core also contains normal cell structures, such as ribosomes and other enzymes, but is not metabolically active.

Up to 20% of the dry weight of the endospore consists of calcium dipicolinate within the core, which is thought to stabilize the DNA. Dipicolinic acid could be responsible for the heat resistance of the spore, and calcium may aid in resistance to heat and oxidizing agents. However, mutants resistant to heat but lacking dipicolinic acid have been isolated, suggesting other mechanisms contributing to heat resistance are also at work. Small acid-soluble proteins (SASPs) are found in endospores. These proteins tightly bind and condense the DNA, and are in part responsible for resistance to UV light and DNA-damaging chemicals.

Visualising endospores under light microscopy can be difficult due to the impermeability of the endospore wall to dyes and stains. While the rest of a bacterial cell may stain, the endospore is left colourless. To combat this, a special stain technique called a Moeller stain is used. That allows the endospore to show up as red, while the rest of the cell stains blue. Another staining technique for endospores is the Schaeffer-Fulton stain, which stains endospores green and bacterial bodies red. The arrangement of spore layers is as follows: