Typically, Gram-positive bacteria are best known for producing intracellular spores called endospores as a survival mechanism. Bacterial spores are the most dormant form of bacteria since they exhibit minimal metabolism and respiration, as well as reduced enzyme production. One of the most common coping mechanisms for bacteria is forming spores to protect themselves against ecological degrading agents. Given the importance of the killing of spores of Bacillus species in the food and medical products industry, a deeper understanding of the mechanisms of spore resistance and killing may lead to improved methods for spore destruction.Bacterial species have different coping mechanisms with selective harsh environmental conditions. Factors important in spore chemical resistance vary with the chemical, but include: (i) the spore coat proteins that likely react with and detoxify chemical agents (ii) the relative impermeability of the spore's inner membrane that restricts access of exogenous chemicals to the spore core (iii) the protection of spore DNA by its saturation with alpha/beta-type SASP and (iv) DNA repair for agents that kill spores via DNA damage. There are also other agents such as glutaraldehyde for which the mechanism of spore killing is unclear. nitrous acid, formaldehyde) again kill spores by DNA damage, while others, in particular oxidizing agents, appear to damage the spore's inner membrane so that this membrane ruptures upon spore germination and outgrowth. ![]() While SP is formed in spores with approximately the same quantum efficiency as that for generation of CPDs and 64PPs in growing cells, SP is repaired rapidly and efficiently in spore outgrowth by a number of repair systems, at least one of which is specific for SP. UV irradiation of spores at 254 nm does not generate the cyclobutane dimers (CPDs) and (6-4)-photoproducts (64PPs) formed between adjacent pyrimidines in growing cells, but rather a thymidyl-thymidine adduct termed spore photoproduct (SP). In contrast, spore UV resistance is due largely to an alteration in spore DNA photochemistry caused by the binding of alpha/beta-type SASP to the DNA, and to a lesser extent to the photosensitizing action of the spore core's large pool of dipicolinic acid. The mechanism of spore resistance to gamma-radiation is not well understood, although the alpha/beta-type SASP are not involved. Both UV and gamma-radiation also kill spores via DNA damage. The alpha/beta-type SASP are also important in spore resistance to dry heat, as is DNA repair in spore outgrowth, as Bacillus subtilis spores are killed by dry heat via DNA damage. However, how wet heat kills spores is not clear, although it is not through DNA damage. The level and type of spore core mineral ions and the intrinsic stability of total spore proteins also play a role in spore wet heat resistance, and the saturation of spore DNA with alpha/beta-type small, acid-soluble spore proteins (SASP) protects DNA against wet heat damage. A lower core water content generally gives more wet heat-resistant spores. ![]() Spore resistance to wet heat is determined largely by the water content of spore core, which is much lower than that in the growing cell protoplast. A number of mechanisms are responsible for the resistance of spores of Bacillus species to heat, radiation and chemicals and for spore killing by these agents.
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