Spores of Bacillus subtilis: their resistance to and killing by radiation, heat and chemicals

Abstract

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. 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 lower core water content generally gives more wet heat-resistant spores. 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 a/b-type small, acid-soluble spore proteins (SASP) protects DNA against wet heat damage. However, how wet heat kills spores is not clear, although it is not through DNA damage. The a/b-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. Both UV and c-radiation also kill spores via DNA damage. The mechanism of spore resistance to c-radiation is not well understood, although the a/b-type SASP are not involved. In contrast, spore UV resistance is due largely to an alteration in spore DNA photochemistry caused by the binding of a/b-type SASP to the DNA, and to a lesser extent to the photosensitizing action of the spore core's large pool of dipicolinic acid. 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 thymidylthymidine adduct termed spore photoproduct (SP). 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. Some chemicals (e.g. 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. There are also other agents such as glutaraldehyde for which the mechanism of spore killing is unclear. 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 a/b-type SASP; and (iv) DNA repair for agents that kill spores via DNA damage.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.

Inactivation of Geobacillus stearothermophilus spores by alkaline hydrolysis applied to medical waste treatment

“…Although the mechanism of spore inactivation by wet heat is not entirely clear yet, it is partially due to the rupture of the spore inner membrane permeability barrier, which causes an increase in the core water content (Setlow, 2006). The spores inactivation by alkaline treatment seems to involve the removal of alkali-soluble coat proteins with consequent inactivation of the lytic enzymes essential for cortex hydrolysis and spore germination (Duncan et al, 1972).…”

“…The low water content in the spore core seems to be the most important factor of a spore wet heat resistance. Indeed, the wet heat resistance correlates negatively with the core water content (Setlow, 2006). The high core mineralization also confers wet heat resistance; ions such as Ca 2+ ensure a higher wet heat protection than Mg 2+ , Mn 2+ , Na + and K + .…”

“…The high core mineralization also confers wet heat resistance; ions such as Ca 2+ ensure a higher wet heat protection than Mg 2+ , Mn 2+ , Na + and K + . Another essential factor to the spore resistance is the high quantity of small acid-soluble spore proteins (SASPs) that protect the spore DNA by its saturation with a/b-type SASP and DNA repair systems (Leggett et al, 2012;Setlow, 2006).…”

Role of the Nfo and ExoA Apurinic/Apyrimidinic Endonucleases in Repair of DNA Damage during Outgrowth of Bacillus subtilis Spores

“…As a consequence, these proteins make major contributions to spore resistance to heat and oxidizing agents (13, 31). The ␣/␤-type SASP are also a major factor in the high resistance of B. subtilis spores to UV light (13,14,28,29,31). However, these DNA binding proteins do not protect DNA against base alkylation (27).…”

Rapid and reliable detection of bacterial endospores in environmental samples by diagnostic electron microscopy combined with X-ray microanalysis

“…Spores contain a considerable amount of calcium within the spore core and sometimes other particular elements (e.g. manganese) (Setlow, 2006), which could help to distinguish spores from other, for instance inorganic particles. Investigation of spores by scanning-transmission electron microscopy and X-ray spectroscopy, using thin cryosections, could reveal the main element composition of the spore and their distribution among the different structures (Scherrer and Gerhardt, 1972, Stewart et al, 1980and Stewart et al, 1981.…”