Endospores of Bacillus spp., especially Bacillus subtilis, have served as experimental models for exploring the molecular mechanisms underlying the incredible longevity of spores and their resistance to environmental insults. In this review we summarize the molecular laboratory model of spore resistance mechanisms and attempt to use the model as a basis for exploration of the resistance of spores to environmental extremes both on Earth and during postulated interplanetary transfer through space as a result of natural impact processes.

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Resistance of Bacillus endospores to extreme terrestrial and extraterrestrial environments.

Sporulation of Bacillus subtilis

Genetic aspects of bacterial endospore formation.

Bacillus subtilis sporulation: regulation of gene expression and control of morphogenesis.

Spores of Bacillus subtilis with a mutation in spoVF cannot synthesize dipicolinic acid (DPA) and are too unstable to be purified and studied in detail. However, the spores of a strain lacking the three major germinant receptors (termed Deltager3), as well as spoVF, can be isolated, although they spontaneously germinate much more readily than Deltager3 spores. The Deltager3 spoVF spores lack DPA and have higher levels of core water than Deltager3 spores, although sporulation with DPA restores close to normal levels of DPA and core water to Deltager3 spoVF spores. The DPA-less spores have normal cortical and coat layers, as observed with an electron microscope, but their core region appears to be more hydrated than that of spores with DPA. The Deltager3 spoVF spores also contain minimal levels of the processed active form (termed P(41)) of the germination protease, GPR, a finding consistent with the known requirement for DPA and dehydration for GPR autoprocessing. However, any P(41) formed in Deltager3 spoVF spores may be at least transiently active on one of this protease's small acid-soluble spore protein (SASP) substrates, SASP-gamma. Analysis of the resistance of wild-type, Deltager3, and Deltager3 spoVF spores to various agents led to the following conclusions: (i) DPA and core water content play no role in spore resistance to dry heat, dessication, or glutaraldehyde; (ii) an elevated core water content is associated with decreased spore resistance to wet heat, hydrogen peroxide, formaldehyde, and the iodine-based disinfectant Betadine; (iii) the absence of DPA increases spore resistance to UV radiation; and (iv) wild-type spores are more resistant than Deltager3 spores to Betadine and glutaraldehyde. These results are discussed in view of current models of spore resistance and spore germination.

Characterization of Spores of Bacillus subtilis Which Lack Dipicolinic Acid

Mutants of Bacillus cereus strain T that produce thermoresistant spores which lack dipicolinic acid and have low levels of calcium and manganese have been isolated. The thermoresistant dipicolinate-less spores lose thermoresistance upon storage in distilled water and do not germinate in any liquid medium tested. All spores germinated and produced colonies on G medium agar. It is suggested that calcium and dipicolinic acid are not required for the acquisition of thermoresistance in bacterial endospores but that they are essential to the maintenance of the thermoresistant state and are required for normal germination.

Hanson Rs

Papers

Mutants of Bacillus cereus strain T that produce thermoresistant spores lacking dipicolinate and have low levels of calcium.