Small, Acid-Soluble Spore Proteins of the alpha/beta Type Do Not Protect the DNA in Bacillus subtilis Spores against Base Alkylation

Bacillus subtilis stress-associated mutagenesis and developmental DNA repair

SUMMARYThe metabolic conditions that prevail during bacterial growth have evolved with the faithful operation of repair systems that recognize and eliminate DNA lesions caused by intracellular and exogenous agents. This idea is supported by the low rate of spontaneous mutations (10-9) that occur in replicating cells, maintaining genome integrity. In contrast, when growth and/or replication cease, bacteria frequently process DNA lesions in an error-prone manner. DNA repairs provide cells with the tools needed for maintaining homeostasis during stressful conditions and depend on the developmental context in which repair events occur. Thus, different physiological scenarios can be anticipated. In nutritionally stressed bacteria, different components of the base excision repair pathway may process damaged DNA in an error-prone approach, promoting genetic variability. Interestingly, suppressing the mismatch repair machinery and activating specific DNA glycosylases promote stationary-phase mutations. Current evidence also suggests that in resting cells, coupling repair processes to actively transcribed genes may promote multiple genetic transactions that are advantageous for stressed cells. DNA repair during sporulation is of interest as a model to understand how transcriptional processes influence the formation of mutations in conditions where replication is halted. Current reports indicate that transcriptional coupling repair-dependent and -independent processes operate in differentiating cells to process spontaneous and induced DNA damage and that error-prone synthesis of DNA is involved in these events. These and other noncanonical ways of DNA repair that contribute to mutagenesis, survival, and evolution are reviewed in this manuscript.

The small acid-soluble proteins of spore-forming organisms: similarities and differences in function

The small acid-soluble proteins are found in all endospore-forming organisms and are a major component of spores. Through their DNA binding capabilities, the SASPs shield the DNA from outside insults (e.g., UV and genotoxic chemicals). The absence of the major SASPs results in spores with reduced viability when exposed to UV light and, in at least one case, the inability to complete sporulation. While the SASPs have been characterized for decades, some evidence suggests that using newer technologies to revisit the roles of the SASPs could reveal novel functions in spore regulation.

Aag Hypoxanthine-DNA Glycosylase Is Synthesized in the Forespore Compartment and Involved in Counteracting the Genotoxic and Mutagenic Effects of Hypoxanthine and Alkylated Bases in DNA during Bacillus subtilis Sporulation

Aag from Bacillus subtilis has been implicated in in vitro removal of hypoxanthine and alkylated bases from DNA. The regulation of expression of aag in B. subtilis and the resistance to genotoxic agents and mutagenic properties of an Aag-deficient strain were studied here. A strain with a transcriptional aag-lacZ fusion expressed low levels of β-galactosidase during growth and early sporulation but exhibited increased transcription during late stages of this developmental process. Notably, aag-lacZ expression was higher inside the forespore than in the mother cell compartment, and this expression was abolished in a sigG-deficient background, suggesting a forespore-specific mechanism of aag transcription. Two additional findings supported this suggestion: (i) expression of an aag-yfp fusion was observed in the forespore, and (ii) in vivo mapping of the aag transcription start site revealed the existence of upstream regulatory sequences possessing homology to σ^G-dependent promoters. In comparison with the wild-type strain, disruption of aag significantly reduced survival of sporulating B. subtilis cells following nitrous acid or methyl methanesulfonate treatments, and the Rif^r mutation frequency was significantly increased in an aag strain. These results suggest that Aag protects the genome of developing B. subtilis sporangia from the cytotoxic and genotoxic effects of base deamination and alkylation.

IMPORTANCE

In this study, evidence is presented revealing that aag, encoding a DNA glycosylase implicated in processing of hypoxanthine and alkylated DNA bases, exhibits a forespore-specific pattern of gene expression during B. subtilis sporulation. Consistent with this spatiotemporal mode of expression, Aag was found to protect the sporulating cells of this microorganism from the noxious and mutagenic effects of base deamination and alkylation.

Clostridium difficile spores: a major threat to the hospital environment

Clostridium difficile is a Gram-positive, anaerobic spore former and is an important nosocomial and community-acquired pathogenic bacterium. C. difficile infections (CDI) are a leading cause of infections worldwide with elevated rates of morbidity. Despite the fact that two major virulence factors, the enterotoxin TcdA and the cytotoxin TcdB, are essential in the development of CDI, C. difficile spores are the main vehicle of infection, and persistence and transmission of CDI and are thought to play an essential role in episodes of CDI recurrence and horizontal transmission. Recent research has unmasked several properties of C. difficile's unique strategy to form highly transmissible spores and to persist in the colonic environment. Therefore, the aim of this article is to summarize recent advances in the biological properties of C. difficile spores, which might be clinically relevant to improve the management of CDI in hospital environments.

Bacterial spore structures and their protective role in biocide resistance

The structure and chemical composition of bacterial spores differ considerably from those of vegetative cells. These differences largely account for the unique resistance properties of the spore to environmental stresses, including disinfectants and sterilants, resulting in the emergence of spore-forming bacteria such as Clostridium difficile as major hospital pathogens. Although there has been considerable work investigating the mechanisms of action of many sporicidal biocides against Bacillus subtilis spores, there is far less information available for other species and particularly for various Clostridia. This paucity of information represents a major gap in our knowledge given the importance of Clostridia as human pathogens. This review considers the main spore structures, highlighting their relevance to spore resistance properties and detailing their chemical composition, with a particular emphasis on the differences between various spore formers. Such information will be vital for the rational design and development of novel sporicidal chemistries with enhanced activity in the future.

Role of the Nfo and ExoA apurinic/apyrimidinic endonucleases in repair of DNA damage during outgrowth of Bacillus subtilis spores

Germination and outgrowth are critical steps for returning Bacillus subtilis spores to life. However, oxidative stress due to full hydration of the spore core during germination and activation of metabolism in spore outgrowth may generate oxidative DNA damage that in many species is processed by apurinic/apyrimidinic (AP) endonucleases. B. subtilis spores possess two AP endonucleases, Nfo and ExoA; the outgrowth of spores lacking both of these enzymes was slowed, and the spores had an elevated mutation frequency, suggesting that these enzymes repair DNA lesions induced by oxidative stress during spore germination and outgrowth. Addition of H2O2 also slowed the outgrowth of nfo exoA spores and increased the mutation frequency, and nfo and exoA mutations slowed the outgrowth of spores deficient in either RecA, nucleotide excision repair (NER), or the DNA-protective alpha/beta-type small acid-soluble spore proteins (SASP). These results suggest that alpha/beta-type SASP protect DNA of germinating spores against damage that can be repaired by Nfo and ExoA, which is generated either spontaneously or promoted by addition of H2O2. The contribution of RecA and Nfo/ExoA was similar to but greater than that of NER in repair of DNA damage generated during spore germination and outgrowth. However, nfo and exoA mutations increased the spontaneous mutation frequencies of outgrown spores lacking uvrA or recA to about the same extent, suggesting that DNA lesions generated during spore germination and outgrowth are processed by Nfo/ExoA in combination with NER and/or RecA. These results suggest that Nfo/ExoA, RecA, the NER system, and alpha/beta-type SASP all contribute to the repair of and/or protection against oxidative damage of DNA in germinating and outgrowing spores.

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

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 alpha/beta-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 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. Both UV and gamma-radiation also kill spores via DNA damage. The mechanism of spore resistance to gamma-radiation is not well understood, although the alpha/beta-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 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. 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). 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 alpha/beta-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.

I will survive: DNA protection in bacterial spores

Dormant spores of Bacillus, Clostridium and related species can survive for years, largely because spore DNA is well protected against damage by many different agents. This DNA protection is partly a result of the high level of Ca(2+)-dipicolinic acid in spores and DNA repair during spore outgrowth, but is primarily caused by the saturation of spore DNA with a group of small, acid-soluble spore proteins (SASP), which are synthesized in the developing spore and then degraded after completion of spore germination. The structure of both DNA and SASP alters upon their association and this causes major changes in the chemical and photochemical reactivity of DNA.

Mechanisms of Bacillus subtilis spore killing by and resistance to an acidic Fe3+-EDTA-iodide-ethanol formulation

AIMS

To determine the mechanisms of Bacillus subtilis spore killing by and resistance to an acidic solution containing Fe(3+), EDTA, KI and ethanol termed the KMT reagent.

METHODS AND RESULTS

Wild-type B. subtilis spores were not mutagenized by the KMT reagent but the wild-type and recA spores were killed at the same rate. Spores (alpha(-)beta(-)) lacking most DNA-protective alpha/beta-type small, acid-soluble spore proteins were less resistant to the KMT reagent than wild-type spores but were also not mutagenized, and alpha(-)beta(-) and alpha(-)beta(-)recA spores exhibited nearly identical resistance. Spore resistance to the KMT reagent was greatly decreased if spores had defective coats. However, the level of unsaturated fatty acids in the inner membrane did not determine spore sensitivity to the KMT reagent. Survivors in spore populations killed by the KMT reagent were sensitized to killing by wet heat or nitrous acid and to high salt in plating medium. KMT reagent-killed spores had not released their dipicolinic acid (DPA), although these killed spores released their DPA more readily when germinated with dodecylamine than did untreated spores. However, KMT reagent-killed spores did not germinate with nutrients or Ca(2+)-DPA and were recovered only poorly by lysozyme treatment in a hypertonic medium.

CONCLUSIONS

The KMT reagent does not kill spores by DNA damage and a major factor in spore resistance to this reagent is the spore coat. KMT reagent treatment damages the spore's ability to germinate, perhaps by damaging the spore's inner membrane. However, this damage is not oxidation of unsaturated fatty acids.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results provide information on the mechanism of spore resistance to and killing by the KMT reagent developed for killing Bacillus spores.

Relevance of diffusion through bacterial spore coats/membranes and the associated concentration boundary layers in the initial lag phase of inactivation: A case study for Bacillus subtilis with ozone and monochloramine

Disinfectants are generally used to inactivate microorganisms in solutions. The process of inactivation involves the disinfectant in the liquid diffusing towards the bacteria sites and thereafter reacting with bacteria at rates determined by the respective reaction rates. Such processes have demonstrated an initial lag phase followed by an active depletion phase of bacteria. This paper attempts to study the importance of the combined effects of diffusion of the disinfectant through the outer membrane of the bacteria and transport through the associated concentration boundary layers (CBLs) during the initial lag phase. Mathematical equations are developed correlating the initial concentration of the disinfectant with time required for reaching a critical concentration (C*) at the inner side of the membrane of the cell based on diffusion of disinfectant through the outer membranes of the bacteria and the formation of concentration boundary layers on both sides of the membranes. Experimental data of the lag phases of inactivation already available in the literature for inactivation of Bacillus subtilis spores with ozone and monochloramine are tested with the equations. The results seem to be in good agreement with the theoretical equations indicating the importance of diffusion process across the outer cell membranes and the resulting CBL's during the lag phase of disinfection.

Analysis of factors that influence the sensitivity of spores of Bacillus subtilis to DNA damaging chemicals

AIMS

To elucidate factors influencing the sensitivity of Bacillus subtilis spores to DNA damaging chemicals.

METHODS AND RESULTS

Wild-type spores of B. subtilis made at lower temperatures were more sensitive to the DNA damaging chemicals formaldehyde and nitrous acid than were spores made at higher temperatures, but this was not the case with the DNA alkylating agents ethylmethanesulphonate and methylmethanesulphonate. Spores lacking most DNA protective alpha/beta-type small, acid-soluble proteins (termed alpha-beta- spores) made at lower temperatures were also more sensitive to killing through DNA damage by hydrogen peroxide than were alpha-beta- spores made at higher temperatures. The spore coat, whose composition varies significantly with sporulation temperature, played only a minor role in spore resistance to these DNA damaging agents. Spores made at lower temperatures exhibited higher permeability to the methylamine and germinated more rapidly with the surfactant dodecylamine than did spores made at higher temperatures. Treatment of spores with the oxidizing agent cumene hydroperoxide sensitized the surviving spores to all these DNA damaging agents. The fatty acid composition of the inner membrane of spores made at different temperatures differed significantly, but levels of unsaturated fatty acids in the inner membrane did not influence spore resistance to DNA damaging agents or the sensitization to such agents by prior treatment with cumene hydroperoxide.

CONCLUSIONS

The higher rates of methylamine permeation across the inner membrane of spores made at lower temperatures and the greater sensitivity of wild-type spores made at lower temperatures to formaldehyde and nitrous acid and of alpha-beta- spores made at lower temperatures to hydrogen peroxide, all agents that must pass through the spore's inner membrane to damage DNA in the spore core, suggest that the permeability of the inner membrane is a significant factor influencing spore sensitivity to these agents. The sensitization of spores to DNA damaging chemicals by pretreatment with an oxidizing agent, a treatment that increases the permeability of the spore's inner membrane, and the more rapid dodecylamine germination of spores made at lower temperatures are consistent with this suggestion.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results in this communication provide new insight into the factors that influence the resistance of spores of Bacillus species to chemicals that kill spores by damaging spore DNA.

Structure of the DNA-SspC complex: implications for DNA packaging, protection, and repair in bacterial spores

Bacterial spores have long been recognized as the sturdiest known life forms on earth, revealing extraordinary resistance to a broad range of environmental assaults. A family of highly conserved spore-specific DNA-binding proteins, termed alpha/beta-type small, acid-soluble spore proteins (SASP), plays a major role in mediating spore resistance. The mechanism by which these proteins exert their protective activity remains poorly understood, in part due to the lack of structural data on the DNA-SASP complex. By using cryoelectron microscopy, we have determined the structure of the helical complex formed between DNA and SspC, a characteristic member of the alpha/beta-type SASP family. The protein is found to fully coat the DNA, forming distinct protruding domains, and to modify DNA structure such that it adopts a 3.2-nm pitch. The protruding SspC motifs allow for interdigitation of adjacent DNA-SspC filaments into a tightly packed assembly of nucleoprotein helices. By effectively sequestering DNA molecules, this dense assembly of filaments is proposed to enhance and complement DNA protection obtained by DNA saturation with the alpha/beta-type SASP.

Mechanisms of Bacillus subtilis spore resistance to and killing by aqueous ozone

AIMS

To determine the mechanisms of Bacillus subtilis spore killing by and resistance to aqueous ozone.

METHODS AND RESULTS

Killing of B. subtilis spores by aqueous ozone was not due to damage to the spore's DNA, as wild-type spores were not mutagenized by ozone and wild-type and recA spores exhibited very similar ozone sensitivity. Spores (termed alpha-beta-) lacking the two major DNA protective alpha/beta-type small, acid-soluble spore proteins exhibited decreased ozone resistance but were also not mutagenized by ozone, and alpha-beta- and alpha-beta-recA spores exhibited identical ozone sensitivity. Killing of spores by ozone was greatly increased if spores were chemically decoated or carried a mutation in a gene encoding a protein essential for assembly of the spore coat. Ozone killing did not cause release of the spore core's large depot of dipicolinic acid (DPA), but these killed spores released all of their DPA after a subsequent normally sublethal heat treatment and also released DPA much more readily when germinated in dodecylamine than did untreated spores. However, ozone-killed spores did not germinate with either nutrients or Ca(2+)-DPA and could not be recovered by lysozyme treatment.

CONCLUSIONS

Ozone does not kill spores by DNA damage, and the major factor in spore resistance to this agent appears to be the spore coat. Spore killing by ozone seems to render the spores defective in germination, perhaps because of damage to the spore's inner membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results provide information on the mechanisms of spore killing by and resistance to ozone.

Mechanisms of killing of Bacillus subtilis spores by Decon and OxoneTM, two general decontaminants for biological agents

AIMS

To determine the mechanisms of Bacillus subtilis spore killing by and resistance to the general biological decontamination agents, Decon and Oxone.

METHODS AND RESULTS

Spores of B. subtilis treated with Decon or Oxone did not accumulate DNA damage and were not mutagenized. Spore killing by these agents was increased if spores were decoated. Spores prepared at higher temperatures were more resistant to these agents, consistent with a major role for spore coats in this resistance. Neither Decon nor Oxone released the spore core's depot of dipicolinic acid (DPA), but Decon- and Oxone-treated spores more readily released DPA upon a subsequent normally sublethal heat treatment. Decon- and Oxone-killed spores initiated germination with dodecylamine more rapidly than untreated spores, but could not complete germination triggered by nutrients or Ca(2+)-DPA and did not degrade their peptidoglycan cortex. However, lysozyme treatment did not recover these spores.

CONCLUSIONS

Decon and Oxone do not kill B. subtilis spores by DNA damage, and a major factor in spore resistance to these agents is the spore coat. Spore killing by both agents renders spores defective in germination, possibly because of damage to the inner membrane of spore.

SIGNIFICANCE AND IMPACT OF STUDY

These results provide information on the mechanisms of the killing of bacterial spores by Decon and Oxone.

Mechanisms of killing of Bacillus subtilis spores by hypochlorite and chlorine dioxide

AIMS

To determine the mechanisms of Bacillus subtilis spore killing by hypochlorite and chlorine dioxide, and its resistance against them.

METHODS AND RESULTS

Spores of B. subtilis treated with hypochlorite or chlorine dioxide did not accumulate damage to their DNA, as spores with or without the two major DNA protective alpha/beta-type small, acid soluble spore proteins exhibited similar sensitivity to these chemicals; these agents also did not cause spore mutagenesis and their efficacy in spore killing was not increased by the absence of a major DNA repair pathway. Spore killing by these two chemicals was greatly increased if spores were first chemically decoated or if spores carried a mutation in a gene encoding a protein essential for assembly of many spore coat proteins. Spores prepared at a higher temperature were also much more resistant to these agents. Neither hypochlorite nor chlorine dioxide treatment caused release of the spore core's large depot of dipicolinic acid (DPA), but hypochlorite- and chlorine dioxide-treated spores much more readily released DPA upon a subsequent normally sub-lethal heat treatment than did untreated spores. Hypochlorite-killed spores could not initiate the germination process with either nutrients or a 1 : 1 chelate of Ca2+-DPA, and these spores could not be recovered by lysozyme treatment. Chlorine dioxide-treated spores also did not germinate with Ca2+-DPA and could not be recovered by lysozyme treatment, but did germinate with nutrients. However, while germinated chlorine dioxide-killed spores released DPA and degraded their peptidoglycan cortex, they did not initiate metabolism and many of these germinated spores were dead as determined by a viability stain that discriminates live cells from dead ones on the basis of their permeability properties.

CONCLUSIONS

Hypochlorite and chlorine dioxide do not kill B. subtilis spores by DNA damage, and a major factor in spore resistance to these agents appears to be the spore coat. Spore killing by hypochlorite appears to render spores defective in germination, possibly because of severe damage to the spore's inner membrane. While chlorine dioxide-killed spores can undergo the initial steps in spore germination, these germinated spores can go no further in this process probably because of some type of membrane damage.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results provide information on the mechanisms of the killing of bacterial spores by hypochlorite and chlorine dioxide.

YqfS from Bacillus subtilis is a spore protein and a new functional member of the type IV apurinic/apyrimidinic-endonuclease family

The enzymatic properties and the physiological function of the type IV apurinic/apyrimidinic (AP)-endonuclease homolog of Bacillus subtilis, encoded by yqfS, a gene specifically expressed in spores, were studied here. To this end, a recombinant YqfS protein containing an N-terminal His6 tag was synthesized in Escherichia coli and purified to homogeneity. An anti-His6-YqfS polyclonal antibody exclusively localized YqfS in cell extracts prepared from B. subtilis spores. The His6-YqfS protein demonstrated enzymatic properties characteristic of the type IV family of DNA repair enzymes, such as AP-endonucleases and 3'-phosphatases. However, the purified protein lacked both 5'-phosphatase and exonuclease III activities. YqfS showed not only a high level of amino acid identity with E. coli Nfo but also a high resistance to inactivation by EDTA, in the presence of DNA containing AP sites (AP-DNA). These results suggest that YqfS possesses a trinuclear Zn center in which the three metal atoms are intimately coordinated by nine conserved basic residues and two water molecules. Electrophoretic mobility shift assays demonstrated that YqfS possesses structural properties that permit it to bind and scan undamaged DNA as well as to strongly interact with AP-DNA. The ability of yqfS to genetically complement the DNA repair deficiency of an E. coli mutant lacking the major AP-endonucleases Nfo and exonuclease III strongly suggests that its product confers protection to cells against the deleterious effects of oxidative promoters and alkylating agents. Thus, we conclude that YqfS of B. subtilis is a spore-specific protein that has structural and enzymatic properties required to participate in the repair of AP sites and 3' blocking groups of DNA generated during both spore dormancy and germination.

Forespore-specific expression of Bacillus subtilis yqfS, which encodes type IV apurinic/apyrimidinic endonuclease, a component of the base excision repair pathway

The temporal and spatial expression of the yqfS gene of Bacillus subtilis, which encodes a type IV apurinic/apyrimidinic endonuclease, was studied. A reporter gene fusion to the yqfS opening reading frame revealed that this gene is not transcribed during vegetative growth but is transcribed during the last steps of the sporulation process and is localized to the developing forespore compartment. In agreement with these results, yqfS mRNAs were mainly detected by both Northern blotting and reverse transcription-PCR, during the last steps of sporulation. The expression pattern of the yqfS-lacZ fusion suggested that yqfS may be an additional member of the Esigma(G) regulon. A primer extension product mapped the transcriptional start site of yqfS, 54 to 55 bp upstream of translation start codon of yqfS. Such an extension product was obtained from RNA samples of sporulating cells but not from those of vegetatively growing cells. Inspection of the nucleotide sequence lying upstream of the in vivo-mapped transcriptional yqfS start site revealed the presence of a sequence with good homology to promoters preceding genes of the sigma(G) regulon. Although yqfS expression was temporally regulated, neither oxidative damage (after either treatment with paraquat or hydrogen peroxide) nor mitomycin C treatment induced the transcription of this gene.

Analysis of the killing of spores of Bacillus subtilis by a new disinfectant, Sterilox

AIMS

To determine the mechanism whereby the new disinfectant Sterilox kills spores of Bacillus subtilis.

METHODS AND RESULTS

Bacillus subtilis spores were readily killed by Sterilox and spore resistance to this agent was due in large part to the spore coats. Spore killing by Sterilox was not through DNA damage, released essentially no spore dipicolinic acid and Sterilox-killed spores underwent the early steps in spore germination, including dipicolinic acid release, cortex degradation and initiation of metabolism. However, these germinated spores never swelled and many had altered permeability properties.

CONCLUSIONS

We suggest that Sterilox treatment kills dormant spores by oxidatively modifying the inner membrane of the spores such that this membrane becomes non-functional in the germinated spore leading to spore death.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work provides information on the mechanism of spore resistance to and spore killing by a new disinfectant.

Resistance of Bacillus endospores to extreme terrestrial and extraterrestrial environments

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.

Mechanisms of killing of spores of Bacillus subtilis by iodine, glutaraldehyde and nitrous acid

Treatment of wild-type spores of Bacillus subtilis with glutaraldehyde or an iodine-based disinfectant (Betadine) did not cause detectable mutagenesis, and spores (termed alpha-beta-) lacking the major DNA-protective alpha/beta-type, small, acid-soluble proteins (SASP) exhibited similar sensitivity to these agents. A recA mutation did not sensitize wild-type or alpha-beta- spores to Betadine or glutaraldehyde, nor did spore treatment with these agents result in significant expression of a recA-lacZ fusion when the treated spores germinated. Spore glutaraldehyde sensitivity was increased dramatically by removal of much spore coat protein, but this treatment had no effect on Betadine sensitivity. In contrast, nitrous acid treatment of wild-type and alpha-beta- spores caused significant mutagenesis, with alpha-beta- spores being much more sensitive to this agent. A recA mutation further sensitized both wild-type and alpha-beta- spores to nitrous acid, and there was significant expression of a recA-lacZ fusion when nitrous acid-treated spores germinated. These results indicate that: (a) nitrous acid kills B. subtilis spores at least in part by DNA damage, and alpha/beta-type SASP protect against this DNA damage; (b) killing of spores by glutaraldehyde or Betadine is not due to DNA damage; and (c) the spore coat protects spores against killing by glutaraldehyde but not Betadine. Further analysis also demonstrated that spores treated with nitrous acid still germinated normally, while those treated with glutaraldehyde or Betadine did not.

Formaldehyde kills spores of Bacillus subtilis by DNA damage and small, acid-soluble spore proteins of the alpha/beta-type protect spores against this DNA damage

Killing of wild-type spores of Bacillus subtilis with formaldehyde also caused significant mutagenesis; spores (termed alpha-beta-) lacking the two major alpha/beta-type small, acid-soluble spore proteins (SASP) were more sensitive to both formaldehyde killing and mutagenesis. A recA mutation sensitized both wild-type and alpha-beta- spores to formaldehyde treatment, which caused significant expression of a recA-lacZ fusion when the treated spores germinated. Formaldehyde also caused protein-DNA cross-linking in both wild-type and alpha-beta- spores. These results indicate that: (i) formaldehyde kills B. subtilis spores at least in part by DNA damage and (b) alpha/beta-type SASP protect against spore killing by formaldehyde, presumably by protecting spore DNA.

Heat killing of Bacillus subtilis spores in water is not due to oxidative damage

The heat resistance of wild-type spores of Bacillus subtilis or spores (termed alpha-beta-) lacking DNA protective alpha/beta-type small, acid-soluble spore proteins was not altered by anaerobiosis or high concentrations of the free radical scavenging agents ethanethiol and ethanedithiol. Heat-killed wild-type and alpha-beta- spores exhibited no increase in either protein carbonyl content or oxidized bases in DNA. These data strongly suggest that oxidative damage to spore macromolecules does not contribute significantly to spore killing by heat.