Role of SpoVA proteins in release of dipicolinic acid during germination of Bacillus subtilis spores triggered by dodecylamine or lysozyme

The release of dipicolinic acid (DPA) during the germination of Bacillus subtilis spores by the cationic surfactant dodecylamine exhibited a pH optimum of approximately 9 and a temperature optimum of 60 degrees C. DPA release during dodecylamine germination of B. subtilis spores with fourfold-elevated levels of the SpoVA proteins that have been suggested to be involved in the release of DPA during nutrient germination was about fourfold faster than DPA release during dodecylamine germination of wild-type spores and was inhibited by HgCl(2). Spores carrying temperature-sensitive mutants in the spoVA operon were also temperature sensitive in DPA release during dodecylamine germination as well as in lysozyme germination of decoated spores. In addition to DPA, dodecylamine triggered the release of amounts of Ca(2+) almost equivalent to those of DPA, and at least one other abundant spore small molecule, glutamic acid, was released in parallel with Ca(2+) and DPA. These data indicate that (i) dodecylamine triggers spore germination by opening a channel in the inner membrane for Ca(2+)-DPA and other small molecules, (ii) this channel is composed at least in part of proteins, and (iii) SpoVA proteins are involved in the release of Ca(2+)-DPA and other small molecules during spore germination, perhaps by being a part of a channel in the spore's inner membrane.     

Analysis of factors influencing the rate of germination of spores of Bacillus subtilis by very high pressure

AIMS: To elucidate the factors that determine the rate of germination of Bacillus subtilis spores with very high pressure (VHP) and the mechanism of VHP germination. METHODS AND RESULTS: Spores of B. subtilis were germinated rapidly with a VHP of 500 MPa at 50 degrees C. This VHP germination did not require the spore's nutrient-germinant receptors, as found previously, and did not require diacylglycerylation of membrane proteins. However, the spore's pool of dipicolinic acid (DPA) was essential. Either of the two redundant enzymes that degrade the spore's peptidoglycan cortex, and thus allow completion of spore germination, was essential for completion of VHP germination. However, neither of these enzymes was needed for DPA release triggered by VHP treatment. Completion of spore germination as well as DPA release with VHP had an optimum temperature of approx. 60 degrees C, in contrast to an optimum temperature of 40 degrees C for germination with the moderately high pressure of 150 MPa. The rate of spore germination by VHP decreased approx. fourfold when the sporulation temperature increased from 23 degrees C to 44 degrees C, and decreased twofold when 1 mol l(-1) salt was present in sporulation. However, large variations in levels of unsaturated fatty acids in the spore's inner membranes did not affect rates of VHP germination. Complete germination of spores by VHP was not inhibited significantly by killing of spores with several oxidizing agents, and was not inhibited by ethanol, octanol or o-chlorophenol at concentrations that abolish nutrient germination. Completion of spore germination by VHP was also inhibited by Hg(2+), but this ion did not inhibit DPA release caused by VHP. In contrast, dodecylamine, a surfactant that can trigger spore germination, strongly inhibited DPA release caused by VHP treatment. CONCLUSIONS: VHP does not cause spore germination by acting upon the spore's nutrient-germinant receptors, but by directly causing DPA release. This DPA release then leads to subsequent completion of germination. VHP likely acts on the spore's inner membrane to cause DPA release, targeting either a membrane protein or the membrane itself. However, the precise identity of this target is not yet clear. SIGNIFICANCE AND IMPACT OF THE STUDY: There is significant interest in the use of VHP to eliminate or reduce levels of bacterial spores in foods. As at least partial spore germination by pressure is almost certainly essential for subsequent spore killing, knowledge of factors involved and the mechanism of VHP germination are crucial to the understanding of spore killing by VHP. This work provides new insight into factors that can affect the rate of B. subtilis spore germination by VHP, and into the mechanism of VHP germination itself.     

Studies of the release of small molecules during pressure germination of spores of Bacillus subtilis

AIMS: To measure rates of release of small molecules during pressure germination of Bacillus subtilis spores, and the role of SpoVA proteins in dipicolinic acid (DPA) release. METHODS AND RESULTS: Rates of DPA release during B. subtilis spore germination with pressures of 150 or 500 megaPascals were much higher in spores with elevated levels of SpoVA proteins, and spores with a temperature-sensitive mutation in the spoVA operon were temperature-sensitive in DPA release during pressure germination. Spores also released arginine and glutamic acid, but not AMP, during pressure germination. CONCLUSIONS: Pressure germination of B. subtilis spores causes release of many small molecules including DPA. SpoVA proteins are involved in the release of DPA, perhaps because SpoVA proteins are a component of a DPA channel in the spore's inner membrane. SIGNIFICANCE AND IMPACT OF THE STUDY: This work provides new insight into the mechanism of pressure germination of spores of Bacillus species, a process that has significant potential for usage in the food industry.     

Role of a SpoVA protein in dipicolinic acid uptake into developing spores of Bacillus subtilis

The proteins encoded by the spoVA operon, including SpoVAD, are essential for the uptake of the 1:1 chelate of pyridine-2,6-dicarboxylic acid (DPA(2,6)) and Ca(2+) into developing spores of the bacterium Bacillus subtilis. The crystal structure of B. subtilis SpoVAD has been determined recently, and a structural homology search revealed that SpoVAD shares significant structural similarity but not sequence homology to a group of enzymes that bind to and/or act on small aromatic molecules. We find that molecular docking placed DPA(2,6) exclusively in a highly conserved potential substrate-binding pocket in SpoVAD that is similar to that in the structurally homologous enzymes. We further demonstrate that SpoVAD binds both DPA(2,6) and Ca(2+)-DPA(2,6) with a similar affinity, while exhibiting markedly weaker binding to other DPA isomers. Importantly, mutations of conserved amino acid residues in the putative DPA(2,6)-binding pocket in SpoVAD essentially abolish its DPA(2,6)-binding capacity. Moreover, replacement of the wild-type spoVAD gene in B. subtilis with any of these spoVAD gene variants effectively eliminated DPA(2,6) uptake into developing spores in sporulation, although the variant proteins were still located in the spore inner membrane. Our results provide direct evidence that SpoVA proteins, in particular SpoVAD, are directly involved in DPA(2,6) movement into developing B. subtilis spores.     

Photosensitization of DNA by dipicolinic acid, a major component of spores of Bacillus species.

The DNA in spores of Bacillus species exhibits a relatively novel photochemistry, as 5-thyminyl-5,6-dihydrothymine (spore photoproduct (SP)) is by far the major UV photoproduct whereas cyclobutane dimers (CPDs) and (6-4) photoproducts (6-4PPs) are the major photoproducts in growing cells. Dehydration and more importantly complexation of DNA by alpha/beta-type small, acid-soluble spore proteins (SASP) have been shown to partly explain the photochemistry of spore DNA. The large amount ( approximately 10% of dry weight) of the spore's dipicolinic acid (DPA) also has been shown to play a role in spore DNA photochemistry. In the present work we showed by exposing spores of various strains of B. subtilis to UVC radiation that DPA photosensitizes spore DNA to damage and favors the formation of SP. The same result was obtained in either the presence or absence of the alpha/beta-type SASP that saturate the spore chromosome. Addition of DPA to dry films of isolated DNA or to frozen solutions of thymidine also led to a higher yield of SP and increased ratio of CPDs to 6-4PPs; DPA also significantly increased the yield of CPDs in thymidine exposed to UVC in liquid solution. These observations strongly support a triplet energy transfer between excited DPA and thymine residues. We further conclude that the combined effects of alpha/beta-type SASP and DPA explain the novel photochemistry of DNA in spores of Bacillus species.     

Role of dipicolinic acid in resistance and stability of spores of Bacillus subtilis with or without DNA-protective alpha/beta-type small acid-soluble proteins

Dipicolinic acid (DPA) comprises approximately 10% of the dry weight of spores of Bacillus species. Although DPA has long been implicated in spore resistance to wet heat and spore stability, definitive evidence on the role of this abundant molecule in spore properties has generally been lacking. Bacillus subtilis strain FB122 (sleB spoVF) produced very stable spores that lacked DPA, and sporulation of this strain with DPA yielded spores with nearly normal DPA levels. DPA-replete and DPA-less FB122 spores had similar levels of the DNA protective alpha/beta-type small acid-soluble spore proteins (SASP), but the DPA-less spores lacked SASP-gamma. The DPA-less FB122 spores exhibited similar UV resistance to the DPA-replete spores but had lower resistance to wet heat, dry heat, hydrogen peroxide, and desiccation. Neither wet heat nor hydrogen peroxide killed the DPA-less spores by DNA damage, but desiccation did. The inability to synthesize both DPA and most alpha/beta-type SASP in strain PS3664 (sspA sspB sleB spoVF) resulted in spores that lost viability during sporulation, at least in part due to DNA damage. DPA-less PS3664 spores were more sensitive to wet heat than either DPA-less FB122 spores or DPA-replete PS3664 spores, and the latter also retained viability during sporulation. These and previous results indicate that, in addition to alpha/beta-type SASP, DPA also is extremely important in spore resistance and stability and, further, that DPA has some specific role(s) in protecting spore DNA from damage. Specific roles for DPA in protecting spore DNA against damage may well have been a major driving force for the spore's accumulation of the high levels of this small molecule.     

Germination of spores of Bacillus subtilis with dodecylamine

AIMS: To determine the properties of Bacillus subtilis spores germinated with the alkylamine dodecylamine, and the mechanism of dodecylamine-induced spore germination. METHODS AND RESULTS: Spores of B. subtilis prepared in liquid medium were germinated efficiently by dodecylamine, while spores prepared on solid medium germinated more poorly with this agent. Dodecylamine germination of spores was accompanied by release of almost all spore dipicolinic acid (DPA), degradation of the spore's peptidoglycan cortex, release of the spore's pool of free adenine nucleotides and the killing of the spores. The dodecylamine-germinated spores did not initiate metabolism, did not degrade their pool of small, acid-soluble spore proteins efficiently and had a significantly lower level of core water than did spores germinated by nutrients. As measured by DPA release, dodecylamine readily induced germination of B. subtilis spores that: (a) were decoated, (b) lacked all the receptors for nutrient germinants, (c) lacked both the lytic enzymes either of which is essential for cortex degradation, or (d) had a cortex that could not be attacked by the spore's cortex-lytic enzymes. The DNA in dodecylamine-germinated wild-type spores was readily stained, while the DNA in dodecylamine-germinated spores of strains that were incapable of spore cortex degradation was not. These latter germinated spores also did not release their pool of free adenine nucleotides. CONCLUSIONS: These results indicate that: (a) the spore preparation method is very important in determining the rate of spore germination with dodecylamine, (b) wild-type spores germinated by dodecylamine progress only part way through the germination process, (c) dodecylamine may trigger spore germination by a novel mechanism involving the activation of neither the spore's nutrient germinant receptors nor the cortex-lytic enzymes, and (d) dodecylamine may trigger spore germination by directly or indirectly activating release of DPA from the spore core, through the opening of channels for DPA in the spore's inner membrane. SIGNIFICANCE AND IMPACT OF THE STUDY: These results provide new insight into the mechanism of spore germination with the cationic surfactant dodecylamine, and also into the mechanism of spore germination in general. New knowledge of mechanisms to stimulate spore germination may have applied utility, as germinated spores are much more sensitive to processing treatments than are dormant spores.     

Role of dipicolinic acid in the germination, stability, and viability of spores of Bacillus subtilis

Spores of Bacillus subtilis spoVF strains that cannot synthesize dipicolinic acid (DPA) but take it up during sporulation were prepared in medium with various DPA concentrations, and the germination and viability of these spores as well as the DPA content in individual spores were measured. Levels of some other small molecules in DPA-less spores were also measured. These studies have allowed the following conclusions. (i) Spores with no DPA or low DPA levels that lack either the cortex-lytic enzyme (CLE) SleB or the receptors that respond to nutrient germinants could be isolated but were unstable and spontaneously initiated early steps in spore germination. (ii) Spores that lacked SleB and nutrient germinant receptors and also had low DPA levels were more stable. (iii) Spontaneous germination of spores with no DPA or low DPA levels was at least in part via activation of SleB. (iv) The other redundant CLE, CwlJ, was activated only by the release of high levels of DPA from spores. (v) Low levels of DPA were sufficient for the viability of spores that lacked most alpha/beta-type small, acid-soluble spore proteins. (vi) DPA levels accumulated in spores prepared in low-DPA-containing media varied greatly between individual spores, in contrast to the presence of more homogeneous DPA levels in individual spores made in media with high DPA concentrations. (vii) At least the great majority of spores of several spoVF strains that contained no DPA also lacked other major spore small molecules and had gone through some of the early reactions in spore germination.     

Characterization of Clostridium perfringens spores that lack SpoVA proteins and dipicolinic acid

Spores of Clostridium perfringens possess high heat resistance, and when these spores germinate and return to active growth, they can cause gastrointestinal disease. Work with Bacillus subtilis has shown that the spore's dipicolinic acid (DPA) level can markedly influence both spore germination and resistance and that the proteins encoded by the spoVA operon are essential for DPA uptake by the developing spore during sporulation. We now find that proteins encoded by the spoVA operon are also essential for the uptake of Ca(2+) and DPA into the developing spore during C. perfringens sporulation. Spores of a spoVA mutant had little, if any, Ca(2+) and DPA, and their core water content was approximately twofold higher than that of wild-type spores. These DPA-less spores did not germinate spontaneously, as DPA-less B. subtilis spores do. Indeed, wild-type and spoVA C. perfringens spores germinated similarly with a mixture of l-asparagine and KCl (AK), KCl alone, or a 1:1 chelate of Ca(2+) and DPA (Ca-DPA). However, the viability of C. perfringens spoVA spores was 20-fold lower than the viability of wild-type spores. Decoated wild-type and spoVA spores exhibited little, if any, germination with AK, KCl, or exogenous Ca-DPA, and their colony-forming efficiency was 10(3)- to 10(4)-fold lower than that of intact spores. However, lysozyme treatment rescued these decoated spores. Although the levels of DNA-protective alpha/beta-type, small, acid-soluble spore proteins in spoVA spores were similar to those in wild-type spores, spoVA spores exhibited markedly lower resistance to moist heat, formaldehyde, HCl, hydrogen peroxide, nitrous acid, and UV radiation than wild-type spores did. In sum, these results suggest the following. (i) SpoVA proteins are essential for Ca-DPA uptake by developing spores during C. perfringens sporulation. (ii) SpoVA proteins and Ca-DPA release are not required for C. perfringens spore germination. (iii) A low spore core water content is essential for full resistance of C. perfringens spores to moist heat, UV radiation, and chemicals.     

How moist heat kills spores of Bacillus subtilis

Populations of Bacillus subtilis spores in which 90 to 99.9% of the spores had been killed by moist heat gave only two fractions on equilibrium density gradient centrifugation: a fraction comprised of less dense spores that had lost their dipicolinic acid (DPA), undergone significant protein denaturation, and were all dead and a fraction with the same higher density as that of unheated spores. The latter fraction from heat-killed spore populations retained all of its DPA, but ≥98% of the spores could be dead. The dead spores that retained DPA germinated relatively normally with nutrient and nonnutrient germinants, but the outgrowth of these germinated spores was significantly compromised, perhaps because they had suffered damage to some proteins such that metabolic activity during outgrowth was greatly decreased. These results indicate that DPA release takes place well after spore killing by moist heat and that DPA release during moist-heat treatment is an all-or-nothing phenomenon; these findings also suggest that damage to one or more key spore proteins causes spore killing by moist heat.     

Analysis of the Loss in Heat and Acid Resistance during Germination of Spores of Bacillus Species

A major event in the nutrient germination of spores of Bacillus species is release of the spores'' large depot of dipicolinic acid (DPA). This event is preceded by both commitment, in which spores continue through germination even if germinants are removed, and loss of spore heat resistance. The latter event is puzzling, since spore heat resistance is due largely to core water content, which does not change until DPA is released during germination. We now find that for spores of two Bacillus species, the early loss in heat resistance during germination is most likely due to release of committed spores'' DPA at temperatures not lethal for dormant spores. Loss in spore acid resistance during germination also paralleled commitment and was also associated with the release of DPA from committed spores at acid concentrations not lethal for dormant spores. These observations plus previous findings that DPA release during germination is preceded by a significant release of spore core cations suggest that there is a significant change in spore inner membrane permeability at commitment. Presumably, this altered membrane cannot retain DPA during heat or acid treatments innocuous for dormant spores, resulting in DPA-less spores that are rapidly killed.     

Bacillus subtilis spore protein SpoVAC functions as a mechanosensitive channel

A critical event during spore germination is the release of Ca‐DPA (calcium in complex with dipicolinic acid). The mechanism of release of Ca‐DPA through the inner membrane of the spore is not clear, but proteins encoded by the Bacillus subtilis spoVA operon are involved in the process. We cloned and expressed the spoVAC gene in Escherichia coli and characterized the SpoVAC protein. We show that SpoVAC protects E. coli against osmotic downshift, suggesting that it might act as a mechanosensitive channel. Purified SpoVAC was reconstituted in unilamellar lipid vesicles to determine the gating mechanism and pore properties of the protein. By means of a fluorescence‐dequenching assay, we show that SpoVAC is activated upon insertion into the membrane of the amphiphiles lysoPC and dodecylamine. Patch clamp experiments on E. coli giant spheroplast as well as giant unilamellar vesicles (GUVs) containing SpoVAC show that the protein forms transient pores with main conductance values of about 0.15 and 0.1 nS respectively. Overall, our data indicate that SpoVAC acts as a mechanosensitive channel and has properties that would allow the release of Ca‐DPA and amino acids during germination of the spore.     

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 Bacillus subtilis spore killing by and resistance to an acidic Fe-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.     

Genetic requirements for induction of germination of spores of Bacillus subtilis by Ca(2+)-dipicolinate

Dormant Bacillus subtilis spores can be induced to germinate by nutrients, as well as by nonmetabolizable chemicals, such as a 1:1 chelate of Ca(2+) and dipicolinic acid (DPA). Nutrients bind receptors in the spore, and this binding triggers events in the spore core, including DPA excretion and rehydration, and also activates hydrolysis of the surrounding cortex through mechanisms that are largely unknown. As Ca(2+)-DPA does not require receptors to induce spore germination, we asked if this process utilizes other proteins, such as the putative cortex-lytic enzymes SleB and CwlJ, that are involved in nutrient-induced germination. We found that Ca(2+)-DPA triggers germination by first activating CwlJ-dependent cortex hydrolysis; this mechanism is different from nutrient-induced germination where cortex hydrolysis is not required for the early germination events in the spore core. Nevertheless, since nutrients can induce release of the spore's DPA before cortex hydrolysis, we examined if the DPA excreted from the core acts as a signal to activate CwlJ in the cortex. Indeed, endogenous DPA is required for nutrient-induced CwlJ activation and this requirement was partially remedied by exogenous Ca(2+)-DPA. Our findings thus define a mechanism for Ca(2+)-DPA-induced germination and also provide the first definitive evidence for a signaling pathway that activates cortex hydrolysis in response to nutrients.     

Role of dipicolinic acid in survival of Bacillus subtilis spores exposed to artificial and solar UV radiation

Pyridine-2,6-dicarboxylic acid (dipicolinic acid [DPA]) constitutes approximately 10% of Bacillus subtilis spore dry weight and has been shown to play a significant role in the survival of B. subtilis spores exposed to wet heat and to 254-nm UV radiation in the laboratory. However, to date, no work has addressed the importance of DPA in the survival of spores exposed to environmentally relevant solar UV radiation. Air-dried films of spores containing DPA or lacking DPA due to a null mutation in the DPA synthetase operon dpaAB were assayed for their resistance to UV-C (254 nm), UV-B (290 to 320 nm), full-spectrum sunlight (290 to 400 nm), and sunlight from which the UV-B portion was filtered (325 to 400 nm). In all cases, air-dried DPA-less spores were significantly more UV sensitive than their isogenic DPA-containing counterparts. However, the degree of difference in UV resistance between the two strains was wavelength dependent, being greatest in response to radiation in the UV-B portion of the spectrum. In addition, the inactivation responses of DPA-containing and DPA-less spores also depended strongly upon whether spores were exposed to UV as air-dried films or in aqueous suspension. Spores lacking the gerA, gerB, and gerK nutrient germination pathways, and which therefore rely on chemical triggering of germination by the calcium chelate of DPA (Ca-DPA), were also more UV sensitive than wild-type spores to all wavelengths tested, suggesting that the Ca-DPA-mediated spore germination pathway may consist of a UV-sensitive component or components.     

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.     

Characterization of spores of Bacillus subtilis which lack dipicolinic acid

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.     

How do spores germinate?

Spore germination, as defined as those events that result in the loss of the spore-specific properties, is an essentially biophysical process. It occurs without any need for new macromolecular synthesis, so the apparatus required is already present in the mature dormant spore. Germination in response to specific chemical nutrients requires specific receptor proteins, located at the inner membrane of the spore. After penetrating the outer layers of spore coat and cortex, germinant interacts with its receptor: one early consequence of this binding is the movement of monovalent cations from the spore core, followed by Ca2(+) and dipicolinic acid (DPA). In some species, an ion transport protein is also required for these early stages. Early events - including loss of heat resistance, ion movements and partial rehydration of the spore core - can occur without cortex hydrolysis, although the latter is required for complete core rehydration and colony formation from a spore. In Bacillus subtilis two crucial cortex lytic enzymes have been identified: one is CwlJ, which is DPA-responsive and is located at the cortex-coat junction. The second, SleB, is present both in outer layers and at the inner spore membrane, and is more resistant to wet heat than is CwlJ. Cortex hydrolysis leads to the complete rehydration of the spore core, and then enzyme activity within the spore protoplast resumes. We do not yet know what activates SleB activity in the spore, and neither do we have any information at all on how the spore coat is degraded.     

Single-spore elemental analyses indicate that dipicolinic acid-deficient Bacillus subtilis spores fail to accumulate calcium

Dipicolinic acid (pyridine-2,6-carboxylic acid; DPA) is a major component of bacterial spores and has been shown to be an important determinant of spore resistance. In the core of dormant Bacillus subtilis spores, DPA is associated with divalent calcium in a 1:1 chelate (Ca–DPA). Spores excrete Ca–DPA during germination, but it is unknown whether Ca and DPA are imported separately or together into the developing spore. Elemental analysis by scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) of wild-type spores and mutant spores lacking the ability to synthesize DPA showed that DPA-less spores also lacked calcium, suggesting that the two compounds may be co-imported.