Characterization of spores of Bacillus subtilis which lack dipicolinic acid

The products of the spoVA operon are involved in dipicolinic acid uptake into developing spores of Bacillus subtilis

Bacillus subtilis cells with mutations in the spoVA operon do not complete sporulation. However, a spoVA strain with mutations that remove all three of the spore's functional nutrient germinant receptors (termed the ger3 mutations) or the cortex lytic enzyme SleB (but not CwlJ) did complete sporulation. ger3 spoVA and sleB spoVA spores lack dipicolinic acid (DPA) and have lower core wet densities and levels of wet heat resistance than wild-type or ger3 spores. These properties of ger3 spoVA and sleB spoVA spores are identical to those of ger3 spoVF and sleB spoVF spores that lack DPA due to deletion of the spoVF operon coding for DPA synthetase. Sporulation in the presence of exogenous DPA restored DPA levels in ger3 spoVF spores to 53% of the wild-type spore levels, but there was no incorporation of exogenous DPA into ger3 spoVA spores. These data indicate that one or more products of the spoVA operon are involved in DPA transport into the developing forespore during sporulation.

UV photochemistry of DNA in vitro and in Bacillus subtilis spores at earth-ambient and low atmospheric pressure: implications for spore survival on other planets or moons in the solar system

Two major parameters influencing the survival of Bacillus subtilis spores in space and on bodies within the Solar System are UV radiation and vacuum, both of which induce inactivating damage to DNA. To date, however, spore survival and DNA photochemistry have been explored only at the extremes of Earth-normal atmospheric pressure (101.3 kPa) and at simulated space vacuum (10(-3)-10(-6) Pa). In this study, wild-type spores, mutant spores lacking alpha/beta-type small, acid-soluble spore proteins (SASP), naked DNA, and complexes between SASP SspC and DNA were exposed simultaneously to UV (254 nm) at intermediate pressure (1-2 Pa), and the UV photoproducts cis,syn-thymine-thymine cyclobutane dimer (c,sTT), trans,syn-thymine-thymine cyclobutane dimer (t,sTT), and "spore photoproduct" (SP) were quantified. At 101.3 kPa, UV-treated wild-type spores accumulated only SP, but spores treated with UV radiation at 1-2 Pa exhibited a spectrum of DNA damage similar to that of spores treated at 10(-6) Pa, with accumulation of SP, c,sTT, and t,sTT. The presence or absence of alpha/beta-type SASP in spores was partly responsible for the shift observed between levels of SP and c,sTT, but not t,sTT. The changes observed in spore DNA photochemistry at 1-2 Pa in vivo were not reproduced by irradiation of naked DNA or SspC:DNA complexes in vitro, suggesting that factors other than SASP are involved in spore DNA photochemistry at low pressure.

Killing of spores of Bacillus subtilis by peroxynitrite appears to be caused by membrane damage

During an infection of a higher eukaryote, dormant spores of a Bacillus species have been previously shown to be present in cells that can generate the toxic agent peroxynitrite (PON). Dormant spores of Bacillus subtilis were much more resistant to killing by PON than were growing cells, and spore-coat alteration or removal greatly decreased PON resistance. Spores were not killed by PON through DNA damage and lost no dipicolinic acid (DPA) during PON treatment. However, PON-killed spores lost DPA during subsequent heat treatments that caused much less DPA release from untreated spores. Although dead, the PON-killed spores germinated and initiated metabolism but never went through outgrowth; the great majority of germinated PON-killed spores also took up propidium iodide, indicating that they had suffered significant membrane damage and were dead. Together these data suggest that spore killing by PON is through some type of damage to the spore's inner membrane.

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.

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.

Analysis of the role of bacterial endospore cortex structure in resistance properties and demonstration of its conservation amongst species

AIMS

The aim of this work was to compare the chemical structure of the spore cortex of a range of species, and to determine any correlation between cortex structure and spore resistance properties.

METHODS AND RESULTS

The fine chemical structure of the cortex of Bacillus subtilis, Bacillus megaterium, Bacillus cereus and Clostridium botulinum was examined by muropeptide analysis using reverse phase HPLC. There is a conserved basic structure between peptidoglycan of these species, with the only difference being the level of de-N-acetylation of an amino sugar. In order to determine if an alteration in cortex structure correlates with heat resistance properties, the peptidoglycan structure and properties of B. subtilis spores prepared under different conditions were compared. Peptidoglycan from spores prepared in Nutrient Broth (NB) showed reduction in single L-alanine substituted muramic acid to only 13.9% compared with 20.6% in CCY-grown spores. NB-prepared spores are also unstable, with 161-fold less heat resistance (60 min, 85 degrees C) and 43 times less Mn(2+) content than CCY-grown spores. Addition of MnCl(2) to NB led to a peptidoglycan profile similar to CCY-grown spores, sevenfold more heat resistance (60 min, 85 degrees C) and an 86-fold increase in Mn(2+) content. Addition of CCY salts to NB led all parameters to be comparable with CCY-grown spore levels.

CONCLUSION

It has been shown that peptidoglycan structure is conserved in four spore-forming bacteria. Also, spore heat resistance is multifactorial and cannot be accounted for by any single parameter.

SIGNIFICANCE AND IMPACT OF THE STUDY

Endospores made by diverse species most likely have common mechanisms of heat resistance. However, the molecular basis for their resistance remains elusive.

Properties of spores of Bacillus subtilis blocked at an intermediate stage in spore germination

Germination of mutant spores of Bacillus subtilis unable to degrade their cortex is accompanied by excretion of dipicolinic acid and uptake of some core water. However, compared to wild-type germinated spores in which the cortex has been degraded, the germinated mutant spores accumulated less core water, exhibited greatly reduced enzyme activity in the spore core, synthesized neither ATP nor reduced pyridine or flavin nucleotides, and had significantly higher resistance to heat and UV irradiation. We propose that the germinated spores in which the cortex has not been degraded represent an intermediate stage in spore germination, which we term stage I.

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.

Resistance of spores of Bacillus species to ultraviolet light

Dormant spores of the various Bacillus species, including B. subtilis, are 5 to 50 times more resistant to UV radiation than are the corresponding growing cells. This elevated spore UV resistance is due to: a) the photochemistry of DNA within spores, as UV generates few if any cyclobutane dimers, but rather a photoproduct (Fig. 1) called spore photoproduct (SP; 5-thyminyl-5,6-dihydrothymine); and b) DNA repair, in particular SP-specific repair, during spore germination. The novel UV photochemistry of spore DNA is largely due to its saturation with a group of small, acid-soluble proteins (SASP), which are unique to spores and whose binding alters the DNA conformation and thus its photochemistry. SP-specific repair is also unique to spores and is carried out by a light-independent SP-lyase, an iron-sulfur protein that utilizes S-adenosylmethionine to catalyze SP monomerization without DNA backbone cleavage.