Prevention of DNA damage in spores and in vitro by small, acid-soluble proteins from Bacillus species

Characterization of high-pressure-treated Bacillus subtilis spore populations using flow cytometry - Shedding light on spore superdormancy at 550 MPa

Mild spore inactivation can be challenging in industry because of the remarkable resistance of bacterial spores. High pressure (HP) can trigger spore germination, which reduces the spore's resistance, and thereby allows mild spore inactivation. However, spore germination is heterogenous. Some slowly germinating or non-germinating spores called superdormant spores remain resistant and can survive. Therefore, superdormant spores need to be characterized to understand the causes of their germination deficiency. Bacillus subtilis spores were pressurized for 50 s - 6 min at a very high pressure (vHP) level of 550 MPa and 60 °C in buffer to trigger germination. For a rapid quantification of the remaining ungerminated superdormant spores, flow cytometry (FCM) analysis was validated using single cell sorting and growth analysis. FCM based on propidium iodide (PI) and SYTO16 can be used for 550 MPa-superdormant spores after short vHP treatments of ≤1 min and post-HP incubation at 37 °C or 60 °C. The need for a post-HP incubation is particular for vHP treatments. The incubation was successful to separate FCM signals from superdormant and germinated spores, thus allowing superdormant spore quantification. The SYTO16 and PI fluorescence levels did not necessarily indicate superdormancy or apparent viability. This highlights the general need for FCM validation for different HP treatment conditions. The ∼7 % of ungerminated, i.e., superdormant, spores were isolated after a vHP treatment (550 MPa, 60 °C, 43-52 s). This allowed the characterization of vHP superdormant spores for the first time. The superdormant spores had a similar dipicolinic acid content as spores of the initial dormant population. Descendants of superdormant spores had a normal vHP germination capacity. The causes of vHP superdormancy were thus unlikely linked to the dipicolinic acid content or a permanent genetic change. Isolated superdormant spores germinated better in a second vHP treatment compared to the initial spore population. This has not been observed for other germination stimuli so far. In addition, the germination capacity of the initial spore population was time-dependent. A vHP germination deficiency can therefore be lost over time and seems to be caused by transient factors. Permanent cellular properties played a minor role as causes of superdormancy under chosen HP treatment conditions. The study gained new fundamental insights in vHP superdormancy which are of applied interest. Understanding superdormancy helps to efficiently develop a strategy to avoid superdormant spores and hence to inactivate all spores. The development of a mild HP spore germination-inactivation process aims at better preserving the food quality.

Mechanistic insight into roles of α/β-type small acid-soluble proteins, RecA, and inner membrane proteins during bacterial spore inactivation by ohmic heating

AIM

Ohmic heating (OH) (i.e. heating by electric field) more effectively kills bacterial spores than traditional wet heating, yet its mechanism remains poorly understood. This study investigates the accelerated spore inactivation mechanism using genetically modified spores.

METHODS AND RESULTS

We investigated the effects of OH and conventional heating (CH) on various genetically modified strains of Bacillus subtilis: isogenic PS533 (wild type_1), PS578 [lacking spores' α/β-type small acid-soluble proteins (SASP)], PS2318 (lacking recA, encoding a DNA repair protein), isogenic PS4461 (wild type_2), and PS4462 (having the 2Duf protein in spores, which increases spore wet heat resistance and decreases spore inner membrane fluidity). Removal of SASP brought the inactivation profiles of OH and CH closer, suggesting the interaction of these proteins with the field. However, the reemergence of a difference between CH and OH killing for SASP-deficient spores at the highest tested field strength suggested there is also interaction of the field with another spore core component. Additionally, RecA-deficient spores yielded results like those with the wild-type spores for CH, while the OH resistance of this mutant increased at the lower tested temperatures, implying that RecA or DNA are a possible additional target for the electric field. Addition of the 2Duf protein markedly increased spore resistance both to CH and OH, although some acceleration of killing was observed with OH at 50 V/cm.

CONCLUSIONS

In summary, both membrane fluidity and interaction of the spore core proteins with electric field are key factors in enhanced spore killing with electric field-heat combinations.

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.

Targetable nano-delivery vehicles to deliver anti-bacterial small acid-soluble spore protein (SASP) genes

Interest in phage-based therapeutics is increasing, at least in part due to the need for new treatment options for infections caused by antibiotic-resistant bacteria. It is possible to use wild-type (WT) phages to treat bacterial infections, but it is also possible to modify WT phages to generate therapeutics with improved features. Here, we will discuss features of Phico Therapeutics' SASPject technology, which modifies phages for use as targetable nano-delivery vehicles (NDV), to introduce antibacterial Small Acid Soluble Spore Protein (SASP) genes into specific target bacteria.

Engineered Bacteriophage as a Delivery Vehicle for Antibacterial Protein, SASP

The difficulties in developing novel classes of antibacterials is leading to a resurgence of interest in bacteriophages as therapeutic agents, and in particular engineered phages that can be optimally designed. Here, pre-clinical microbiology assessment is presented of a Staphylococcus aureus phage engineered to deliver a gene encoding an antibacterial small acid soluble spore protein (SASP) and further, rendered non-lytic to give product SASPject PT1.2. PT1.2 has been developed initially for nasal decolonisation of S. aureus, including methicillin-resistant S. aureus. Time-kill curve assays were conducted with PT1.2 against a range of staphylococcal species, and serial passaging experiments were conducted to investigate the potential for resistance to develop. SASPject PT1.2 demonstrates activity against 100% of 225 geographically diverse S. aureus isolates, exquisite specificity for S. aureus, and a rapid speed of kill. The kinetics of S. aureus/PT1.2 interaction is examined together with demonstrating that PT1.2 activity is unaffected by the presence of human serum albumin. SASPject PT1.2 shows a low propensity for resistance to develop with no consistent shift in sensitivity in S. aureus cells passaged for up to 42 days. SASPject PT1.2 shows promise as a novel first-in-class antibacterial agent and demonstrates potential for the SASPject platform.

The small acid-soluble proteins of Clostridioides difficile are important for UV resistance and serve as a check point for sporulation

Clostridioides difficile is a nosocomial pathogen which causes severe diarrhea and colonic inflammation. C. difficile causes disease in susceptible patients when endospores germinate into the toxin-producing vegetative form. The action of these toxins results in diarrhea and the spread of spores into the hospital and healthcare environments. Thus, the destruction of spores is imperative to prevent disease transmission between patients. However, spores are resilient and survive extreme temperatures, chemical exposure, and UV treatment. This makes their elimination from the environment difficult and perpetuates their spread between patients. In the model spore-forming organism, Bacillus subtilis, the small acid-soluble proteins (SASPs) contribute to these resistances. The SASPs are a family of small proteins found in all endospore-forming organisms, C. difficile included. Although these proteins have high sequence similarity between organisms, the role(s) of the proteins differ. Here, we investigated the role of the main α/β SASPs, SspA and SspB, and two annotated putative SASPs, CDR20291_1130 and CDR20291_3080, in protecting C. difficile spores from environmental insults. We found that SspA is necessary for conferring spore UV resistance, SspB minorly contributes, and the annotated putative SASPs do not contribute to UV resistance. In addition, the SASPs minorly contribute to the resistance of nitrous acid. Surprisingly, the combined deletion of sspA and sspB prevented spore formation. Overall, our data indicate that UV resistance of C. difficile spores is dependent on SspA and that SspA and SspB regulate/serve as a checkpoint for spore formation, a previously unreported function of SASPs.

DNA Damage Kills Bacterial Spores and Cells Exposed to 222-Nanometer UV Radiation

This study examined the microbicidal activity of 222-nm UV radiation (UV₂₂₂), which is potentially a safer alternative to the 254-nm UV radiation (UV₂₅₄) that is often used for surface decontamination. Spores and/or growing and stationary-phase cells of Bacillus cereus, Bacillus subtilis, Bacillus thuringiensis, Staphylococcus aureus, and Clostridioides difficile and a herpesvirus were all killed or inactivated by UV₂₂₂ and at lower fluences than with UV₂₅₄B. subtilis spores and cells lacking the major DNA repair protein RecA were more sensitive to UV₂₂₂, as were spores lacking their DNA-protective proteins, the α/β-type small, acid-soluble spore proteins. The spore cores' large amount of Ca²⁺-dipicolinic acid (∼25% of the core dry weight) also protected B. subtilis and C. difficile spores against UV₂₂₂, while spores' proteinaceous coat may have given some slight protection against UV₂₂₂ Survivors among B. subtilis spores treated with UV₂₂₂ acquired a large number of mutations, and this radiation generated known mutagenic photoproducts in spore and cell DNA, primarily cyclobutane-type pyrimidine dimers in growing cells and an α-thyminyl-thymine adduct termed the spore photoproduct (SP) in spores. Notably, the loss of a key SP repair protein markedly decreased spore UV₂₂₂ resistance. UV₂₂₂-treated B. subtilis spores germinated relatively normally, and the generation of colonies from these germinated spores was not salt sensitive. The latter two findings suggest that UV₂₂₂ does not kill spores by general protein damage, and thus, the new results are consistent with the notion that DNA damage is responsible for the killing of spores and cells by UV₂₂₂IMPORTANCE Spores of a variety of bacteria are resistant to common decontamination agents, and many of them are major causes of food spoilage and some serious human diseases, including anthrax caused by spores of Bacillus anthracis Consequently, there is an ongoing need for efficient methods for spore eradication, in particular methods that have minimal deleterious effects on people or the environment. UV radiation at 254 nm (UV₂₅₄) is sporicidal and commonly used for surface decontamination but can cause deleterious effects in humans. Recent work, however, suggests that 222-nm UV (UV₂₂₂) may be less harmful to people than UV₂₅₄ yet may still kill bacteria and at lower fluences than UV₂₅₄ The present work has identified the damage by UV₂₂₂ that leads to the killing of growing cells and spores of some bacteria, many of which are human pathogens, and UV₂₂₂ also inactivates a herpesvirus.

Freezing Tolerance of Thermophilic Bacterial Endospores in Marine Sediments

Dormant endospores of anaerobic, thermophilic bacteria found in cold marine sediments offer a useful model for studying microbial biogeography, dispersal, and survival. The dormant endospore phenotype confers resistance to unfavorable environmental conditions, allowing dispersal to be isolated and studied independently of other factors such as environmental selection. To study the resilience of thermospores to conditions relevant for survival in extreme cold conditions, their viability following different freezing treatments was tested. Marine sediment was frozen at either −80°C or −20°C for 10 days prior to pasteurization and incubation at +50°C for 21 days to assess thermospore viability. Sulfate reduction commenced at +50°C following both freezing pretreatments indicating persistence of thermophilic endospores of sulfate-reducing bacteria. The onset of sulfate reduction at +50°C was delayed in −80°C pretreated microcosms, which exhibited more variability between triplicates, compared to −20°C pretreated microcosms and parallel controls that were not frozen in advance. Microbial communities were evaluated by 16S rRNA gene amplicon sequencing, revealing an increase in the relative sequence abundance of thermophilic endospore-forming Firmicutes in all microcosms. Different freezing pretreatments (−80°C and −20°C) did not appreciably influence the shift in overall bacterial community composition that occurred during the +50°C incubations. Communities that had been frozen prior to +50°C incubation showed an increase in the relative sequence abundance of operational taxonomic units (OTUs) affiliated with the class Bacilli, relative to unfrozen controls. These results show that freezing impacts but does not obliterate thermospore populations and their ability to germinate and grow under appropriate conditions. Indeed the majority of the thermospore OTUs detected in this study (21 of 22) could be observed following one or both freezing treatments. These results are important for assessing thermospore viability in frozen samples and following cold exposure such as the very low temperatures that would be encountered during panspermia.

Observations on research with spores of Bacillales and Clostridiales species

The purpose of this article is to highlight some areas of research with spores of bacteria of Firmicute species in which the methodology too commonly used is not optimal and generates misleading results. As a consequence, conclusions drawn from data obtained are often flawed or not appropriate. Topics covered in the article include the following: (i) the importance of using well-purified bacterial spores in studies on spore resistance, composition, killing, disinfection and germination; (ii) methods for obtaining good purification of spores of various species; (iii) appropriate experimental approaches to determine mechanisms of spore resistance and spore killing by a variety of agents, as well as known mechanisms of spore resistance and killing; (iv) common errors made in drawing conclusions about spore killing by various agents, including failure to neutralize chemical agents before plating for viable spore enumeration, and equating correlations between changes in spore properties accompanying spore killing with causation. It is hoped that a consideration of these topics will improve the quality of spore research going forward.

Mechanism of inactivation of Bacillus subtilis spores by high pressure CO2 at high temperature

Spores of wild-type Bacillus subtilis and some isogenic mutant strains were treated by high pressure CO₂ (HPCD) at high temperature (HT) (HPCD + HT) at 20 MPa and 84-86 °C for 0-60 min, and centrifuged on a high density solution to obtain pelleted spores that retained CaDPA and light spores that lost CaDPA. All treated spores were analyzed for viability, and tested for germination, outgrowth, core protein damage, mutagenesis and inner membrane (IM) properties. The results showed that (i) with HPCD + HT treated spores, most pelleted spores and all light spores were dead; ii) a significant amount of dead HPCD + HT-treated spores that retained CaDPA germinated, but outgrowth was blocked; (iii) minimal mutants were generated in survivors of HPCD + HT treatment; (iv) the GFP fluorescence decrease in HPCD + HT-treated spores with high GFP levels was slower than spore inactivation; (v) the IM of HPCD + HT-treated spores that retained CaDPA lost its ability to retain CaDPA at 85 °C, and almost all of these spores' outgrowth in high salt was blocked; and (vi) HPCD + HT-treated spores that retained CaDPA germinated with l-valine or AGFK were almost all stained with propidium iodide. These results indicated that HPCD + HT inactivated B. subtilis spores by damaging spores' IM, thus blocking spore outgrowth after germination.

Geobacillus and Bacillus Spore Inactivation by Low Energy Electron Beam Technology: Resistance and Influencing Factors

Low energy electron beam (LEEB) treatment is an emerging non-thermal technology that performs surface decontamination with a minimal influence on food quality. Bacterial spore resistance toward LEEB treatment and its influencing factors were investigated in this study. Spores from Geobacillus and Bacillus species were treated with a lab-scale LEEB at energy levels of 80 and 200 keV. The spore resistances were expressed as D-values (the radiation dose required for one log10 reduction at a given energy level) calculated from the linear regression of log10 reduction against absorbed dose of the sample. The results revealed that the spore inactivation efficiency by LEEB is comparable to that of other ionizing radiations and that the inactivation curves are mostly log10-linear at the investigated dose range (3.8 - 8.2 kGy at 80 keV; 6.0 - 9.8 kGy at 200 keV). The D-values obtained from the wildtype strains varied from 2.2 - 3.0 kGy at 80 keV, and from 2.2 - 3.1 kGy at 200 keV. Bacillus subtilis mutant spores lacking α/β-type small, acid-soluble spore proteins showed decreased D-values (1.3 kGy at 80 and 200 keV), indicating that spore DNA is one of the targets for LEEB spore inactivation. The results revealed that bacterial species, sporulation conditions and the treatment dose influence the spore LEEB inactivation. This finding indicates that for the application of this emerging technology, special attention should be paid to the choice of biological indicator, physiological state of the indicator and the processing settings. High spore inactivation efficiency supports the application of LEEB for the purpose of food surface decontamination. With its environmental, logistical, and economic advantages, LEEB can be a relevant technology for surface decontamination to deliver safe, minimally processed and additive-free food products.

Application of High Pressure with Homogenization, Temperature, Carbon Dioxide, and Cold Plasma for the Inactivation of Bacterial Spores: A Review

Formation of highly resistant spores is a concern for the safety of low-acid foods as they are a perfect vehicle for food spoilage and/or human infection. For spore inactivation, the strategy usually applied in the food industry is the intensification of traditional preservation methods to sterilization levels, which is often accompanied by decreases of nutritional and sensory properties. In order to overcome these unwanted side effects in food products, novel and emerging sterilization technologies are being developed, such as pressure-assisted thermal sterilization, high-pressure carbon dioxide, high-pressure homogenization, and cold plasma. In this review, the application of these emergent technologies is discussed, in order to understand the effects on bacterial spores and their inactivation and thus ensure food safety of low-acid foods. In general, the application of these novel technologies for inactivating spores is showing promising results. However, it is important to note that each technique has specific features that can be more suitable for a particular type of product. Thus, the most appropriate sterilization method for each product (and target microorganisms) should be assessed and carefully selected.

Sporicidal activity of ceragenin CSA-13 against Bacillus subtilis

Spore-forming bacteria are a class of microorganisms that possess the ability to survive in extreme environmental conditions. Morphological features of spores assure their resistance to stress factors such as high temperature, radiation, disinfectants, and drying. Consequently, spore elimination in industrial and medical environments is very challenging. Ceragenins are a new class of cationic lipids characterized by a broad spectrum of bactericidal activity resulting from amphipathic nature and membrane-permeabilizing properties. To assess the impact of ceragenin CSA-13 on spores formed by Bacillus subtilis (ATCC 6051), we performed the series of experiments confirming that amphipathic and membrane-permeabilizing properties of CSA-13 are sufficient to disrupt the structure of B. subtilis spores resulting in decreased viability. Raman spectroscopy analysis provided evidence that upon CSA-13 treatment the number of CaDPA-positive spores was clearly diminished. As a consequence, a loss of impermeability of the inner membranes of spores, accompanied by a decrease in spore resistance and killing take place. In addition to their broad antimicrobial spectrum, ceragenins possess great potential for development as new sporicidal agents.

Identity, Abundance, and Reactivation Kinetics of Thermophilic Fermentative Endospores in Cold Marine Sediment and Seawater

Cold marine sediments harbor endospores of fermentative and sulfate-reducing, thermophilic bacteria. These dormant populations of endospores are believed to accumulate in the seabed via passive dispersal by ocean currents followed by sedimentation from the water column. However, the magnitude of this process is poorly understood because the endospores present in seawater were so far not identified, and only the abundance of thermophilic sulfate-reducing endospores in the seabed has been quantified. We investigated the distribution of thermophilic fermentative endospores (TFEs) in water column and sediment of Aarhus Bay, Denmark, to test the role of suspended dispersal and determine the rate of endospore deposition and the endospore abundance in the sediment. We furthermore aimed to determine the time course of reactivation of the germinating TFEs. TFEs were induced to germinate and grow by incubating pasteurized sediment and water samples anaerobically at 50°C. We observed a sudden release of the endospore component dipicolinic acid immediately upon incubation suggesting fast endospore reactivation in response to heating. Volatile fatty acids (VFAs) and H2 began to accumulate exponentially after 3.5 h of incubation showing that reactivation was followed by a short phase of outgrowth before germinated cells began to divide. Thermophilic fermenters were mainly present in the sediment as endospores because the rate of VFA accumulation was identical in pasteurized and non-pasteurized samples. Germinating TFEs were identified taxonomically by reverse transcription, PCR amplification and sequencing of 16S rRNA. The water column and sediment shared the same phylotypes, thereby confirming the potential for seawater dispersal. The abundance of TFEs was estimated by most probable number enumeration, rates of VFA production, and released amounts of dipicolinic acid during germination. The surface sediment contained ∼105-106 inducible TFEs cm-3. TFEs thus outnumber thermophilic sulfate-reducing endospores by an order of magnitude. The abundance of cultivable TFEs decreased exponentially with sediment depth with a half-life of 350 years. We estimate that 6 × 109 anaerobic thermophilic endospores are deposited on the seafloor per m2 per year in Aarhus Bay, and that these thermophiles represent >10% of the total endospore community in the surface sediment.

Mechanism of Bacillus subtilis spore inactivation by and resistance to supercritical CO2 plus peracetic acid

AIMS

Determine how supercritical CO2 (scCO2 ) plus peracetic acid (PAA) inactivates Bacillus subtilis spores, factors important in spore resistance to scCO2 -PAA, and if spores inactivated by scCO2 -PAA are truly dead.

METHODS AND RESULTS

Spores of wild-type B. subtilis and isogenic mutants lacking spore protective proteins were treated with scCO2 -PAA in liquid or dry at 35°C. Wild-type wet spores (aqueous suspension) were more susceptible than dry spores. Treated spores were examined for viability (and were truly dead), dipicolinic acid (DPA), mutations, permeability to nucleic acid stains, germination under different conditions, energy metabolism and outgrowth. ScCO2 -PAA-inactivated spores retained DPA, and survivors had no notable DNA damage. However, DPA was released from inactivated spores at a normally innocuous temperature (85°C), and colony formation from treated spores was salt sensitive. The inactivated spores germinated but did not outgrow, and these germinated spores had altered plasma membrane permeability and defective energy metabolism. Wet or dry coat-defective spores had increased scCO2 -PAA sensitivity, and dry spores but not wet spores lacking DNA protective proteins were more scCO2 -PAA sensitive.

CONCLUSIONS

These findings suggest that scCO2 -PAA inactivates spores by damaging spores' inner membrane. The spore coat provided scCO2 -PAA resistance for both wet and dry spores. DNA protective proteins provided scCO2 -PAA resistance only for dry spores.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results provide information on mechanisms of spore inactivation of and resistance to scCO2 -PAA, an agent with increasing use in sterilization applications.

Impact of surface structure and feed gas composition on Bacillus subtilis endospore inactivation during direct plasma treatment

This study investigated the inactivation efficiency of cold atmospheric pressure plasma treatment on Bacillus subtilis endospores dependent on the used feed gas composition and on the surface, the endospores were attached on. Glass petri-dishes, glass beads, and peppercorns were inoculated with the same endospore density and treated with a radio frequency plasma jet. Generated reactive species were detected using optical emission spectroscopy. A quantitative polymerase chain reaction (qPCR) based ratio detection system was established to monitor the DNA damage during the plasma treatment. Argon + 0.135% vol. oxygen + 0.2% vol. nitrogen as feed gas emitted the highest amounts of UV-C photons and considerable amount of reactive oxygen and nitrogen species. Plasma generated with argon + 0.135% vol. oxygen was characterized by the highest emission of reactive oxygen species (ROS), whereas the UV-C emission was negligible. The use of pure argon showed a negligible emission of UV photons and atomic oxygen, however, the emission of vacuum (V)UV photons was assumed. Similar maximum inactivation results were achieved for the three feed gas compositions. The surface structure had a significant impact on the inactivation efficiency of the plasma treatment. The maximum inactivation achieved was between 2.4 and 2.8 log10 on glass petri-dishes and 3.9 to 4.6 log10 on glass beads. The treatment of peppercorns resulted in an inactivation lower than 1.0 log10. qPCR results showed a significant DNA damage for all gas compositions. Pure argon showed the highest results for the DNA damage ratio values, followed by argon + 0.135% vol. oxygen + 0.2% vol. nitrogen. In case of argon + 0.135% vol. oxygen the inactivation seems to be dominated by the action of ROS. These findings indicate the significant role of VUV and UV photons in the inactivation process of B. subtilis endospores.

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.

Validated empirical models describing the combined effect of water activity and pH on the heat resistance of spores of a psychrotolerant Bacillus cereus strain in broth and béchamel sauce

The major objective of this study was to evaluate and model the combined effect of the water activity (a(w)) and pH of the heating menstrum on the heat resistance of spores of a psychrotolerant Bacillus cereus strain isolated from béchamel sauce. Two models, a quadratic polynomial equation and a reparameterized function, were assessed for their ability to describe the combined influence of a(w) and pH on the D(85°C)-values of the B. cereus isolate in tryptone soy broth. The performance of the models was validated by challenging the models with data independently collected in broth and béchamel sauce. Both models were found to adequately describe the validation data obtained in broth. However, it was determined that in béchamel sauce the predictions of the polynomial function not only showed bias (bias factor = 1.156) but were also fail-dangerous, as they deviated from the validation data by 17.2%. The reparameterized function was determined to be a good predictor of the D(85°C)-values in béchamel sauce as it showed no bias (bias factor = 1.033) and its predictions differed by only 7.9% from the validation data. The reparameterized function can be used to provide estimates of the minimum processing conditions required to achieve desired levels of spore inactivation within the a(w) and pH ranges studied and to determine the potential changes in heat resistance of B. cereus spores when a(w) and pH are changed, for example, during product reformulation. As validation of heat resistance models is rarely performed, let alone in actual food products, the models evaluated and validated in this study (in particular the reparameterized function) are of immediate relevance to the food industry.

Monitoring the wet-heat inactivation dynamics of single spores of Bacillus species by using Raman tweezers, differential interference contrast microscopy, and nucleic acid dye fluorescence microscopy

Dynamic processes during wet-heat treatment of individual spores of Bacillus cereus, Bacillus megaterium, and Bacillus subtilis at 80 to 90°C were investigated using dual-trap Raman spectroscopy, differential interference contrast (DIC) microscopy, and nucleic acid stain (SYTO 16) fluorescence microscopy. During spore wet-heat treatment, while the spores' 1:1 chelate of Ca(2+) with dipicolinic acid (CaDPA) was released rapidly at a highly variable time T(lag), the levels of spore nucleic acids remained nearly unchanged, and the T(lag) times for individual spores from the same preparation were increased somewhat as spore levels of CaDPA increased. The brightness of the spores' DIC image decreased by ~50% in parallel with CaDPA release, and there was no spore cortex hydrolysis observed. The lateral diameters of the spores' DIC image and SYTO 16 fluorescence image also decreased in parallel with CaDPA release. The SYTO 16 fluorescence intensity began to increase during wet-heat treatment at a time before T(lag) and reached maximum at a time slightly later than T(release). However, the fluorescence intensities of wet-heat-inactivated spores were ~15-fold lower than those of nutrient-germinated spores, and this low SYTO 16 fluorescence intensity may be due in part to the low permeability of the dormant spores' inner membranes to SYTO 16 and in part to nucleic acid denaturation during the wet-heat treatment.

Association between increased DNA mutational frequency and thermal inactivation of aerosolized Bacillus spores exposed to dry heat

Inactivation of viable bioaerosol particles, especially stress-resistant microorganisms, has important implications for biodefense and air quality control. It has earlier been shown that the loss of viability of bacterial endospores due to exposure to dry heat is associated with mutational damage. Previous studies, however, used non-aerosolized spores, long exposure times, and moderately elevated temperatures. This study was designed to investigate the mechanism of inactivation of aerosolized Bacillus endospores exposed to high temperatures for sub-second time periods. Bioaerosol was tested in a continuous air flow chamber under two flow rates, 18 L/min and 36 L/min. The chamber had a cylindrical electric heating element installed along its axis. The estimated characteristic exposure temperature (T _exposure ) ranged from 164°C to 277°C (with an uncertainty of 21-26°C). To quantify mutational frequency, spores were cultivated after dry heat exposure on tryptic-soy agar and on antibiotic nalidixic acid media. Increases in the exposure temperature caused viability loss and increase in mutational frequency of the spore DNA. Significant association was found between the inactivation factor and the mutational frequency ratio (heat exposed versus non-exposed) with R² of 0.985 for both flow rates combined. The results suggest that mutational damage is involved in the causal chain of events leading to inactivation of aerosolized endospores exposed to heat for sub-second time periods.

Characterization of wet-heat inactivation of single spores of bacillus species by dual-trap Raman spectroscopy and elastic light scattering

Dual-trap laser tweezers Raman spectroscopy (LTRS) and elastic light scattering (ELS) were used to investigate dynamic processes during high-temperature treatment of individual spores of Bacillus cereus, Bacillus megaterium, and Bacillus subtilis in water. Major conclusions from these studies included the following. (i) After spores of all three species were added to water at 80 to 90 degrees C, the level of the 1:1 complex of Ca(2+) and dipicolinic acid (CaDPA; approximately 25% of the dry weight of the spore core) in individual spores remained relatively constant during a highly variable lag time (T(lag)), and then CaDPA was released within 1 to 2 min. (ii) The T(lag) values prior to rapid CaDPA release and thus the times for wet-heat killing of individual spores of all three species were very heterogeneous. (iii) The heterogeneity in kinetics of wet-heat killing of individual spores was not due to differences in the microscopic physical environments during heat treatment. (iv) During the wet-heat treatment of spores of all three species, spore protein denaturation largely but not completely accompanied rapid CaDPA release, as some changes in protein structure preceded rapid CaDPA release. (v) Changes in the ELS from individual spores of all three species were strongly correlated with the release of CaDPA. The ELS intensities of B. cereus and B. megaterium spores decreased gradually and reached minima at T(1) when approximately 80% of spore CaDPA was released, then increased rapidly until T(2) when full CaDPA release was complete, and then remained nearly constant. The ELS intensity of B. subtilis spores showed similar features, although the intensity changed minimally, if at all, prior to T(1). (vi) Carotenoids in B. megaterium spores' inner membranes exhibited two changes during heat treatment. First, the carotenoid's two Raman bands at 1,155 and 1,516 cm(-1) decreased rapidly to a low value and to zero, respectively, well before T(lag), and then the residual 1,155-cm(-1) band disappeared, in parallel with the rapid CaDPA release beginning at T(lag).

An evolutionary analysis of lateral gene transfer in thymidylate synthase enzymes

Thymidylate synthases (Thy) are key enzymes in the synthesis of deoxythymidylate, 1 of the 4 building blocks of DNA. As such, they are essential for all DNA-based forms of life and therefore implicated in the hypothesized transition from RNA genomes to DNA genomes. Two evolutionally unrelated Thy enzymes, ThyA and ThyX, are known to catalyze the same biochemical reaction. Both enzymes are sporadically distributed within each of the 3 domains of life in a pattern that suggests multiple nonhomologous lateral gene transfer (LGT) events. We present a phylogenetic analysis of the evolution of the 2 enzymes, aimed at unraveling their entangled evolutionary history and tracing their origin back to early life. A novel probabilistic evolutionary model was developed, which allowed us to compute the posterior probabilities and the posterior expectation of the number of LGT events. Simulation studies were performed to validate the model's ability to accurately detect LGT events, which have occurred throughout a large phylogeny. Applying the model to the Thy data revealed widespread nonhomologous LGT between and within all 3 domains of life. By reconstructing the ThyA and ThyX gene trees, the most likely donor of each LGT event was inferred. The role of viruses in LGT of Thy is finally discussed.

Characterization of Bacillus subtilis spore inactivation in low-pressure, low-temperature gas plasma sterilization processes

AIMS

To identify structural components of Bacillus subtilis spores serving as targets for sterilization with microwave induced low-pressure, low-temperature nitrogen-oxygen plasma.

METHODS AND RESULTS

The inactivation of spores followed a biphasic kinetics consisting of a log-linear phase with rapid inactivation followed by a slow inactivation phase. In the course of plasma treatment, damage to DNA, proteins and spore membranes were observed by monitoring the occurrence of auxotrophic mutants, inactivation of catalase (KatX) activity and the leakage of dipicolinic acid, respectively. Spores of the wild-type strain showed the highest resistance to plasma treatment. Spores of mutants defective in nucleotide excision repair (uvrA) and small acid-soluble proteins (Delta sspA Delta sspB) were more sensitive than those defective in the coat protein CotE or spore photoproduct repair (splB). Exclusion of reactive particles and spectral fractions of UV radiation from access to the spores revealed that UV-C radiation is the most effective inactivation agent in the plasma, whereby the splB and Delta cotE mutant spores were equally and slightly less sensitive, respectively, than the wild-type spores. Finally, the extent of damages in the spore DNA determined by quantitative PCR correlated with the spore inactivation.

CONCLUSIONS

Spore inactivation was efficiently mediated by a combination of DNA damage and protein inactivation. DNA was identified to be the primary target for spore inactivation by UV radiation emitted by the plasma. Coat proteins were found to constitute a protective layer against the action of the plasma.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results provide new evidence to the understanding of plasma sterilization processes. This knowledge supports the identification of useful parameters for novel plasma sterilization equipment to control process safety.

Structure of a protein-DNA complex essential for DNA protection in spores of Bacillus species

The DNA-binding alpha/beta-type small acid-soluble proteins (SASPs) are a major factor in the resistance and long-term survival of spores of Bacillus species by protecting spore DNA against damage due to desiccation, heat, toxic chemicals, enzymes, and UV radiation. We now report the crystal structure at 2.1 A resolution of an alpha/beta-type SASP bound to a 10-bp DNA duplex. In the complex, the alpha/beta-type SASP adopt a helix-turn-helix motif, interact with DNA through minor groove contacts, bind to approximately 6 bp of DNA as a dimer, and the DNA is in an A-B type conformation. The structure of the complex provides important insights into the molecular details of both DNA and alpha/beta-type SASP protection in the complex and thus also in spores.

Inactivation kinetics of avirulent Bacillus anthracis spores in milk with a combination of heat and hydrogen peroxide

The combined effect of heat and hydrogen peroxide (HP) on the inactivation of avirulent Bacillus anthracis spores (Sterne strain 7702; strain ANR-1, an avirulent Ames derivative lacking the pXO2 plasmid; and strain 9131, a plasmid-less Sterne strain) was evaluated in milk. The study temperature ranged from 90 to 95 degrees C, and the concentration of added HP varied from 0.05 to 0.5%. Decimal reduction times (D-values) were determined using a sealed capillary tube technique. The mean D- and z-values of hydrated freeze-dried spores of all three strains in milk ranged from 550 s at 90 degrees C to 180 s at 94 degrees C and from 8.6 to 9.0 degrees C, respectively. When 0.05% HP was added to the milk, the D-values were decreased at least threefold, and at 0.5% HP the D-values ranged from 1 to 10 s. At 90 degrees C, all three strains had similar D-values when 0.05% HP was added. Increasing the concentration of HP to 0.5% had a greater reducing effect on the D-value for strain 7702 than on the values for strains ANR-1 and 9131. The rate of inactivation of each strain followed first-order reaction kinetics at each temperature-peroxide combination. Equations in the form of D = Constant x (HP concentration)n had R2 values greater than 0.97 for strains ANR-1 and 7702 and of at least 0.7 for strain 9131. This study suggests that a combination of high temperature (from 90 to 95 degrees C) and HP could be used for inactivation of B. anthracis spores in the event of deliberate contamination of milk such that the contaminated milk could be disposed of safely.

Effect of small, acid-soluble proteins on spore resistance and germination under a combination of pressure and heat treatment

To find the range of pressure required for effective high-pressure inactivation of bacterial spores and to investigate the role of alpha/beta-type small, acid-soluble proteins (SASP) in spores under pressure treatment, mild heat was combined with pressure (room temperature to 65 degrees C and 100 to 500 MPa) and applied to wild-type and SASP-alpha-/beta- Bacillus subtilis spores. On the one hand, more than 4 log units of wild-type spores were reduced after pressurization at 100 to 500 MPa and 65 degrees C. On the other hand, the number of surviving mutant spores decreased by 2 log units at 100 MPa and by more than 5 log units at 500 MPa. At 500 MPa and 65 degrees C, both wild-type and mutant spore survivor counts were reduced by 5 log units. Interestingly, pressures of 100, 200, and 300 MPa at 65 degrees C inactivated wild-type SASP-alpha+/beta+ spores more than mutant SASP-alpha-/beta- spores, and this was attributed to less pressure-induced germination in SASP-alpha-/beta- spores than in wild-type SASP-alpha+/beta+ spores. However, there was no difference in the pressure resistance between SASP-alpha+/beta+ and SASP-alpha-/beta- spores at 100 MPa and ambient temperature (approximately 22 degrees C) for 30 min. A combination of high pressure and high temperature is very effective for inducing spore germination, and then inactivation of the germinated spore occurs because of the heat treatment. This study showed that alpha/beta-type SASP play a role in spore inactivation by increasing spore germination under 100 to 300 MPa at high temperature.

Formulation of stable Bacillus subtilis AH18 against temperature fluctuation with highly heat-resistant endospores and micropore inorganic carriers

To survive the commercial market and to achieve the desired effect of beneficial organisms, the strains in microbial products must be cost-effectively formulated to remain dormant and hence survive through high and low temperatures of the environment during transportation and storage. Dormancy and stability of Bacillus subtilis AH18 was achieved by producing endospores with enhanced heat resistance and using inorganic carriers. Heat stability assays, at 90 degrees C for 1 h, showed that spores produced under a sublethal temperature of 57 degrees C was 100 times more heat-resistant than the ones produced by food depletion at the growing temperature of 37 degrees C. When these highly heat-resistant endospores were formulated with inorganic carriers of natural and synthetic zeolite or kaolin clay minerals having substantial amount of micropores, the dormancy of the endospores was maintained for 6 months at 15-25 degrees C. Meanwhile, macroporous perlite carriers with average pore diameter larger than 3.7 microm stimulated the germination of the spores and rapid proliferation of the bacteria. These results indicated that a B. subtilis AH18 product that can remain dormant and survive through environmental temperature fluctuation can be formulated by producing heat-stressed endospores and incorporating inorganic carriers with micropores in the formulation step.

Mode of Salmonella and Escherichia coli O157:H7 inactivation by a stabilized oxychloro-based sanitizer

AIM

To determine the mechanisms by which a stabilized oxychloro (SOC)-based sanitizer, applied to decontaminate seeds destined for sprout production, inactivates Escherichia coli O157:H7 ph1 and Salmonella serotype Meleagridis.

MATERIALS AND RESULTS

The action of SOC on the metabolism, membrane and DNA integrity of Salmonella and E. coli O157:H7 was studied. In both pathogens, there was an oxidative burst and depletion of intracellular glutathione (GSH) upon initial exposure to 200 ppm SOC. Metabolic activity, measured via bioluminescence, decreased over a 4-h period in E. coli O157:H7 ph1 cells exposed to SOC. Membrane integrity, assessed through viability staining, decreased progressively over 23 h when exposed to SOC. The appearance of auxotrophic mutants suggested that DNA damage had also occurred. Enzymes rich in disulfide bonds (alkaline phosphatase and protease) were sensitive to the chlorite-based sanitizer. Through challenging other microbial types, it was found that Gram positive had higher tolerance to SOC than Gram negatives with the exception of Salmonella. MS2 bacteriophage was highly sensitive; however, Bacillus endospores were not inactivated by SOC.

CONCLUSIONS

SOC inactivates E. coli O157:H7 and Salmonella through GSH oxidation and disruption of disulfide bonds. Ultimately, membrane damage resulting from prolonged exposure to SOC leads to the loss of cell viability.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results provide a basis for understanding why extended treatment times are required to inactivate bacteria using SOC.

Mechanisms of killing of spores of Bacillus subtilis by dimethyldioxirane

AIMS

To determine the mechanisms of Bacillus subtilis spore resistance to and killing by a novel sporicide, dimethyldioxirane (DMDO) that was generated in situ from acetone and potassium peroxymonosulfate at neutral pH.

METHODS AND RESULTS

Spores of B. subtilis were effectively killed by DMDO. Rates of killing by DMDO of spores lacking most DNA protective alpha/beta-type small, acid-soluble spore proteins (alpha- beta- spores) or the major DNA repair protein, RecA, were very similar to that of wild-type spore killing. Survivors of wild-type and alpha- beta- spores treated with DMDO also exhibited no increase in mutations. Spores lacking much coat protein due either to mutation or chemical decoating were much more sensitive to DMDO than were wild-type spores, but were more resistant than growing cells. Wild-type spores killed with this reagent retained their large pool of dipicolinic acid (DPA), and the survivors of spores treated with DMDO were sensitized to wet heat. The DMDO-killed spores germinated with nutrients, albeit more slowly than untreated spores, but germinated faster than untreated spores with dodecylamine. The killed spores were also germinated by very high pressures and by lysozyme treatment in hypertonic medium, but many of these spores lysed shortly after their germination, and none of these treatments were able to revive the DMDO-killed spores.

CONCLUSIONS

DMDO is an effective reagent for killing B. subtilis spores. The spore coat is a major factor in spore resistance to DMDO, which does not kill spores by DNA damage or by inactivating some component needed for spore germination. Rather, this reagent appears to kill spores by damaging the spore's inner membrane in some fashion.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work demonstrates that DMDO is an effective decontaminant for spores of Bacillus species that can work under mild conditions, and the killed spores cannot be revived. Evidence has also been obtained on the mechanisms of spore resistance to and killing by this reagent.

Killing of spores of Bacillus subtilis by tert-butyl hydroperoxide plus a TAML�activator

AIMS

To determine the effectiveness of tert-butyl hydroperoxide (tBHP) plus the cationic surfactant cetyltrimethyl ammonium bromide (CTAB) and a tetra-amido macrocyclic ligand (TAML) activator in killing spores of Bacillus subtilis and the mechanisms of spore resistance to and killing by this reagent.

METHODS AND RESULTS

Killing of spores of B. subtilis by tBHP was greatly stimulated by the optimum ratio of concentrations of a TAML activator (1.7 micromol l(-1)) to tBHP (4.4%, vol/vol) plus a low level (270 mg l(-1)) of CTAB. Rates of killing of spores lacking most DNA protective alpha/beta-type small, acid-soluble spore proteins (alpha(-)beta(-) spores) or the major DNA repair protein, RecA, by tBHP plus CTAB and a TAML activator were essentially identical to that of wild-type spore killing. Survivors of wild-type and alpha(-)beta(-) spores treated with tBHP plus CTAB and a TAML activator also exhibited no increase in mutations. Spores lacking much coat protein either because of mutation or chemical decoating were much more sensitive to this reagent than were wild-type spores, but were more resistant than growing cells. Wild-type spores killed with this reagent retained their large pool of dipicolinic acid (DPA), and the survivors of spores treated with this reagent were sensitized to wet heat. The tBHP plus CTAB and TAML activator-killed spores germinated with nutrients, albeit more slowly than untreated spores, but germinated faster than untreated spores with dodecylamine. The killed spores were also germinated by application of 150 and 500 megaPascals of pressure for 15 min and by lysozyme treatment in hypertonic medium, but these spores lysed shortly after their germination.

CONCLUSIONS

The combination of tBHP plus CTAB and a TAML activator is effective in killing B. subtilis spores. The spore coat is a major factor in spore resistance to this reagent system, which does not kill spores by DNA damage or by inactivating some component needed for spore germination. Rather, this reagent system appears to kill spores by damaging the spore's inner membrane in some fashion.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work demonstrates that tBHP plus CTAB and a TAML activator is an effective and mild decontaminant for spores of Bacillus species. Evidence has also been obtained on the mechanisms of spore resistance to and killing by this reagent system.

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.

Investigating the role of small, acid-soluble spore proteins (SASPs) in the resistance of Clostridium perfringens spores to heat

Background

Clostridium perfringens type A food poisoning is caused by enterotoxigenic C. perfringens type A isolates that typically possess high spore heat-resistance. The molecular basis for C. perfringens spore heat-resistance remains unknown. In the current study, we investigated the role of small, acid-soluble spore proteins (SASPs) in heat-resistance of spores produced by C. perfringens food poisoning isolates.

Results

Our current study demonstrated the presence of all three SASP-encoding genes (ssp1, 2 and 3) in five surveyed C. perfringens clinical food poisoning isolates. β-Glucuronidase assay showed that these ssp genes are expressed specifically during sporulation. Consistent with these expression results, our study also demonstrated the production of SASPs by C. perfringens food poisoning isolates. When the heat sensitivities of spores produced by a ssp3 knock-out mutant of a C. perfringens food poisoning isolate was compared with that of spores of the wild-type strain, spores of the ssp3 mutant were found to exhibit a lower decimal reduction value (D value) at 100°C than exhibited by the spores of wild-type strain. This effect was restored by complementing the ssp3 mutant with a recombinant plasmid carrying wild-type ssp3, suggesting that the observed differences in D values between spores of wild-type versus ssp3 mutant was due to the specific inactivation of ssp3. Furthermore, our DNA protection assay demonstrated that C. perfringens SASPs can protect DNA from DNase I digestion.

Conclusion

The results from our current study provide evidences that SASPs produced by C. perfringens food poisoning isolates play a role in protecting their spores from heat-damage, which is highly significant and relevant from a food safety perspective. Further detailed studies on mechanism of action of SASPs from C. perfringens should help in understanding the mechanism of protection of C. perfringens spores from heat-damage.

Levels of glycine betaine in growing cells and spores of Bacillus species and lack of effect of glycine betaine on dormant spore resistance

Bacteria of various Bacillus species are able to grow in media with very high osmotic strength in part due to the accumulation of low-molecular-weight osmolytes such as glycine betaine (GB). Cells of Bacillus species grown in rich and minimal media contained low levels of GB, but GB levels were 4- to 60-fold higher in cells grown in media with high salt. GB levels in Bacillus subtilis cells grown in minimal medium were increased approximately 7-fold by GB in the medium and 60-fold by GB plus high salt. GB was present in spores of Bacillus species prepared in media with or without high salt but at lower levels than in comparable growing cells. With spores prepared in media with high salt, GB levels were highest in B. subtilis spores and > or =20-fold lower in B. cereus and B. megaterium spores. Although GB levels in B. subtilis spores were elevated 15- to 30-fold by GB plus high salt in sporulation media, GB levels did not affect spore resistance. GB levels were similar in wild-type B. subtilis spores and spores that lacked major small, acid-soluble spore proteins but were much lower in spores that lacked dipicolinic acid.

The Forespore Line of Gene Expression in Bacillus subtilis

Endospore formation by Bacillus subtilis involves three differentiating cell types, the predivisional cell, the mother cell, and the forespore. Here we report the program of gene expression in the forespore, which is governed by the RNA polymerase sigma factors sigma(F) and sigma(G) and the DNA-binding proteins RsfA and SpoVT. The sigma(F) factor turns on about 48 genes, including the gene for RsfA, which represses a gene in the sigma(F) regulon, and the gene for sigma(G). The sigma(G) factor newly activates 81 genes, including the gene for SpoVT, which turns on (in nine cases) or stimulates (in 11 cases) the expression of 20 genes that had been turned on by sigma(G) and represses the expression of 27 others. The forespore line of gene expression consists of many genes that contribute to morphogenesis and to the resistance and germination properties of the spore but few that have metabolic functions. Comparative genomics reveals a core of genes in the sigma(F) and sigma(G) regulons that are widely conserved among endospore-forming species but are absent from closely related, but non-spore-forming Listeria spp. Two such partially conserved genes (ykoU and ykoV), which are members of the sigma(G) regulon, are shown to confer dry-heat resistance to dormant spores. The ykoV gene product, a homolog of the non-homologous end-joining protein Ku, is shown to associate with the nucleoid during germination. Extending earlier work on gene expression in the predivisional cell and the mother cell, we present an integrated overview of the entire program of sporulation gene expression.

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.

Role of the Nfo (YqfS) and ExoA apurinic/apyrimidinic endonucleases in protecting Bacillus subtilis spores from DNA damage

The Bacillus subtilis enzymes ExoA and Nfo (originally termed YqfS) are endonucleases that can repair apurinic/apyrimidinic (AP) sites and strand breaks in DNA. We have analyzed how the lack of ExoA and Nfo affects the resistance of growing cells and dormant spores of B. subtilis to a variety of treatments, some of which generate AP sites and DNA strand breaks. The lack of ExoA and Nfo sensitized spores (termed alpha-beta-) lacking the majority of their DNA-protective alpha/beta-type small, acid-soluble spore proteins (SASP) to wet heat. However, the lack of these enzymes had no effect on the wet-heat resistance of spores that retained alpha/beta-type SASP. The lack of either ExoA or Nfo sensitized wild-type spores to dry heat, but loss of both proteins was necessary to sensitize alpha-beta- spores to dry heat. The lack of ExoA and Nfo also sensitized alpha-beta-, but not wild-type, spores to desiccation. In contrast, loss of ExoA and Nfo did not sensitize growing cells or wild-type or alpha-beta- spores to hydrogen peroxide or t-butylhydroperoxide. Loss of ExoA and Nfo also did not increase the spontaneous mutation frequency of growing cells. exoA expression took place not only in growing cells, but also in the forespore compartment of the sporulating cell. These results, together with those from previous work, suggest that ExoA and Nfo are additional factors that protect B. subtilis spores from DNA damage accumulated during spore dormancy.

Treatment with oxidizing agents damages the inner membrane of spores of Bacillus subtilis and sensitizes spores to subsequent stress

AIMS

To determine if treatment of Bacillus subtilis spores with a variety of oxidizing agents causes damage to the spore's inner membrane.

METHODS AND RESULTS

Spores of B. subtilis were killed 80-99% with wet heat or a variety of oxidizing agents, including betadine, chlorine dioxide, cumene hydroperoxide, hydrogen peroxide, Oxone, ozone, sodium hypochlorite and t-butylhydroperoxide, and the agents neutralized and/or removed. Survivors of spores pretreated with oxidizing agents exhibited increased sensitivity to killing by a normally minimal lethal heat treatment, while spores pretreated with wet heat did not. In addition, spores treated with wet heat or the oxidizing agents, except sodium hypochlorite, were more sensitive to high NaCl in plating media than were untreated spores. The core region of spores treated with at least two oxidizing agents was also penetrated much more readily by methylamine than was the core of untreated spores, and spores treated with oxidizing agents but not wet heat germinated faster with dodecylamine than did untreated spores. Spores of strains with very different levels of unsaturated fatty acids in their inner membrane exhibited essentially identical resistance to oxidizing agents.

CONCLUSIONS

Treatment of spores with oxidizing agents has been suggested to cause damage to the spore's inner membrane, a membrane whose integrity is essential for spore viability. The sensitization of spores to killing by heat and to high salt after pretreatment with oxidizing agents is consistent with and supports this suggestion. Presumably mild pretreatment with oxidizing agents causes some damage to the spore's inner membrane. While this damage may not be lethal under normal conditions, the damaged inner membrane may be less able to maintain its integrity, when dormant spores are exposed to high temperature or when germinated spores are faced with osmotic stress. Triggering of spore germination by dodecylamine likely involves action by this agent on the spore's inner membrane allowing release of the spore core's depot of dipicolinic acid. Presumably dodecylamine more readily alters the permeability of a damaged inner membrane and thus more readily triggers germination of spores pretreated with oxidizing agents. Damage to the inner spore membrane by oxidizing agents is also consistent with the more rapid penetration of methylamine into the core of treated spores, as the inner membrane is likely the crucial permeability barrier to methylamine entry into the spore core. As spores of strains with very different levels of unsaturated fatty acids in their inner membrane exhibited essentially identical resistance to oxidizing agents, it is not through oxidation of unsaturated fatty acids that oxidizing agents kill and/or damage spores. Perhaps these agents work by causing oxidative damage to key proteins in the spore's inner membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

The more rapid heat killing and germination with dodecylamine, the greater permeability of the spore core and the osmotic stress sensitivity in outgrowth of spores pretreated with oxidizing agents is consistent with such agents causing damage to the spore's inner membrane, even if this damage is not lethal under normal conditions. It may be possible to take advantage of this phenomenon to devise improved, less costly regimens for spore inactivation.

Killing of Bacillus subtilis spores by a modified Fenton reagent containing CuCl2 and ascorbic acid

Bacillus subtilis spores were killed by CuCl(2)-ascorbic acid, chloride ions were essential for killing of spores, and spores with defective coats were killed more rapidly. CuCl(2)-ascorbic acid did not damage spore DNA, and spores killed by this reagent initiated germination. However, spores killed by CuCl(2)-ascorbic acid may have damage to their inner membrane.

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.