Structural analysis of high pressure treated Bacillus subtilis spores

Ultrasonic-Assisted Marine Antifouling Strategy on Gel-like Epoxy Primer

Ultrasonic technology has drawn extensive interests for its great potential in marine antifouling applications. However, its effects on the adhesion behavior of marine fouling organisms on marine structures remain underexplored. This work investigated how ultrasonic treatment impacted the adhesion of Pseudoalteromonas on a gel-like marine epoxy primer. And the process parameters for ultrasonic treatment were optimized using response surface analysis with Design-Expert software 11. The results revealed that ultrasonic treatment disrupted the cellular structure of Pseudoalteromonas, causing the deformation and fragmentation of the cell membrane, leading to bacterial death. Additionally, ultrasonic treatment reduced the particle size and Zeta potential value of Pseudoalteromonas, which disrupted the stability of bacterial suspensions. It also increased the relative surface hydrophobicity of Pseudoalteromonas cells, resulting in a reduction in adhesion to the gel-like marine epoxy primer. This study demonstrated that ultrasonic treatment significantly disturbed the adhesion behavior of microorganisms like Pseudoalteromonas on the gel-like marine epoxy primer, which provided an effective approach for controlling marine biofouling.

New insights into peroxide toxicology: sporulenes help Bacillus subtilis endospores from hydrogen peroxide

AIM

The purpose of the present study was to understand the possible events involved in the toxicity of hydrogen peroxide (H2O2) to wild and sporulene-deficient spores of Bacillus subtilis, as H2O2 was previously shown to have deleterious effects.

METHODS AND RESULTS

The investigation utilized two strains of B. subtilis, namely the wild-type PY79 (WT) and the sporulene-deficient TB10 (ΔsqhC mutant). Following treatment with 0.05% H2O2 (v/v), spore viability was assessed using a plate count assay, which revealed a significant decrease in cultivability of 80% for the ΔsqhC mutant spores. Possible reasons for the loss of spore viability were investigated with microscopic analysis, dipicholinic acid (DPA) quantification and propidium iodide (PI) staining. Microscopic examinations revealed the presence of withered and deflated morphologies in spores of ΔsqhC mutants treated with H2O2, indicating a compromised membrane permeability. This was further substantiated by the absence of DPA and a high frequency (50%-75%) of PI infiltration. The results of fatty acid methyl ester analysis and protein profiling indicated that the potentiation of H2O2-induced cellular responses was manifested in the form of altered spore composition in ΔsqhC B. subtilis. The slowed growth rates of the ΔsqhC mutant and the heightened sporulene biosynthesis pathways in the WT strain, both upon exposure to H2O2, suggested a protective function for sporulenes in vegetative cells.

CONCLUSIONS

Sporulenes serve as a protective layer for the inner membrane of spores, thus assuming a significant role in mitigating the adverse effects of H2O2 in WT B. subtilis. The toxic effects of H2O2 were even more pronounced in the spores of the ΔsqhC mutant, which lacks this protective barrier of sporulenes.

The underlying mechanism of bacterial spore germination: An update review

Bacterial spores are highly resilient and universally present on earth and can irreversibly enter the food chain to cause food spoilage or foodborne illness once revived to resume vegetative growth. Traditionally, extensive thermal processing has been employed to efficiently kill spores; however, the relatively high thermal load adversely affects food quality attributes. In recent years, the germination-inactivation strategy has been developed to mildly kill spores based on the circumstance that germination can decrease spore-resilient properties. However, the failure to induce all spores to geminate, mainly owing to the heterogeneous germination behavior of spores, hampers the success of applying this strategy in the food industry. Undoubtedly, elucidating the detailed germination pathway and underlying mechanism can fill the gap in our understanding of germination heterogeneity, thereby facilitating the development of full-scale germination regimes to mildly kill spores. In this review, we comprehensively discuss the mechanisms of spore germination of Bacillus and Clostridium species, and update the molecular basis of the early germination events, for example, the activation of germination receptors, ion release, Ca-DPA release, and molecular events, combined with the latest research evidence. Moreover, high hydrostatic pressure (HHP), an advanced non-thermal food processing technology, can also trigger spore germination, providing a basis for the application of a germination-inactivation strategy in HHP processing. Here, we also summarize the diverse germination behaviors and mechanisms of spores of Bacillus and Clostridium species under HHP, with the aim of facilitating HHP as a mild processing technology with possible applications in food sterilization. Practical Application: This work provides fundamental basis for developing efficient killing strategies of bacterial spores in food industry.

Post-high-pressure temperature and time - Overlooked parameters in high pressure treatment of bacterial spores

High pressure (HP) processing has high potential for bacterial spore inactivation with minimal thermal input. To advance HP germination and subsequent inactivation of spores, this study explored the physiological state of HP-treated spores using flow cytometry (FCM). Bacillus subtilis spores were treated at 550 MPa and 60 °C (very HP (vHP)) in buffer, incubated after the HP treatment, and stained for FCM analysis with SYTO16 indicating germination and propidium iodide (PI) indicating membrane damage. FCM subpopulations were analyzed depending on the HP dwell time (≤20 min), post-HP temperature (ice, 37 °C, 60 °C) and time (≤4 h), germination-relevant cortex-lytic enzymes (CLEs) and small-acid-soluble-proteins-(SASP)-degrading enzymes by using deletion strains. The effect of post-HP temperatures (ice, 37 °C) was additionally studied for moderate HP (150 MPa, 38 °C, 10 min). Post-HP incubation conditions strongly influenced the prevalence of five observed FCM subpopulations. Post-HP incubation on ice did not or only slowly shifted SYTO16-positive spores to higher SYTO16 levels. At 37 °C post-HP, this shift accelerated, and a shift to high PI intensities occurred depending on the HP dwell time. At 60 °C post-HP, the main shift was from SYTO16-positive to PI-positive subpopulations. The enzymes CwlJ and SleB, which are CLEs, seemed both necessary for PI or SYTO16 uptake, and to have different sensitivities to 550 MPa and 60 °C. Different extents of SASP degradation might explain the existence of two SYTO16-positive subpopulations. Shifts to higher SYTO16 intensities during post-HP incubation on ice or at 37 °C might rely on the activity and recovery of CLEs, SASP-degrading enzymes or their associated proteins from reversible HP-induced structural changes. These enzymes seemingly become active only during decompression or after vHP treatments (550 MPa, 60 °C). Based on our results, we provide a refined model of HP germination-inactivation of B. subtilis spores and an optimized FCM method for quantification of the safety-relevant subpopulation, i.e., vHP (550 MPa, 60 °C) superdormant spores. This study contributes to the development of mild spore inactivation processes by shedding light on overlooked parameters: post-HP incubation conditions. Post-HP conditions significantly influenced the physiological state of spores, likely due to varying enzymatic activity. This finding may explain inconsistencies in previous research and shows the importance of reporting post-HP conditions in future research. Furthermore, the addition of post-HP conditions as HP process parameter may open up new possibilities to optimize HP-based inactivation of spores for potential industrial applications in the food industry.

High-Pressure-Based Strategies for the Inactivation of Bacillus subtilis Endospores in Honey

Honey is a value-added product rich in several types of phenolic compounds, enzymes, and sugars recently explored in biomedical and food applications. Nevertheless, even though it has a low water activity (a_W ≈ 0.65) that hinders the development of pathogenic and spoilage microorganisms, it is still prone to contamination by pathogenic microorganisms (vegetative and spores) and may constitute harm to special groups, particularly by immunosuppressed people and pregnant women. Thus, an efficient processing methodology needs to be followed to ensure microbial safety while avoiding 5-hydroxymethylfurfural (HMF) formation and browning reactions, with a consequent loss of biological value. In this paper, both thermal (pressure-assisted thermal processing, PATP) and nonthermal high-pressure processing (HPP), and another pressure-based methodology (hyperbaric storage, HS) were used to ascertain their potential to inactivate Bacillus subtilis endospores in honey and to study the influence of a_W on the inactivation on this endospore. The results showed that PATP at 600 MPa/15 min/75 °C of diluted honey (52.9 °Brix) with increased a_W (0.85 compared to ≈0.55, the usual honey a_W) allowed for inactivating of at least 4.0 log units of B. subtilis spores (to below detection limits), while HS and HPP caused neither the germination nor inactivated spores (i.e., there was neither a loss of endospore resistance after heat shock nor endospore inactivation as a consequence of the storage methodology). PATP of undiluted honey even at harsh processing conditions (600 MPa/15 min/85 °C) did not impact the spore load. The results for diluted honey open the possibility of its decontamination by spores’ inactivation for medical and pharmaceutical applications.

The effect of high pressure combined with moderate temperature and peptidoglycan fragments on spore inactivation

High pressure processing (HPP) is a promising non-thermal processing method for food production. However, extremely high pressure and temperature are often required to achieve spores inactivation and commercial sterilization using HPP. In this study, the combined treatment of HPP, moderate temperature, and peptidoglycan fragments (PGF) for spore inactivation was investigated. The combined treatment of 200 MPa and 1 mg/mL PGF at 80 °C for 20 min resulted in 8.6 log inactivation of Bacillus subtilis 168 and more than 5 log reductions of Clostridium sporogenes PA3679 spores, respectively. A strong synergistic effect on spore inactivation among HPP, PGF, and temperature was observed. By comparing the effect of the treatment on the fluidity of the inner membrane and structural change of spores using fluorescence assay, a probable inactivation mechanism was proposed. It was concluded that the spores were firstly triggered to enter the Stage I of the germination process by HPP and PGF, and then immediately inactivated by the mild heat. This novel processing method could be an alternative to ensure commercial sterilization in the food industry.

Bacillus spore germination at moderate high pressure: A review on underlying mechanisms, influencing factors, and its comparison with nutrient germination

Spore-forming bacteria are resistant to stress conditions owing to their ability to form highly resistant dormant spores. These spores can survive adverse environmental conditions in nature, as well as decontamination processes in the food and related industries. Bacterial spores may return to their vegetative state through a process called germination. As spore germination is critical for the loss of resistance, outgrowth, and development of pathogenicity and spoilage potential, the germination pathway has piqued the interest of the scientific community. The inhibition and induction of germination have critical applications in the food industry. Targeted germination can aid in decreasing the resistance of spores and allow the application of milder inactivation procedures. This germination-inactivation strategy allows better maintenance of important food quality attributes. Different stimuli are reported to trigger germination. Among those, isostatic high pressure (HP) has gained increasing attention due to its potential applications in industrial processes. However, pressure-mediated spore germination is extremely heterogeneous as some spores germinate rapidly, while others exhibit slow germination or do not undergo germination at all. The successful and safe implementation of the germination-inactivation strategy, however, depends on the germination of all spores. Therefore, there is a need to elucidate the mechanisms of HP-mediated germination. This work aimed to critically review the current state of knowledge on Bacillus spore germination at a moderate HP of 50-300 MPa. In this review, the germination mechanism, heterogeneity, and influencing factors have been outlined along with knowledge gaps.

Ultraviolet-C inactivation and hydrophobicity of Bacillus subtilis and Bacillus velezensis spores isolated from extended shelf-life milk

Bacterial spores are important in food processing due to their ubiquity, resistance to high temperature and chemical inactivation. This work aims to study the effect of ultraviolet C (UVC) on the spores of Bacillus subtilis and Bacillus velezensis at a molecular and individual level to guide in deciding on the right parameters that must be applied during the processing of liquid foods. The spores were treated with UVC using phosphate buffer saline (PBS) as a suspension medium and their lethality rate was determined for each sample. Purified spore samples of B. velezensis and B. subtilis were treated under one pass in a UVC reactor to inactivate the spores. The resistance pattern of the spores to UVC treatment was determined using dipicolinic acid (Ca-DPA) band of spectral analysis obtained from Raman spectroscopy. Flow cytometry analysis was also done to determine the effect of the UVC treatment on the spore samples at the molecular level. Samples were processed for SEM and the percentage spore surface hydrophobicity was also determined using the Microbial Adhesion to Hydrocarbon (MATH) assay to predict the adhesion strength to a stainless-steel surface. The result shows the maximum lethality rate to be 6.5 for B. subtilis strain SRCM103689 (B47) and highest percentage hydrophobicity was 54.9% from the sample B. velezensis strain LPL-K103 (B44). The difference in surface hydrophobicity for all isolates was statistically significant (P < 0.05). Flow cytometry analysis of UVC treated spore suspensions clarifies them further into sub-populations unaccounted for by plate counting on growth media. The Raman spectroscopy identified B4002 as the isolate possessing the highest concentration of Ca-DPA. The study justifies the critical role of Ca-DPA in spore resistance and the possible sub-populations after UVC treatment that may affect product shelf-life and safety. UVC shows a promising application in the inactivation of resistant spores though there is a need to understand the effects at the molecular level to design the best parameters during processing.

Efficient reduction in vegetative cells and spores of Bacillus subtilis by essential oil components-coated silica filtering materials

Inactivation of bacterial spores is a key objective for developing novel food preservation technologies. In this work, the removal properties of filtering materials based on silica microparticles functionalized with essential oil components (EOCs) (carvacrol, eugenol, thymol, and vanillin) against Bacillus subtilis, a spore-forming bacterium, in two liquid matrices were investigated. The viability of vegetative cells and spores after treatment was also evaluated. The results exhibited marked removal effectiveness against B. subtilis vegetative cells and spores after filtration with the different silica supports coated with EOCs in either sterile water or nutrient broth, with reductions of 3.2 to 4.9 log units and 3.7 to 5.0 log units for vegetative cells and spores, respectively. The fluorescent viability images revealed the poor viability of the treated B. subtilis vegetative cells and spores due to damage to the cell envelope when coming into contact with the immobilized antimicrobials. The culture counts results revealed the great inhibitory capacity of the EOC-functionalized silica microparticles against B. subtilis vegetative cells and spores after a single filtration. Hence, the present work suggests the feasibility of using EOC-functionalized supports as filtering aids to enhance the microbial quality of liquid matrices with spore-forming microorganisms. PRACTICAL APPLICATION: The developed antimicrobial-coated filters have shown remarkable removal properties against an important spore-forming bacterium in food industry. These filters may be used as a potential sterilization technique for preservation of different beverages alone or in combination with other mild-thermal or nonthermal techniques.

Opportunities for the application of real-time bacterial cell analysis using flow cytometry for the advancement of sterilization microbiology

Medical devices provide critical care and diagnostic applications through patient contact. Sterility assurance level (SAL) may be defined as the probability of a single viable micro-organism occurring on an item after a sterilization process. Sterilization microbiology often relies upon using an overkill validation method where a 12-log reduction in recalcitrant bacterial endospore population occurs during the process that exploits conventional laboratory-based culture media for enumeration. This timely review explores key assumptions underpinning use of conventional culture-based methods in sterilization microbiology. Consideration is given to how such methods may limit the ability to fully appreciate the inactivation kinetics of a sterilization process such as vaporized hydrogen peroxide (VH2O2) sterilization, and consequently design efficient sterilization processes. Specific use of the real-time flow cytometry (FCM) is described by way of elucidating the practical relevance of these limitation factors with implications and opportunities for the sterilization industry discussed. Application of FCM to address these culture-based limitation factors will inform real-time kinetic inactivation modelling and unlock potential to embrace emerging opportunities for pharma, medical device and sterilization industries including potentially disruptive applications that may involve reduced usage of sterilant.

Effects of Heavy Ion Particle Irradiation on Spore Germination of Bacillus spp. from Extremely Hot and Cold Environments

Extremophiles are optimal models in experimentally addressing questions about the effects of cosmic radiation on biological systems. The resistance to high charge energy (HZE) particles, and helium (He) ions and iron (Fe) ions (LET at 2.2 and 200 keV/µm, respectively, until 1000 Gy), of spores from two thermophiles, Bacillushorneckiae SBP3 and Bacilluslicheniformis T14, and two psychrotolerants, Bacillus sp. A34 and A43, was investigated. Spores survived He irradiation better, whereas they were more sensitive to Fe irradiation (until 500 Gy), with spores from thermophiles being more resistant to irradiations than psychrotolerants. The survived spores showed different germination kinetics, depending on the type/dose of irradiation and the germinant used. After exposure to He 1000 Gy, D-glucose increased the lag time of thermophilic spores and induced germination of psychrotolerants, whereas L-alanine and L-valine increased the germination efficiency, except alanine for A43. FTIR spectra showed important modifications to the structural components of spores after Fe irradiation at 250 Gy, which could explain the block in spore germination, whereas minor changes were observed after He radiation that could be related to the increased permeability of the inner membranes and alterations of receptor complex structures. Our results give new insights on HZE resistance of extremophiles that are useful in different contexts, including astrobiology.

Hyperbaric storage at room like temperatures as a possible alternative to refrigeration: evolution and recent advances

From 2012, the preservation of food products under pressure has been increasingly studied and the knowledge acquired has enlarged since several food products have been studied at different storage conditions. This new food preservation methodology concept called Hyperbaric Storage (HS) has gain relevance due to its potential as a replacement or an improvement to the conventional cold storage processes, such as the traditional refrigeration (RF), or even frosting, from the energetic savings to the reduction of the carbon foot-print. Briefly, HS is capable to inhibit the microbial proliferation or its inactivation which results in the extension of the shelf-life of several food products when compared to RF. Moreover, the overall quality parameters seem not to be affected by HS, being the differences detected on samples over storage similar to lower when compared to the ones stored at RF. This review paper aims to gather data from all studies carried out so far regarding HS performance, mainly at room temperature on fruit juices, meat and fisheries, as well on dairy products and ready-to-eat meals. The HS advantages as a new food preservation methodology are presented and explained, being also discussed the industrial viability and environmental impact of this methodology, as well its limitations.

Endospore Inactivation by Emerging Technologies: A Review of Target Structures and Inactivation Mechanisms

Recent developments in preservation technologies allow for the delivery of food with nutritional value and superior taste. Of special interest are low-acid, shelf-stable foods in which the complete control or inactivation of bacterial endospores is the crucial step to ensure consumer safety. Relevant preservation methods can be classified into physicochemical or physical hurdles, and the latter can be subclassified into thermal and nonthermal processes. The underlying inactivation mechanisms for each of these physicochemical or physical processes impact different morphological or molecular structures essential for spore germination and integrity in the dormant state. This review provides an overview of distinct endospore defense mechanisms that affect emerging physical hurdles as well as which technologies address these mechanisms. The physical spore-inactivation technologies considered include thermal, dynamic, and isostatic high pressure and electromagnetic technologies, such as pulsed electric fields, UV light, cold atmospheric pressure plasma, and high- or low-energy electron beam.

Flow Cytometry Combined With Single Cell Sorting to Study Heterogeneous Germination of Bacillus Spores Under High Pressure

Isostatic high pressure (HP) of 150 MPa can trigger the germination of bacterial spores, making them lose their extreme resistance to stress factors, and increasing their susceptibility to milder inactivation strategies. However, germination response of spores within a population is very heterogeneous, and tools are needed to study this heterogeneity. Here, classical methods were combined with more recent and powerful techniques such as flow cytometry (FCM) and fluorescence activated cell sorting (FACS) to investigate spore germination behavior under HP. Bacillus subtilis spores were treated with HP at 150 MPa and 37°C, stained with SYTO16 and PI, and analyzed via FCM. Four sub-populations were detected. These sub-populations were for the first time isolated on single cell level using FACS and characterized in terms of their heat resistance (80°C, 10 min) and cultivability in a nutrient-rich environment. The four isolated sub-populations were found to include (1) heat-resistant and mostly cultivable superdormant spores, i.e., spores that remained dormant after this specific HP treatment, (2) heat-sensitive and cultivable germinated spores, (3) heat-sensitive and partially-cultivable germinated spores, and (4) membrane-compromised cells with barely detectable cultivability. Of particular interest was the physiological state of the third sub-population, which was previously referred to as "unknown". Moreover, the kinetic transitions between different physiological states were characterized. After less than 10 min of HP treatment, the majority of spores germinated and ended up in a sublethally damaged stage. HP treatment at 150 MPa and 37°C did not cause inactivation of all geminated spores, suggesting that subsequent inactivation strategies such as mild heat inactivation or other inactivation techniques are necessary to control spores in food. This study validated FCM as a powerful technique to investigate the heterogeneous behavior of spores under HP, and provided a pipeline using FACS for isolation of different sub-populations and subsequent characterization to understand their physiological states.

Building of Pressure-Assisted Ultra-High Temperature System and Its Inactivation of Bacterial Spores

The pressure-assisted ultra-high temperature (PAUHT) system was built by using soybean oil as pressure-transmitting medium, and the multiple regression equation of soybean oil temperature change (ΔTP ) during pressurization as a function of initial temperature (Ti ) and set pressure (P) was developed: ΔTP = -13.45 + 0.46 Ti + 0.0799 P - 0.0037T i 2 - 2.83 × 10-5 P2. The fitted model indicated that the temperature of the system would achieve ≥121°C at 600 MPa when the initial temperature of soybean oil was ≥84°C. The PAUHT system could effectively inactivate spores of Bacillus subtilis 168 and Clostridium sporogenes PA3679 (less than 1 min). Treatment of 600 MPa and 121°C with no holding time resulted in a 6.75 log reductions of B. subtilis 168 spores, while treatment of 700 MPa and 121°C with pressure holding time of 20 s achieved more than 5 log reductions of C. sporogenes PA3679 spores. By comparing the PAUHT treatment with high pressure or thermal treatment alone, and also studying the effect of compression on spore inactivation during PAUHT treatment, the inactivation mechanism was further discussed and could be concluded as follows: both B. subtilis 168 and C. sporogenes PA3679 spores were triggered to germinate firstly by high pressure, which was enhanced by increased temperature, then the germinated spores were inactivated by heat.

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.

Effects of High Pressure on Bacillus licheniformis Spore Germination and Inactivation

Bacillus and Clostridium species form spores, which pose a challenge to the food industry due to their ubiquitous nature and extreme resistance. Pressurization at <300 MPa triggers spore germination by activating germination receptors (GRs), while pressurization at >300 MPa likely triggers germination by opening dipicolinic acid (DPA) channels present in the inner membrane of the spores. In this work, we expose spores of Bacillus licheniformis, a species associated with food spoilage and occasionally with food poisoning, to high pressure (HP) for holding times of up to 2 h. By using mutant spores lacking one or several GRs, we dissect the roles of the GerA, Ynd, and GerK GRs in moderately HP (mHP; 150 MPa)-induced spore germination. We show that Ynd alone is sufficient for efficient mHP-induced spore germination. GerK also triggers germination with mHP, although at a reduced germination rate compared to that of Ynd. GerA stimulates mHP-induced germination but only in the presence of either the intact GerK or Ynd GR. These results suggests that the effectiveness of the individual GRs in mHP-induced germination differs from their effectiveness in nutrient-induced germination, where GerA plays an essential role. In contrast to Bacillus subtilis spores, treatment with very HP (vHP) of 550 MPa at 37°C did not promote effective germination of B. licheniformis spores. However, treatment with vHP in combination with elevated temperatures (60°C) gave a synergistic effect on spore germination and inactivation. Together, these results provide novel insights into how HP affects B. licheniformis spore germination and inactivation and the role of individual GRs in this process.IMPORTANCE Bacterial spores are inherently resistant to food-processing regimes, such as high-temperature short-time pasteurization, and may therefore compromise food durability and safety. The induction of spore germination facilitates subsequent inactivation by gentler processing conditions that maintain the sensory and nutritional qualities of the food. High-pressure (HP) processing is a nonthermal food-processing technology used to eliminate microbes from food. The application of this technology for spore eradication in the food industry requires a better understanding of how HP affects the spores of different bacterial species. The present study provides novel insights into how HP affects Bacillus licheniformis spores, a species associated with food spoilage and occasionally food poisoning. We describe the roles of different germination receptors in HP-induced germination and the effects of two different pressure levels on the germination and inactivation of spores. This study will potentially contribute to the effort to implement HP technology for spore inactivation in the food industry.

The Synergistic Effect of High Pressure CO2 and Nisin on Inactivation of Bacillus subtilis Spores in Aqueous Solutions

The inactivation effects of high pressure CO₂ + nisin (simultaneous treatment of HPCD and nisin, HPCD + nisin), HPCD→nisin (HPCD was followed by nisin), and nisin→HPCD (nisin was followed by HPCD) treatments on Bacillus subtilis spores in aqueous solutions were compared. The spores were treated by HPCD at 6.5 or 20 MPa, 84–86°C and 0–30 min, and the concentration of nisin was 0.02%. Treated spores were examined for the viability, the permeability of inner membrane (IM) using flow cytometry method and pyridine-2, 6-dicarboxylic acid (DPA) release, and structural damage by transmission electron microscopy. A synergistic effect of HPCD + nisin treatment on inactivation of the spores was found, and the inactivation efficiency of the spores was HPCD + nisin > HPCD→nisin or nisin→HPCD. Moreover, HPCD + nisin caused higher IM permeability and DPA release of the spores than HPCD. A possible action mode of nisin-enhanced inactivation of the spores was suggested as that HPCD firstly damaged the coat and cortex of spores, and nisin penetrated into and acted on the IM of spores, which increased the damage to the IM of spores, and resulted in higher inactivation of the spores.

Investigating the Inactivation Mechanism of Bacillus subtilis Spores by High Pressure CO2

The objective of this study was to investigate the inactivation mechanism of Bacillus subtilis spores by high pressure CO₂ (HPCD) processing. The spores of B. subtilis were subjected to heat at 0.1 MPa or HPCD at 6.5-20 MPa, and 64-86°C for 0-120 min. The germination, the permeability of inner membrane (IM) and cortex, the release of pyridine-2, 6-dicarboxylic acid (DPA), and changes in the morphological and internal structures of spores were investigated. The HPCD-treated spores did not lose heat resistance and their DPA release was lower than the inactivation, suggesting that spores did not germinate during HPCD. The flow cytometry analysis suggested that the permeability of the IM and cortex of HPCD-treated spores was increased. Furthermore, the DPA of the HPCD-treated spores were released in parallel with their inactivation and the fluorescence photomicrographs showed that these treated spores were stained by propidium iodide, ensuring that the permeability of IM of spores was increased by HPCD. The scanning electron microscopy photomicrographs showed that spores were crushed into debris or exhibited a hollowness on the surface, and the transmission electron microscopy photomicrographs exhibited an enlarged core, ruptured and indistinguishable IM and a loss of core materials in the HPCD-treated spores, indicating that HPCD damaged the structures of the spores. These findings suggested that HPCD inactivated B. subtilis spores by directly damaging the structure of the spores, rather than inducing germination of the spores.

Involvement of Coat Proteins in Bacillus subtilis Spore Germination in High-Salinity Environments

The germination of spore-forming bacteria in high-salinity environments is of applied interest for food microbiology and soil ecology. It has previously been shown that high salt concentrations detrimentally affect Bacillus subtilis spore germination, rendering this process slower and less efficient. The mechanistic details of these salt effects, however, remained obscure. Since initiation of nutrient germination first requires germinant passage through the spores' protective integuments, the aim of this study was to elucidate the role of the proteinaceous spore coat in germination in high-salinity environments. Spores lacking major layers of the coat due to chemical decoating or mutation germinated much worse in the presence of NaCl than untreated wild-type spores at comparable salinities. However, the absence of the crust, the absence of some individual nonmorphogenetic proteins, and the absence of either CwlJ or SleB had no or little effect on germination in high-salinity environments. Although the germination of spores lacking GerP (which is assumed to facilitate germinant flow through the coat) was generally less efficient than the germination of wild-type spores, the presence of up to 2.4 M NaCl enhanced the germination of these mutant spores. Interestingly, nutrient-independent germination by high pressure was also inhibited by NaCl. Taken together, these results suggest that (i) the coat has a protective function during germination in high-salinity environments; (ii) germination inhibition by NaCl is probably not exerted at the level of cortex hydrolysis, germinant accessibility, or germinant-receptor binding; and (iii) the most likely germination processes to be inhibited by NaCl are ion, Ca(2+)-dipicolinic acid, and water fluxes.

Carvacrol suppresses high pressure high temperature inactivation of Bacillus cereus spores

The inactivation of bacterial spores generally proceeds faster and at lower temperatures when heat treatments are conducted under high pressure, and high pressure high temperature (HPHT) processing is, therefore, receiving an increased interest from food processors. However, the mechanisms of spore inactivation by HPHT treatment are poorly understood, particularly at moderately elevated temperature. In the current work, we studied inactivation of the spores of Bacillus cereus F4430/73 by HPHT treatment for 5 min at 600MPa in the temperature range of 50-100°C, using temperature increments of 5°C. Additionally, we investigated the effect of the natural antimicrobial carvacrol on spore germination and inactivation under these conditions. Spore inactivation by HPHT was less than about 1 log unit at 50 to 70°C, but gradually increased at higher temperatures up to about 5 log units at 100°C. DPA release and loss of spore refractility in the spore population were higher at moderate (≤65°C) than at high (≥70°C) treatment temperatures, and we propose that moderate conditions induced the normal physiological pathway of spore germination resulting in fully hydrated spores, while at higher temperatures this pathway was suppressed and replaced by another mechanism of pressure-induced dipicolinic acid (DPA) release that results only in partial spore rehydration, probably because spore cortex hydrolysis is inhibited. Carvacrol strongly suppressed DPA release and spore rehydration during HPHT treatment at ≤65°C and also partly inhibited DPA release at ≥65°C. Concomitantly, HPHT spore inactivation was reduced by carvacrol at 65-90°C but unaffected at 95-100°C.