Effect of the osmotic conditions during sporulation on the subsequent resistance of bacterial spores

Effects of Na+ adaptation on Bacillus cereus endospores inactivation and transcriptome changes

As Bacillus cereus endospores exist in various vegetables grown in soil, the possibility of contamination in food products with high salt concentrations cannot be ignored. Recent studies revealed that harsh conditions affect the resistance of bacteria; thus, we investigated the developmental aspect of heat resistance of B. cereus after sporulation with high NaCl concentration. RNA sequencing was conducted for transcriptomic changes when B. cereus endospores formed at high salinity, and membrane fluidity and hydrophobicity were measured to verify the transcriptomic analysis. Our data showed that increasing NaCl concentration in sporulation media led to a decrease in heat resistance. Also, endospore hydrophobicity, membrane fluidity, and endospore density decreased with sporulation at higher NaCl concentrations. When the transcript changes of B. cereus sporulated at NaCl concentrations of 0.5 and 7% were analyzed by transcriptome analysis, it was confirmed that the NaCl 7% endospores had significantly lower expression levels (FDR<0.05) of genes related to sporulation stages 3 and 4, which led to a decrease in expression of spore-related genes such as coat proteins and small acid-soluble proteins. Our findings indicated that high NaCl concentrations inhibited sporulation stages 3 and 4, thereby preventing proper cell maturation in the forespores and adequate formation of the coat protein and cortex. This inhibition led to decreased endospore density and hydrophobicity, ultimately resulting in reduced heat resistance.resistanceWe expect that this study will be utilized as a baseline for further studies and enhance sterilization strategies.

Role of water activity on sporulation traits and resistance to 915 MHz microwave in the emetic type of Bacillus cereus on rice

The objective of this study is to investigate the influence of water activity on the sporulation of emetic strains of Bacillus cereus and the subsequent susceptibility of sporulated B. cereus to 915 MHz microwave treatment. Water activity levels were manipulated in the sporulation medium by adjusting glycerol concentrations to 0 %, 3 %, 7.5 %, and 10 %, resulting in corresponding water activities of 0.996, 0.981, 0.971, and 0.960, respectively and sporulated at 30℃. These changes in water activity were intended to simulate the variations in water activity that can occur during the cultivation and processing of rice, where B. cereus is commonly found. Sporulation rates increased with higher water activity, achieving over 90 % of total cells after 3, 5, 5, and 6 days of incubation at water activities of 0.996, 0.981, 0.971, and 0.960, respectively. Resistance to microwave treatment increased with higher water activity levels during sporulation. Microwave treatment for 5 min yielded a reduction of 3.03 log CFU/g at a water activity of 0.960 and 1.98 log CFU/g at 0.996 during sporulation. Lower water activity led to higher % dipicolinic acid (DPA) release during microwave treatment, as measured using a spectrometer, indicating greater membrane damage. To investigate the morphology of spores, transmission electron microscopy was used. Spores produced at lower water activity levels were observed to be enveloped by the mother cell, whereas those sporulated at higher water activity levels were distinct from the mother cell. Additionally, spores sporulated at higher water activity levels exhibited higher wet density in Percoll gradient. The levels of germination were also compared after sporulation under different water activity medium using L-alanine. The spores evolved under low water activity exhibited higher germination level compared to those from high water activity conditions. These findings suggested that water activity conditions during sporulation induce variations in morphology, density, germination level, and resistance to 915 MHz microwave. Our experimental results indicated that, even with the same sporulation percentage, there can be differences in the maturation stage, indicating that the level of 915 MHz microwave treatment should be adjusted according to the sporulation conditions of B. cereus.

Pilot-scale production of Bacillus subtilis MSCL 897 spore biomass and antifungal secondary metabolites in a low-cost medium

OBJECTIVES

Bacillus subtilis is a plant growth promoting bacterium (PGPB) that acts as a microbial fertilizer and biocontrol agent, providing benefits such as boosting crop productivity and improving nutrient content. It is able to produce secondary metabolites and endospores simultaneously, enhancing its ability to survive in unfavorable conditions and eliminate competing microorganisms. Optimizing cultivation methods to produce B. subtilis MSCL 897 spores on an industrial scale, requires a suitable medium, typically made from food industry by-products, and optimal temperature and pH levels to achieve high vegetative cell and spore densities with maximum productivity.

RESULTS

This research demonstrates successful pilot-scale (100 L bioreactor) production of a biocontrol agent B. subtilis with good spore yields (1.5 × 109 spores mL-1) and a high degree of sporulation (>80%) using a low-cost cultivation medium. Culture samples showed excellent antifungal activity (1.6-2.3 cm) against several phytopathogenic fungi. An improved methodology for inoculum preparation was investigated to ensure an optimal seed culture state prior to inoculation, promoting process batch-to-batch repeatability. Increasing the molasses concentration in the medium and operating the process in fed-batch mode with additional molasses feed, did not improve the overall spore yield, hence, process operation in batch mode with 10 g molasses L-1 is preferred. Results also showed that the product quality was not significantly impacted for up to 12 months of storage at room temperature.

CONCLUSION

An economically-feasible process for B. subtilis-based biocontrol agent production was successfully developed at the pilot scale.

Endospore production of Bacillus spp. for industrial use

The increased occurrence of antibiotic resistance and the harmful use of pesticides are a major problem of modern times. A ban on the use of antibiotics as growth promoters in animal breeding has put a focus on the probiotics market. Probiotic food supplements are versatile and show promising results in animal and human nutrition. Chemical pesticides can be substituted by biopesticides, which are very effective against various pests in plants due to increased research. What these fields have in common is the use of spore-forming bacteria. The endospore-forming Bacillus spp. belonging to this group offer an effective solution to the aforementioned problems. Therefore, the biotechnological production of sufficient qualities of such endospores has become an innovative and financially viable field of research. In this review, the production of different Bacillus spp. endospores will be reviewed. For this purpose, the media compositions, cultivation conditions and bioprocess optimization methods of the last 20 years are presented and reflected.

Bioprocess development for endospore production by Bacillus coagulans using an optimized chemically defined medium

Bacillus coagulans is a promising probiotic, because it combines probiotic properties of Lactobacillus and the ability of Bacillus to form endospores. Due to this hybrid relationship, cultivation of this organism is challenging. As the probiotics market continues to grow, there is a new focus on the production of these microorganisms. In this work, a strain-specific bioprocess for B. coagulans was developed to support growth on one hand and ensure sporulation on the other hand. This circumstance is not trivial, since these two metabolic states are contrary. The developed bioprocess uses a modified chemically defined medium which was further investigated in a one-factor-at-a-time assay after adaptation. A transfer from the shake flask to the bioreactor was successfully demonstrated in the scope of this work. The investigated process parameters included temperature, agitation and pH-control. Especially the pH-control improved the sporulation in the bioreactor when compared to shake flasks. The bioprocess resulted in a sporulation efficiency of 80%-90%. This corresponds to a sevenfold increase in sporulation efficiency due to a transfer to the bioreactor with pH-control. Additionally, a design of experiment (DoE) was conducted to test the robustness of the bioprocess. This experiment validated the beforementioned sporulation efficiency for the developed bioprocess. Afterwards the bioprocess was then scaled up from a 1 L scale to a 10 L bioreactor scale. A comparable sporulation efficiency of 80% as in the small scale was achieved. The developed bioprocess facilitates the upscaling and application to an industrial scale, and can thus help meet the increasing market for probiotics.

Moderate high-pressure superdormancy in Bacillus spores: properties of superdormant spores and proteins potentially influencing moderate high-pressure germination

Resistant bacterial spores are a major concern in industrial decontamination processes. An approach known as pressure-mediated germination-inactivation strategy aims to artificially germinate spores by isostatic pressure to mitigate their resistance to inactivation processes. The successful implementation of such a germination-inactivation strategy relies on the germination of all spores. However, germination is heterogeneous, with some “superdormant” spores germinating extremely slowly or not at all. The present study investigated potential underlying reasons for moderate high-pressure (150 MPa; 37°C) superdormancy of Bacillus subtilis spores. The water and dipicolinic acid content of superdormant spores was compared with that of the initial dormant spore population. The results suggest that water and dipicolinic acid content are not major drivers of moderate high-pressure superdormancy. A proteomic analysis was used to identify proteins that were quantified at significantly different levels in superdormant spores. Subsequent validation of the germination capacity of deletion mutants revealed that the presence of protein YhcN is required for efficient moderate high-pressure germination and that proteins MinC, cse60, and SspK may also play a role, albeit a minor one.

IMPORTANCE Spore-forming bacteria are ubiquitous in nature and, as a consequence, inevitably enter the food chain or other processing environments. Their presence can lead to significant spoilage or safety-related issues. Intensive treatment is usually required to inactivate them; however, this treatment harms important product quality attributes. A pressure-mediated germination-inactivation approach can balance the need for effective spore inactivation and retention of sensitive ingredients. However, superdormant spores are the bottleneck preventing the successful and safe implementation of such a strategy. An in-depth understanding of moderate high-pressure germination and the underlying causes of superdormancy is necessary to advance the development of mild high pressure-based spore control technologies. The approach used in this work allowed the identification of proteins that have not yet been associated with reduced germination at moderate high pressure. This research paves the way for further studies on the germination and superdormancy mechanisms in spores, assisting the development of mild spore inactivation strategies.

Optimization and comparison of ℽ-aminobutyric acid (GABA) production by LAB in soymilk using RSM and ANN models

Background

ℽ-Aminobutyric acid (GABA) is a non-proteinaceous amino acid. In the mammalian nervous system, GABA functions as an inhibitory neurotransmitter. The present study focused on screening and optimization of ℽ-aminobutyric acid (GABA) yield by lactic acid bacteria by using soymilk as basal media. Lactobacillus fermentum (Lb. fermentum) was isolated from sourdough. The qualitative confirmation of GABA production by Lb. fermentum was observed by detecting colored spots on thin layer chromatography plate (TLC) and comparing it with standard GABA spot. The GABA from bacteria is confirmed by its molecular mass using mass spectrophotometry analysis (MS analysis). Single variable experiments were conducted for various physical and nutritional parameters, and determined the GABA content produced from Lb. fermentum, viable bacterial count, and pH of the fermented soymilk medium. Experimental data were authenticated by using response surface method (RSM) and artificial neural network (ANN) model.

Results

The results demonstrated that through single variable experiments, the yield of GABA and the viable bacterial cells increased in soymilk containing one percent of glucose, monosodium glutamate (MSG), and inoculum volume incubated at 37 °C, 48 h at pH 5. According to RSM results, the interaction of the highest concentration of MSG (1.5%) and mid glucose concentration (1.156%) yielded maximum GABA (5.54 g/L). The experimental data were in good agreement with two optimization models. The RSM models showed less error percentage than that of the ANN model.

Conclusion

This study indicates that soymilk is the best basal substrate for GABA production and growth of Lb. fermentum compared to synthetic media. Lb. fermentum can be explored further by food and pharmaceutical industries for the development of functional foods and therapeutic purposes.

Understanding the Effects of High Pressure on Bacterial Spores Using Synchrotron Infrared Spectroscopy

Bacterial spores are extremely resistant life-forms that play an important role in food spoilage and foodborne disease. The return of spores to a vegetative cell state is a three-step process, these being activation, germination, and emergence. High-pressure (HP) processing is known to induce germination in part of the spore population and even to inactivate a high number of Bacillus spores when combined with other mild treatments such as the addition of nisin. The aim of the present work was to investigate the mechanisms involved in the sensitization of spores to nisin following HP treatment at ambient temperature or with moderate heating leading to a heterogeneous spore response. Bacillus subtilis spores were subjected to HP treatment at 500 MPa at 20 and 50°C. The physiological state of different subpopulations was characterized. Then Fourier transform infrared (FTIR) microspectroscopy coupled to a synchrotron infrared source was used to explore the heterogeneity of the biochemical signatures of the spores after the same HP treatments. Our results confirm that HP at 50°C induces the germination of a large proportion of the spore population. HP treatment at 20°C generated a subpopulation of ungerminated spores reversibly sensitized to the presence of nisin in their growth medium. Regarding infrared spectra of individual spores, spores treated by HP at 50°C and germinated spores had similar spectral signatures involving the same structural properties. However, after HP was performed at 20°C, two groups of spores were distinguished; one of these groups was clearly identified as germinated spores. The second group displayed a unique spectral signature, with shifts in the spectral bands corresponding to changes in membrane fluidity. Besides, spores spectra in the amide region could be divided into several groups close to spectral properties of dormant, germinated, or inactivated spores. The part of the spectra corresponding to α-helix and β-sheet-structures contribute mainly to the spectral variation between spores treated by HP at 20°C and other populations. These changes in the lipid and amide regions could be the signature of reversible changes linked to spore activation.

Effect of cultivation pH on the surface hydrophobicity of Bacillus subtilis spores

Bacillus subtilis spores are often used as biological indicators (BI) to monitor decontamination processes with gaseous hydrogen peroxide. Results in practical inactivation validation tests, however, vary considerably with no available explanation so far. This study reports on the effect of cultivation pH on spore surface hydrophobicity. Surface hydrophobicity is suspected to have an impact on the decontamination of technical surfaces such as packaging material when gaseous, condensing hydrogen peroxide is applied. It is the aim of this study to examine the impact of different cultivation pH levels on surface hydrophobicity and resistance of B. subtilis spores. Submersed cultivation of B. subtilis in bioreactors at controlled conditions with different static pH levels led to contact angles ranged between 50° and 80°, which was analyzed with water on a homogeneous layer of spores on a filter sheet. Resistance of spores was also affected by the cultivation pH. The results show that the culturing conditions during BI production should be controlled to obtain BI with specified characteristics in inactivation validation tests.

Accumulation of γ-aminobutyric acid by Enterococcus avium 9184 in scallop solution in a two-stage fermentation strategy

In this study, a new bacterial strain having a high ability to produce γ‐aminobutyric acid (GABA) was isolated from naturally fermented scallop solution and was identified as Enterococcus avium. To the best of our knowledge, this is the first study to prove that E. avium possesses glutamate decarboxylase activity. The strain was then mutagenized with UV radiation and was designated as E. avium 9184. Scallop solution was used as the culture medium to produce GABA. A two‐stage fermentation strategy was applied to accumulate GABA. In the first stage, cell growth was regulated. Optimum conditions for cell growth were pH, 6.5; temperature, 37°C; and glucose concentration, 10 g·L⁻¹. This produced a maximum dry cell mass of 2.10 g·L⁻¹. In the second stage, GABA formation was regulated. GABA concentration reached 3.71 g·L⁻¹ at 96 h pH 6.0, 37°C and initial l‐monosodium glutamate concentration of 10 g·L⁻¹. Thus, compared with traditional one‐stage fermentation, the two‐stage fermentation significantly increased GABA accumulation. These results provide preliminary data to produce GABA using E. avium and also provide a new approach to process and utilize shellfish.

Vulnerability of Bacillus spores and of related genera to physical impaction injury with particular reference to spread-plating

AIMS

To examine whether bacterial spores are vulnerable to impaction injury during standard spread-plating or to other modes of physical impaction.

METHODS AND RESULTS

Employing heat-challenged spores of Bacillus pumilus, Bacillus subtilis, Bacillus thuringiensis, Lysinibacillus, Paenibacillus and Brevibacillus spp. from day-4 to day-10 nutrient agar (NA) plates in 50% ethanol, plating the spore suspension to the extent of just drying the agar surface on fresh NA (50-60 s; SP-B) was tested in comparison with the spreader-independent approach of spotting-and-tilt-spreading (SATS), or a brief plating (<10 s; SP-A). Spore CFU was significantly reduced with SP-B in different organisms (23-40%) over SATS independent of the spore size. Comparing 4-, 7- and 10-day-old B. pumilus spores, the former two displayed significant CFU reduction in SP-B indicating a spore age-related effect. Continuous plating for 2-5 min showed a reduction in spore CFU in all organisms depending on plating duration. CFU reduction effect with SP-B was less manifest on refrigerated plates where no friction was experienced but acute on prewarmed and surface-dried plates. Spreader movement over agar surface subsequent to the exhaustion of free moisture proved highly detrimental to spores. A simulated plating study by plating the spores over a plastic film till drying showed a significant reduction in spore CFU. DAPI staining and glass bead-vortexing studies confirmed spore disruption through physical impaction.

CONCLUSIONS

Bacterial spores are vulnerable to injury during spread-plating or with other forms of physical impaction with variable effects on different genotypes independent of the spore size but altered by spore age.

SIGNIFICANCE AND IMPACT OF THE STUDY

Implications during spore CFU estimations employing spread-plating and during spore surveillance, and the recommendation of SATS as an easier and safer alternative for spore CFU enumeration.

Life cycle and spore resistance of spore-forming Bacillus atrophaeus

Bacillus endospores have a wide variety of important medical and industrial applications. This is an overview of the fundamental aspects of the life cycle, spore structure and factors that influence the spore resistance of spore-forming Bacillus. Bacillus atrophaeus was used as reference microorganism for this review because their spores are widely used to study spore resistance and morphology. Understanding the mechanisms involved in the cell cycle and spore survival is important for developing strategies for spore killing; producing highly resistant spores for biodefense, food and pharmaceutical applications; and developing new bioactive molecules and methods for spore surface display.

Glycerol-based sterilization bioindicator system from Bacillus atrophaeus: development, performance evaluation, and cost analysis

The development of new value-added applications for glycerol is of worldwide interest because of the environmental and economic problems that may be caused by an excess of glycerol generated from biodiesel production. A novel use of glycerol as a major substrate for production of a low-cost sterilization biological indicator system (BIS; spores on a carrier plus a recovery medium) was investigated. A sequential experimental design strategy was applied for product development and optimization. The proposed recovery medium enables germination and outgrowth of heat-damaged spores, promoting a D (160 °C) value of 6.6 ± 0.1 min. Bacillus atrophaeus spores production by solid-state fermentation reached a 2.3 ± 1.2 × 10(8) CFU/g dry matter. Sporulation kinetics results allowed this process to be restricted in 48 h. Germination kinetics demonstrated the visual identification of nonsterile BIS within 24 h. Performance evaluation of the proposed BIS against dry-heat and ethylene oxide sterilization showed compliance with the regulatory requirements. Cost breakdowns were from 41.8 (quality control) up to 72.8 % (feedstock). This is the first report on sterilization BIS production that uses glycerol as a sole carbon source, with significant cost reduction and the profitable use of a biodiesel byproduct.

Sporulation boundaries and spore formation kinetics of Bacillus spp. as a function of temperature, pH and a(w)

Sporulation niches in the food chain are considered as a source of hazard and are not clearly identified. Determining the sporulation environmental boundaries could contribute to identify potential sporulation niches. Spore formation was determined in a Sporulation Mineral Buffer. The effect of incubation temperature, pH and water activity on time to one spore per mL, maximum sporulation rate and final spore concentration was investigated for a Bacillus weihenstephanensis and a Bacillus licheniformis strain. Sporulation boundaries of B. weihenstephanensis and of B. licheniformis were similar to, or included within, the range of temperatures, pH and water activities supporting growth. For instance, sporulation boundaries of B. weihenstephanensis were evaluated at 5°C, 35°C, pH 5.2 and a(w) 0.960 while growth boundaries were observed at 5°C, 37°C, pH 4.9 and a(w) 0.950. Optimum spore formation was determined at 30°C pH 7.2 for B. weihenstephanensis and at 45°C pH 7.2 for B. licheniformis. Lower temperatures and pH delayed the sporulation process. For instance, the time to one spore per mL was tenfold longer when sporulation occurred at 10°C and 20°C, for each strain respectively, than at optimum sporulation temperature. The relative effect of temperature and pH on sporulation rates and on growth rates is similar. This work suggests that the influence of environmental factors on the quantitative changes in sporulation boundaries and rates was similar to their influence on changes in growth rate.

High gas pressure: an innovative method for the inactivation of dried bacterial spores

In this article, an original non-thermal process to inactivate dehydrated bacterial spores is described. The use of gases such as nitrogen or argon as transmission media under high isostatic pressure led to an inactivation of over 2 logs CFU/g of Bacillus subtilis spores at 430 MPa, room temperature, for a 1 min treatment. A major requirement for the effectiveness of the process resided in the highly dehydrated state of the spores. Only a water activity below 0.3 led to substantial inactivation. The solubility of the gas in the lipid components of the spore and its diffusion properties was essential to inactivation. The main phenomenon involved seems to be the sorption of the gas under pressure by the spores' structures such as residual pores and plasma membranes, followed by a sudden drop in pressure. Observation by phase-contrast microscopy suggests that internal structures have been affected by the treatment. Some parallels with polymer permeability to gas and rigidity at various water activities offer a few clues about the behavior of the outer layers of spores in response to this parameter and provide a good explanation for the sensitivity of spores to high gas pressure discharge at low hydration levels. Specificity of microorganisms such as size, organization, and composition could help in understanding the differences between spores and yeast regarding the parameters required for inactivation, such as pressure or maintenance time.

Effects of sporulation conditions on the germination and germination protein levels of Bacillus subtilis spores

Bacillus subtilis spores prepared in rich medium germinated faster with nutrient germinants than poor-medium spores as populations in liquid and multiple individual spores on a microscope slide. Poor-medium spores had longer average lag times between mixing of spores with nutrient germinants and initiation of Ca-dipicolinic acid (CaDPA) release. Rich-medium spores made at 37°C germinated slightly faster with nutrient germinants than 23°C spores in liquid, but not when spores germinated on a slide. The difference in germination characteristics of these spore populations in liquid was paralleled by changes in expression levels of a transcriptional lacZ fusion to the gerA operon, encoding a germinant receptor (GR). Levels of GR subunits were 3- to 8-fold lower in poor-medium spores than rich-medium spores and 1.6- to 2-fold lower in 23°C spores than 37°C spores, and levels of the auxiliary germination protein GerD were 3.5- to 4-fold lower in poor medium and 23°C spores. In contrast, levels of another likely germination protein, SpoVAD, were similar in all these spores. These different spores germinated similarly with CaDPA, and poor-medium and 23°C spores germinated faster than rich-medium and 37°C spores, respectively, with dodecylamine. Since spore germination with CaDPA and dodecylamine does not require GerD or GRs, these results indicate that determinants of rates of nutrient germination of spores prepared differently are primarily the levels of the GRs that bind nutrient germinants and trigger germination and secondarily the levels of GerD.

Evaluation of a stochastic inactivation model for heat-activated spores of Bacillus spp

Heat activates the dormant spores of certain Bacillus spp., which is reflected in the "activation shoulder" in their survival curves. At the same time, heat also inactivates the already active and just activated spores, as well as those still dormant. A stochastic model based on progressively changing probabilities of activation and inactivation can describe this phenomenon. The model is presented in a fully probabilistic discrete form for individual and small groups of spores and as a semicontinuous deterministic model for large spore populations. The same underlying algorithm applies to both isothermal and dynamic heat treatments. Its construction does not require the assumption of the activation and inactivation kinetics or knowledge of their biophysical and biochemical mechanisms. A simplified version of the semicontinuous model was used to simulate survival curves with the activation shoulder that are reminiscent of experimental curves reported in the literature. The model is not intended to replace current models to predict dynamic inactivation but only to offer a conceptual alternative to their interpretation. Nevertheless, by linking the survival curve's shape to probabilities of events at the individual spore level, the model explains, and can be used to simulate, the irregular activation and survival patterns of individual and small groups of spores, which might be involved in food poisoning and spoilage.

Germination and outgrowth of spores of Bacillus cereus group members: diversity and role of germinant receptors

Bacillus cereus is a gram-positive, facultative anaerobic, endospore-forming toxicogenic human pathogen. Endospores are highly specialized, metabolically dormant cell types that are resistant to extreme environmental conditions, including heat, dehydration and other physical stresses. B. cereus can enter a range of environments, and can in its spore form, survive harsh conditions. If these conditions become favorable, spores can germinate and grow out and reach considerable numbers in a range of environments including processed foods. Certainly the last decade, when consumer preferences have shifted to mildly processed food, new opportunities arose for spore-forming spoilage and pathogenic organisms. Only rigorous methods have been shown to be capable of destroying all spores present in food, thus a shift toward e.g., milder heat preservation strategies, may result in low but significant amounts of viable spores in food products. Hence, the need for a mild spore destruction strategy is eminent including control of spore outgrowth. Consequently, there is a large interest in triggering spore germination in foodstuffs, since germinated spores have lost the extreme resistance of dormant spores and are relatively easy to kill. Another option could be to prevent germination so that no dangerous levels can be reached. This contribution will focus on germination and outgrowth characteristics of B. cereus and other members of the B. cereus group, providing an overview of the niches these spore-formers can occupy, the signals that trigger germination, and how B. cereus copes with these wake-up calls in different environments including foods, during food processing and upon interaction with the human host.