The underlying mechanism of bacterial spore germination: An update review

Sporulation at reduced water activity impairs germination kinetics of Bacillus subtilis spores

While the effect of sporulation temperature on spore germination has been extensively studied, the effect of other relevant environmental factors such as water activity (aw) has been overlooked, despite the fact that sporulation niches with lower humidity than ideal culture media, such as soil, are common in nature. In this work, we characterized the germination kinetics of B. subtilis 168 spores produced at reduced aw (0.98) using either glycerol (Sgly spores) or NaCl (Ssalt spores) in comparison to spores prepared under optimal conditions (Scontrol spores, aw ~0.99) in a variety of nutrient and chemical stimuli, along with the effect of thermal activation. Spores produced at reduced aw showed impaired germination to varying degrees depending on the nutrient and solute used to depress aw. While Sgly spores exhibited germination defects in all the nutrients tested (a rich growth medium, L-alanine, L-valine, and the AGFK mixture) compared to Scontrol spores, Ssalt populations showed an impaired response to L-alanine and L-valine. These nutrient germination defects of the Sgly and Ssalt spores could not be reversed by heat activation. In addition, both populations displayed impaired germination in Ca-DPA, but increased germination rate in dodecylamine. The phenotypes of spores produced at reduced aw suggested plausible alterations in their coat properties. Using mutant spores with coat morphogenesis defects and a 4 kDa FITC-dextran probe permeability test, we could infer that the impaired germination of Ssalt spores in nutrients and Ca-DPA may involve alterations in the crust and/or outer coat, leading to reduced permeability.IMPORTANCEBacterial spores are causative agents of relevant zoonoses and foodborne diseases and are involved in food spoilage. Natural sporulation niches, such as soil, are exposed to a variety of changing environmental conditions, such as temperature and water activity (aw), which strongly influence the dynamics of spore germination. This work provides the first data on the effect of lowering the aw of the sporulation medium on spore germination kinetics, which, together with previous data on the effect of other environmental sporulation conditions and inter- and intraspecific variability, will help accurately predict germination and thus prevent the negative impacts of pathogenic and food spoiling spores. Furthermore, we inferred that alterations related to the coat are associated with impaired nutrient germination of spores produced at reduced aw with the addition of NaCl. These findings may help develop novel and efficient strategies to control germination or eradicate spores.

High-fertility sows reshape gut microbiota: the rise of serotonin-related bacteria and its impact on sustaining reproductive performance

BACKGROUND

Compelling evidence has established a strong link between the gut microbiota and host reproductive health. However, the specific regulatory roles of individual bacterial species on reproductive performance are not well-understood. In the present study, Jinhua sows with varying reproductive performances under the same diet and management conditions were selected to explore potential mechanisms on the intricate relationship between the gut microbiome and host reproductive performance using 16S rRNA sequencing, metagenomics and serum metabolomics.

RESULTS

Our findings revealed that the KEGG pathways for base excision repair and DNA replication were enriched, along with gene-level enhancements in spore formation, in sows with higher reproductive performance, indicating that the gut microbiome experiences stress. Further analysis showed a positive correlation between these changes and litter size, indicating that the host acts as a stressor, reshaping the microbiome. This adaptation allows the intestinal microbes in sows with high reproductive performance to enrich specific serotonin-related bacteria, such as Oxalobacter formigenes, Ruminococcus sp. CAG 382, Clostridium leptum, and Clostridium botulinum. Subsequently, the enriched microbiota may promote host serotonin production, which is positively correlated with reproductive performance in our study, known to regulate follicle survival and oocyte maturation.

CONCLUSION

Our study provides a theoretical basis for understanding the interactions between gut microbes and the host. It highlights new insights into reassembling gut microbiota in sows with higher litter sizes and the role of serotonin-related microbiota and serotonin in fertility.

Pathogen and Spoilage Microorganisms in Meat and Dairy Analogues: Occurrence and Control Strategies

Recent advances in the production of meat and dairy analogues and plant-based products have introduced new food safety challenges, as these foods are susceptible to contamination by both pathogens and spoilage microorganisms originating from raw materials and processing environments. In addition, plant-based analogues often exhibit physicochemical properties such as high water activity, near-neutral pH, and elevated protein and moisture content that favour the survival and growth of microorganisms, as evidenced by the detection of Salmonella sp., Listeria monocytogenes, and Enterobacteriaceae in commercial products. While extrusion and thermal processing effectively reduce microbial loads, these treatments may not inactivate all spores, including spore-forming bacteria like Bacillus cereus and Clostridium spp. Critical findings seem to highlight that some protein isolates are particularly prone to higher microbial contamination, and that outbreaks linked to pathogens such as Salmonella, L. monocytogenes, and E. coli have already occurred in plant-based analogues in Europe and North America. Here we discuss the microbiology and sources of microbial contamination of these products. In addition, we further discuss the integration of non-thermal technologies and biocontrol methods, such as protective cultures, bacteriocins, and bacteriophages, as promising approaches to enhance food safety while addressing clean-label demands.

Recent progress in proteins regulating the germination of Bacillus subtilis spores

Bacterial spores can remain dormant for years, but they maintain the ability to recommence life through a process termed germination. Although spore germination has been reviewed many times, recent work has provided novel conceptual and molecular understandings of this important process. By using Bacillus subtilis as a model organism, here we thoroughly describe the signal transduction pathway and events that lead to spore germination, incorporating the latest findings on transcription and translation that are likely detected during germination. Then, we comprehensively review the proteins associated with germination and their respective functions. Notably, the typical germinant receptor GerA and the SpoVAF/FigP complex have been newly established as channels for ions release at early stage of germination. Moreover, given that germination is also affected by spore quality, such as molecular cargo, we collect the data about the proteins regulating sporulation to affect spore quality. Specifically, RocG-mediated glutamate catabolism during sporulation to ensure spore quality; GerE-regulated coat protein expression, and CotH-modified coat protein by phosphorylation to ensure normal coat assembly; and RNase Y-degraded RNA in newly released spores to promote dormancy. The latest progress in our understanding of these germination proteins provides valuable insights into the mechanism underlying germination.

Newly identified SpoVAF/FigP complex: the role in Bacillus subtilis spore germination at moderate high pressure and influencing factors

The SpoVAF/FigP complex, a newly identified dormant spore ion channel, has been shown to amplify the response of germinant receptors (GRs) to nutrient germinants. However, its contribution to high-pressure-induced germination remains unexplored. In this study, we discovered that the 5AF/FigP complex played an important role in the GR-dependent germination of Bacillus subtilis spores under moderate high pressure (MHP) by facilitating the release of ions, such as potassium (K+), a mechanism in parallel with its role in nutrient-induced germination. Despite its predicted function as an ion channel, the 5AF/FigP complex failed to be activated by MHP in the absence of GerA-type GRs. We quantitatively examined the factors that influence the 5AF/FigP complex's function in MHP-induced germination using modeling and fitting techniques. Our results indicated that the complex's amplification effect was both enhanced and accelerated as pressure levels increase from 50 to 200 MPa. However, raising the MHP treatment temperature from 22°C to 30°C only speeded up the complex's function without enhancing its effectiveness. Moreover, extreme conditions of higher pressure (300 MPa) and temperature (34°C-37°C) could diminish the complex's functionality. Additionally, the amplification effect was weakened in spores produced at both elevated and reduced sporulation temperatures. Taken together, our findings highlight the essential role of the 5AF/FigP complex in boosting the efficiency of MHP-induced germination. This revelation has enriched our understanding of the intricate mechanisms underlying GR-dependent germination in Bacillus spores, offering valuable insights that can be utilized to refine the germination-inactivation strategies within the food industry.

IMPORTANCE

High-pressure-induced spore germination has been discovered for more than half a century, but the signal transduction pathway of the process still needs to be refined. In this study, for the first time, we revealed the role of the newly identified SpoVAF/FigP complex in high-pressure-induced spore germination, as well as the factors influencing its function in this process. The new findings in this work not only enhance the theoretical understanding of spore germination mechanisms under high pressure but also pave the way for developing novel strategies to inactivate spores during high-pressure food processing, a technology that is gaining popularity in the food industry as a promising non-thermal preservation method.

Inactivation effects of combined thermosonication and potassium sorbate treatments on Bacillus subtilis spores

This study aimed to investigate the inactivation effect of combined TS (thermosonication) and PS (potassium sorbate) treatments on Bacillus subtilis spores. The inactivation effect and potential mechanisms were examined using plate counts, OD600 values, nucleic acid leakage, DPA (dipicolinic acid) leakage, flow cytometry, and FTIR (Fourier transform infrared spectroscopy). The results showed that, after TS + PS treatments, the integrity of the inner membrane was lost, the permeability of the inner membrane to water molecules was increased, and the intraspore substances leaked. Furthermore, the OD600 value was reduced, indicating that the spore core hydration was enhanced. Spores proportion with damaged inner membrane was significantly increased to 66%, the ordered secondary structure of the protein was changed into a disordered structure and nucleic acid was fragmented after TS + PS treatment. The results indicated that the combined TS and PS treatments may be a useful method for inactivating bacterial spores in food processing and sterilization.

The Novel Concept of Synergically Combining: High Hydrostatic Pressure and Lytic Bacteriophages to Eliminate Vegetative and Spore-Forming Bacteria in Food Products

The article focuses on the ongoing challenge of eliminating vegetative and spore-forming bacteria from food products that exhibit resistance to the traditional preservation methods. In response to this need, the authors highlight an innovative approach based on the synergistic utilization of high-hydrostatic-pressure (HHP) and lytic bacteriophages. The article reviews the current research on the use of HHP and lytic bacteriophages to combat bacteria in food products. The scope includes a comprehensive review of the existing literature on bacterial cell damage following HHP application, aiming to elucidate the synergistic effects of these technologies. Through this in-depth analysis, the article aims to contribute to a deeper understanding of how these innovative techniques can improve food safety and quality. There is no available research on the use of HHP and bacteriophages in the elimination of spore-forming bacteria; however, an important role of the synergistic effect of HHP and lytic bacteriophages with the appropriate adjustment of the parameters has been demonstrated in the more effective elimination of non-spore-forming bacteria from food products. This suggests that, when using this approach in the case of spore-forming bacteria, there is a high chance of the effective inactivation of this biological threat.

Characteristic substance analysis and rapid detection of bacteria spores in cooked meat products by surface enhanced Raman scattering based on Ag@AuNP array substrate

BACKGROUND

Bacterial spores are the main potential hazard in medium- and high-temperature sterilized meat products, and their germination and subsequent reproduction and metabolism can lead to food spoilage. Moreover, the spores of some species pose a health and safety threat to consumers. The rapid detection, prevention, and control of bacterial spores has always been a scientific problem and a major challenge for the medium and high-temperature meat industry. Early and sensitive identification of spores in meat products is a decisive factor in contributing to consumer health and safety.

RESULTS

In this study, we developed a novel and stable Ag@AuNP array substrate by using a two-step synthesis approach and a liquid-interface self-assembly method that can directly detect bacterial spores in actual meat product samples without the need for additional in vitro bacterial culture. The results indicate that the Ag@AuNP array substrate exhibits high reproducibility and Raman enhancement effects (1.35 × 105). The differentiation in the Surface enhanced Raman scattering (SERS) spectra of five bacterial spores primarily arises from proteins in the spore coat and inner membrane, peptidoglycan of cortex, and Ca2⁺-DPA within the spore core. The correct recognition rate of linear discriminant analysis for spores in the meat product matrix can reach 100 %. The average recovery accuracy of the SERS quantitative model was at around 101.77 %, and the limit of detection can reach below 10 CFU/mL.

SIGNIFICANCE

It provides a promising technological strategy for the characteristic substance analysis and timely monitoring of spores in meat products.

High pressure processing plus technologies: Enhancing the inactivation of vegetative microorganisms

High pressure processing (HPP) is a non-thermal technology that can ensure microbial safety without compromising food quality. However, the presence of pressure-resistant sub-populations, the revival of sub-lethally injured (SLI) cells, and the resuscitation of viable but non-culturable (VBNC) cells pose challenges for its further development. The combination of HPP with other methods such as moderate temperatures, low pH, and natural antimicrobials (e.g., bacteriocins, lactate, reuterin, endolysin, lactoferrin, lactoperoxidase system, chitosan, essential oils) or other non-thermal processes (e.g., CO2, UV-TiO2 photocatalysis, ultrasound, pulsed electric fields, ultrafiltration) offers feasible alternatives to enhance microbial inactivation, termed as "HPP plus" technologies. These combinations can effectively eliminate pressure-resistant sub-populations, reduce SLI or VBNC cell populations, and inhibit their revival or resuscitation. This review provides an updated overview of microbial inactivation by "HPP plus" technologies and elucidates possible inactivation mechanisms.

Characterization of individual spores of two biological insecticides, Bacillus thuringiensis and Lysinibacillus sphaericus, in response to glutaraldehyde using single-cell optical approaches

Bacillus thuringiensis (Bt) and Lysinibacillus sphaericus (Ls) are the most widely used microbial insecticides. Both encounter unfavorable environmental factors and pesticides in the field. Here, the responses of Bt and Ls spores to glutaraldehyde were characterized using Raman spectroscopy and differential interference contrast imaging at the single-cell level. Bt spores were more sensitive to glutaraldehyde than Ls spores under prolonged exposure: <1.0% of Bt spores were viable after 10 min of 0.5% (v/v) glutaraldehyde treatment, compared to ~ 20% of Ls spores. The Raman spectra of glutaraldehyde-treated Bt and Ls spores were almost identical to those of untreated spores; however, the germination process of individual spores was significantly altered. The time to onset of germination, the period of rapid Ca2+-2,6-pyridinedicarboxylic acid (CaDPA) release, and the period of cortex hydrolysis of treated Bt spores were significantly longer than those of untreated spores, with dodecylamine germination being particularly affected. Similarly, the germination of treated Ls spores was significantly prolonged, although the prolongation was less than that of Bt spores. Although the interiors of Bt and Ls spores were undamaged and CaDPA did not leak, proteins and structures involved in spore germination could be severely damaged, resulting in slower and significantly prolonged germination. This study provides insights into the impact of glutaraldehyde on bacterial spores at the single cell level and the variability in spore response to glutaraldehyde across species and populations.

Inactivation Effect of Germination Combined with Cold Plasma Treatment on Bacillus licheniformis Spores

Food spoilage, primarily caused by spore-forming bacteria, has become a critical concern since it results in substantial economic losses within the food industry. Past investigations have successfully identified Bacillus licheniformis as the main bacterium responsible for spoilage in roast chicken. In this study, we screened a new sterilization combination from 16 germinants and 4 cold plasma conditions, respectively. Among them, the combination of "A"GFNa-1 (composed of 60 mmol/L L-alanine, 10 mmol/L D-glucose, 10 mmol/L D-fructose, and 1 g/L NaCl) with cold plasma treatment (packed with 100% argon at 70 kV) proved effective in deactivating B. licheniformis spores, resulting in a reduction of approximately 2.1 log CFU/mL. Furthermore, we exposed the spores to different conditions: CK (no germination, no cold plasma), MF (germination only), CP (no germination, 100% argon packed, 70 kV cold plasma treatment for 3 min), and MF + CP (germination for 5 h, 100% argon packed, 70 kV cold plasma treatment for 3 min). The results of heat inactivation and dipicolinic acid (DPA) release rate demonstrated that cold plasma treatment effectively inactivated both spores and vegetative cells without inducing germination. Additionally, the reduced survival under hyperosmotic conditions and the presence of distinct red fluorescence patterns observed through confocal laser scanning microscopy (CLSM) collectively suggest that cold plasma treatment disrupts the inner membrane structure and leads to the inactivation of B. licheniformis. Overall, our findings indicate a spore clearance rate of 99.2% and suggest that the combination of efficient germinants and cold plasma treatment holds promise as a viable approach to mitigate spore contamination in the food industry.

Bacillus spore germination: mechanisms, identification, and antibacterial strategies

Bacterial spores are metabolically inactive and highly resistant to harsh environmental conditions in nature and during decontamination processes in food and related industries. However, inducing germination using specific germinants in dormant spores can convert them into vegetative cells which are metabolically active and fragile. The potential utility of a "germinate to eradicate" strategy, also known as germination-inactivation, has been validated in foods. Meanwhile, the strategy has sparked much interest in triggering and maximizing spore germination. Although many details of the spore germination process have been identified over the past decades, there remain many uncertainties, including some signal transduction mechanisms involved in germination. In addition, the successful implementation of the germination-inactivation strategy relies on the sensitive detection of germinative biomarkers within minutes of germination initiation and the optimal timing for the subsequent inactivation step. Meanwhile, the emergence of biomarkers has renewed attention to the practical application of the spore germination process. Here, this review presents the current knowledge of the germination mechanisms of Bacillus spore, influencing factors, and germination biomarkers. It also covers a detailed discussion on the development of germination-inactivation as a spore eradication strategy.