SpoVAF and FigP assemble into oligomeric ion channels that enhance spore germination

The Functional Network of PrkC and Its Interaction Proteins in Bacillus subtilis Spores

In the food industry, food spoilage caused by spores is a pressing scientific challenge that needs to be addressed urgently, and spore germination is a key approach to solving this problem. Studies have shown that peptidoglycan-induced spore germination represents a novel mechanism of action, which can bind to the PASTA domain of the serine/threonine kinase PrkC. However, the signaling mechanism of peptidoglycan-induced spore germination remains unclear. This study focuses on Bacillus subtilis, using pull-down experiments to screen for proteins interacting with PrkC. There are 80 interaction proteins of PrkC that were identified in the spore. GO analysis reveals that PrkC-interacting proteins in the spore are mainly involved in metabolic processes, cell part and catalysis. KEGG results indicate that PrkC-interacting proteins in the spore are mainly involved in RNA degradation, quorum sensing, oxidative phosphorylation, etc. Additionally, proteins are categorized into six groups by function based on events that may be associated with post-germination triggered by peptidoglycan-induced activation of the PrkC signaling pathway, including "stimulate translation initiation" and "ATP synthesis and energy metabolism". The experimental results provide a theoretical basis for further elucidating the signaling mechanism of PrkC, revealing the signaling pathway of peptidoglycan-induced spore germination, and identifying targeted inducers and repressors of spore germination.

Germination of Bacillus spores by LiCl

Spores of Bacillus subtilis have been found to germinate when incubated with LiCl, but not with other monovalent or divalent metal cations. Bacillus megaterium spores also germinated with LiCl, but B. cereus spores did not. In B. subtilis, the LiCl germination was via the activation of spores' GerA germinant receptor (GR), and in B. megaterium, it was the GerU GR. Notably, LiCl germination was much slower than normal physiological germinant triggered GR germination. In B. subtilis spores, rates of LiCl germination were increased in spores with a more fluid IM and decreased in spores with a less fluid IM. Analyses of the GerA germinant binding site suggested that Li+ could bind in a specific site in the B. subtilis GerAB subunit where normally a Na+ likely binds. Importantly, NaCl strongly inhibited LiCl germination of B. subtilis spores, much more so than the larger cation in KCl, although neither salt inhibited L-alanine germination via the GerA GR. These findings increase the understanding of features of mechanisms of germination of Bacillus spores.IMPORTANCEThe ability of some bacteria to form spores upon nutrient starvation confers properties of metabolic dormancy and enhanced resistance to environmental stressors that would otherwise kill vegetative cells. Since spore-forming bacteria include several notable pathogens and economically significant spoilage organisms, insight into how spores are stimulated to germinate and form new vegetative cells is important. Here, we reveal that relatively high concentrations of the inorganic salt lithium chloride trigger the germination of Bacillus subtilis and Bacillus megaterium spores by stimulating one of the spores of each species cohort of nutrient germinant receptors. This is significant since novel germinants and increased knowledge of the germination process should provide opportunities for improved control of spores in healthcare, food, and environmental sectors.

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.

Identification and characterization of the Bacillus subtilis spore germination protein GerY

In response to starvation, endospore-forming bacteria differentiate into stress-resistant spores that can remain dormant for years yet rapidly germinate and resume growth when nutrients become available. To identify uncharacterized factors involved in the exit from dormancy, we performed a transposon-sequencing screen taking advantage of the loss of spore heat resistance that accompanies germination. We reasoned that transposon insertions that impair but do not block germination will lose resistance more slowly than wild type after exposure to nutrients and will therefore survive heat treatment. Using this approach, we identified most of the known germination genes and several new ones. We report an initial characterization of 15 of these genes and a more detailed analysis of one (ymaF). Spores lacking ymaF (renamed gerY) are impaired in germination in response to both L-alanine and L-asparagine, D-glucose, D-fructose, and K+. GerY is a soluble protein synthesized under σE control in the mother cell. A YFP-GerY fusion localizes around the developing and mature spore in a manner that depends on CotE and SafA, indicating that it is a component of the spore coat. Coat proteins encoded by the gerP operon and gerT are also required for efficient germination, and we show that spores lacking two or all three of these loci have more severe defects in the exit from dormancy. Our data are consistent with a model in which GerY, GerT, and the GerP proteins are required for efficient transit of nutrients through the coat to access the germination receptors, but each acts independently in this process.

IMPORTANCE

Pathogens in the orders Bacillales and Clostridiales resist sterilization by differentiating into stress-resistant spores. Spores are metabolically inactive and can remain dormant for decades, yet upon exposure to nutrients, they rapidly resume growth, causing food spoilage, food-borne illness, or life-threatening disease. The exit from dormancy, called germination, is a key target in combating these important pathogens. Here, we report a high-throughput genetic screen using transposon sequencing to identify novel germination factors that ensure the efficient exit from dormancy. We identify several new factors and characterize one in greater detail. This factor, renamed GerY, is part of the proteinaceous coat that encapsulates the dormant spore. Our data suggest that GerY enables efficient transit of nutrients through the coat to trigger germination.

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.

Spore germination: Two ion channels are better than one

Germination is the process by which spores emerge from dormancy. Although spores can remain dormant for decades, the study of germination is an active field of research. In this issue of Genes & Development, Gao and colleagues (pp. 31-45) address a perplexing question: How can a dormant spore initiate germination in response to environmental cues? Three distinct complexes are involved: GerA, a germinant-gated ion channel; 5AF/FigP, a second ion channel required for amplification; and SpoVA, a channel for dipicolinic acid (DPA). DPA release is followed by rehydration of the spore core, thus allowing the resumption of metabolic activity.