Germination of Bacillus Spores

Abstract

In response to starvation, metabolically active vegetative cells of Bacillus species differentiate into specialised, highly resistant dormant cells called spores. Spores can survive almost any environmental assault, including UV radiation, heat, organic chemicals and degradative enzymes such as lysozyme. This spore resistance is derived from a series of concentrically arranged protective structures. In response to the presence of nutrients, spores can break their dormancy and resume metabolic activity through a process called germination. Germination can be broken down into a series of stages or steps: commitment, stage I, stage II and outgrowth. The process begins when germinants bind to germinant receptors. In response, monovalent ions are released and dipiclonic acid chelating calcium (Ca‐DPA) leaks from the core. The release of Ca‐DPA from the core stimulates the degradation of the cortex, which sheds the coat layers and results in core rehydration then outgrowth. After germination, the new cell resumes vegetative growth.

Key Concepts

  • Bacillus species have a survival mechanism called sporulation that is a developmental process, which results in a metabolically inactive cell (spore).
  • Spores can persist in nutrient‐poor situations for very long periods due to their ability to resist very harsh environmental conditions that usually damage living cells.
  • When spores sense nutrients they transition back to metabolically active and growing cells, by a process called germination.
  • Spores have specific receptors (germinant receptors) to sense the return of nutrients (amino acids, nucleosides and sugars).
  • Differences in germination receptors and nutrient stimulus exist among Bacillus species.
  • Bacillus spores can also germinate in response to nonnutrient triggers.
  • Key changes in the spore during germination decrease overall resistance.
  • Ca‐DPA facilitates the dehydration of the spore core and activates CwlJ for cortex hydrolysis.

Keywords: Bacillus; cortex hydrolysis; dipicolinic acid (DPA); germination; spores; commitment; germinant; outgrowth; germinant receptors

Figure 1. Key players involved in the germination of Bacillus spores and general structure of layers (sizes not to scale). The central core contains the Ca‐DPA. Location of the germination receptors and Ca‐DPA/ion channels is the inner membrane (IM). The SleB/YpeB proteins are shown in both the inner and outer cortex and CwlJ/GerQ is in the outer cortex. GerP proteins are in the coat to assist germinants. The outermost layer, the exosporium, is not shown. Setlow . Reproduced with permission of Elsevier.
Figure 2. Proposed model of germination receptor pathways in Bacillus anthracis. For each germinant combination, the germination receptors required are shown in the boxes. GerS or GerX can be utilised for the various combinations within the AAID‐1 pathway. The combination of alanine and the purine inosine can utilise both AAID‐1 pathways (Fisher and Hanna, ). Fisher and Hanna . Adapted with permission of American Society for Microbiology.
Figure 3. Overview of the major events of nutrient‐mediated Bacillus spore germination. The spore is first activated by heat or an unknown process. The spore commits to germination after germinant addition as it releases monovalent ions and leaks Ca‐DPA from the core. Stage I consists of rapid release of Ca‐DPA and subsequent hydration of the core. In stage II SleB and CwlJ have become activated and hydrolyse the cortex, allowing the core to expand and hydrate fully. During outgrowth, the hydrated spore begins to elongate and resumes metabolic activity resembling a vegetative cell. Hydration and resistance scales are generalised as the precise measurements of resistance loss and water content have not been thoroughly determined. Resistance scale does represent transition from phase bright to phase dark spore under phase‐contrast microscopy. Setlow . Reproduced with permission of American Society for Microbiology.
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Further Reading

Abee T, Groot MN, Tempelaars M, et al. (2011) Germination and outgrowth of spores of Bacillus cereus group members: diversity and role of germinant receptors. Food Microbiology 28 (2): 199–208.

Abhyankar WR, Wen J, Swarge BN, et al. (2019) Proteomics and microscopy tools for the study of antimicrobial resistance and germination mechanisms of bacterial spores. Food Microbiology 81: 89–96.

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Roser, Andrew J, and Giorno, Rebecca(Mar 2020) Germination of Bacillus Spores. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028938]