ENDOSPORES

An endospore is a dormant, tough, and non reproductive structure produced by a small number of bacteria from the firmicute phylum. The primary function of most endospore is to ensure the survival of a bacterium through periods of environmental stress. They are therefore resistance to ultraviolet and gamma radiation, desiccation, lysozyme, temperature, starvation and chemical disinfectant. Endospores are commonly found in a soil and water, where they may survive for long period of time. Some bacteria produce exospores or cysts instead.

            Certain genera of gram positive bacteria such as Bacillus, Clostridium, Sporohalobactar, Anaerobacter and Heliobacterium, can form highly resistant, dormant structure called endospores. In almost all cases, one endospore is formed and this is not a reproductive process, although Anaerobacter can make up to seven endospores in a single cell. Endospores have a central core of cytoplasm containing DNA ribosome surrounded by a cortex layer and protected by an impermeable and rigid coat.

            Endospore show no detectable metabolism and can survive extreme physical and chemical stresses, such as high level of UV light, gamma radiation, detergent, disinfectant, heat, pressure and desiccation. In this dormant state, these organisms may remain viable for millions of years and endospores even allow bacteria to survive exposure to the vacuumed and radiation in space. Endospore forming bacteria can also cause disease: for e.g., anthrax can be contracted by the inhalation of Bacillus anthraces endospores, and contamination of deep puncture wounds with Clostridium tetani endospores cause tetanus.

Endospore formation: The process of forming and endospore is complex. The model organism used to study endospore formation is Bacillus subtilis. When a bacterium detects environmental condition are becoming unfavorable it may start the process of sporulation, which takes about eight hours. Key morphological changes in the process have been used as markers to define stages of development.

            As a cell begins the process of forming an endospores DNA is replicated, it divides a symmetrically and a membrane wall known as a spore septum begins to form between it and the rest of the cell (Stage II). This result in the creation of two compartments, the larger mother cell a and the smaller forespore (the plasma membrane of the cell surrounds this wall and pinches off to leave a double membrane around the DNA, and the developing structure is now known as a forespore). Calcium dipicolinate is incorporated in to the fore spore during this time. These two cells have different developmental fates.

            Next (stage III) the peptidoglycan in the septum is degraded and the forespore is engulfed by the mother cell, forming a cell within a cell.

            The activities of the mother cell and forespore lead to the synthesis of the endospore-specific compounds, formation of cortex and deposition of the cote (stage IV + V).

            This is a followed by the final dehydration and maturation of the endospore (stage VI+VII).

            Finally, the mother cell is destroyed in a programmed cell death, and the endospore is released in to the environment. The endospore will remain dormant until it senses the return of more favorable condition.

Structure

            In contrast to eukaryotic spore, which are produced by many eukaryotes for reproductive purpose, bacteria will produce a single endospore internally. The spore is often surrounded by thin covering known as exosporium (glycoprotein layer), which overlies the spore coat. The outer protein spore coat impermeable to many toxic molecules and may also contain enzyme that are involved in germination. Proper cortex formation is needed for dehydration of spore core, which aids to resistant to high temperature the cortex lies beneath the spore coat and consist of peptidoglycan. A germ cell wall resides under the cortex. This layer of peptidoglycan will become the cell wall of the bacterium after the endospore germinates. The inner membrane, under the germ cell wall, is a major permeability barrier against several potentially damaging chemical. The core wall lies beneath the cortex and surround the protoplast or core of the endospore. It exists in a very dehydrated state and houses the cell’s DNA, ribosome’s and large no of dipicolanic acid, but is metabolically inactive. Small acid-soluble proteins (SASPs) are also only found in endospore.

            Up to 15% of the dry weight of the endospore consists of calcium dipicolinate within the core, which is thought to stabilize the DNA. Dipicolanic acid cooled is responsible for the heat resistant of the spore, and calcium may aid in resistance to heat and oxidizing agents.

  Location

            The position of the endospore differs among bacterial species and is useful in identification. The main types within the cell are terminal, sub terminal and centrally placed endospores. Lateral endospores are seen occasionally.

            Example of bacteria having terminal endospore includes Clostridium tetani, the pathogen which causes the disease tetanus. Bacteria having centrally placed endospore include Bacillus cereus, and those having a sub terminal endospore include Bacillus subtilis. Some time the endospore can be so large the cell can be distended around the endospore; this is typical of clostridium tetani. In figure the position the endospore, (1, 4) central endospore, (2,3,5) terminal endospore, (6) lateral endospore.

            Visualizing endospores under the light microscope can be difficult due to the impermeability of the endospore wall to dyes and stains. While the rest of bacterial cell may stain, the endospore is left colorless. To combat this, a special stain technique called a Moeller stain is used. That allows the endospore to show up as red, while the rest of the cell stains blue. Another staining technique for endospores is the Schaffer-Fulton stain, which stains endospores green and bacterial bodies red.

Reactivation 

            Reactivation of the endospore occurs when condition are more favorable and involves the activation, germination, and outgrowth. Even if an endospore is located in plentiful nutrients, it may fail to germinate unless activation has taken place. This may be triggered by heating the endospore. Germination involves the dormant endospore starting metabolic activity and thus braking hibernation. It is commonly characterized by rupture or absorption of the spore coat, swelling of the endospore, an increase in metabolic activity, and loss of resistance to environmental stress. Out growth proceeds germination and involves the core of the endospore manufacturing new chemical components and existing the old spore coat to develop in to a fully functional vegetative bacterial cell, which can divide to produce more cells.

Endospore-forming bacteria

Example of Endospore-forming bacteria include the genera

·         Bacillus

·         Clostridium

·         Desulfotomacunum

·         Sporolactobacillus

·         Sporosarcina

·         Thermoactinomyces

            Endospores are resistant to most agents which would normally kill the vegetative cell they formed from household cleaning products generally have no effects, nor do most alcohols, quaternary ammonium compounds or detergents. Alkylating agents however, such as ethylene oxide are effective against endospores. Endospores can be destroyed by burning or autoclaving. Exposure to extreme heat for a long enough periods will generally have some effect, though many endospores can survive hours of boiling or cooking. Prolonged exposure to high energy radiation such as X rays and gamma rays will also kill most endospore.

Importance

1.      As a simplified model for cellular differentiation, the molecular details of endospore formation have been extensively studied in the model organism Bacillus subtilis.

2.      In understanding of gene expression, transcription factors, and the sigma factor sub unit of RNA polymerase. (Sigma factor: - A small protein that directs RNA polymerase to specific sites on the DNA to initiate gene expression.)