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Germination of Bacterial Spore

In this germination of bacterial spore post we have briefly explained about bacterial spore formation, stages of sporulation, germination of bacterial spore, demonstration endospores, and uses of bacterial spore formation.

Bacterial Spore Formation

A variety of gram-positive bacteria, including those from the genera Clostridium and Bacillus, can form a special resistant dormant structure known as an endospore, or simply spores. 

Endospores form when vital nutrients are depleted. During sporulation, each vegetative cell produces only one spore, and each spore produces a single vegetative cell during subsequent germination. Sporulation in bacteria is thus a method of preservation rather than reproduction.

Sporulation

Bacterial spore formation, sporogenesis or sporulation normally commences when growth ceases due to lack of nutrients, depletion of the nitrogen or carbon source (or both) being the most significant factor. New antigens appear on sporulation that is not found in the vegetative cell.

Stages of Sporulation

Stages of bacterial spore formation: Because it is a complicated process, it may be divided into several stages.

bacterial spores

Bacterial spore formation: The stages of endospore formation

Spore septum

In the first observable stage of sporulation, a newly replicated bacterial chromosome and a small portion of cytoplasm are isolated by an ingrowth of the plasma membrane called a spore septum.

Forespore

The spore septum becomes a double layered membrane that surrounds the chromosome and cytoplasm. Structure, entirely enclosed within the original cell, is called a forespore.

Spore coat

The forespore is subsequently completely encircled by dividing septum as a double layered membrane. 

The two spore membranes now engage in active synthesis of various layers of the spore. The inner layer becomes the inner membrane. 

Between the two layers is laid spore cortex and outer layer is transformed into spore coat which consists of several layers. 

In some species from outer layer also develops exosporium which bears ridges and folds.

Free endospore

Free endospore: Finally exosporium disintegrates and the endospore is freed.

Properties of Endospores

Core

The fully developed spore has the core which is the spore protoplast containing the normal cell structures but is metabolically inactive.

Spore wall

The innermost layer surrounding the inner spore membrane is called the spore wall. It contains normal peptidoglycan and becomes the cell wall of the germinating vegetative cell.

Bacterial Spores

Bacterial Spore Formation: Bacterial spores (cross-section)

Cortex

The cortex is the thickest layer of the spore envelope. Cortex peptidoglycan is extremely sensitive to lysozyme, and its autolysins play a role in spore germination.

Spore coat

The spore coat is the thick layer found beneath the perispore of some eukaryotic spores and bacterial mature spores. Cortex, in turn, is enclosed by fairly thick spore coat.

Exosporium

Spores of some species have an additional; apparently rather loose covering known as the exosporium, which may have distinctive ridges and grooves.

Germination of Bacterial Spore

Germination is the process of conversion of a spore into vegetative cells under suitable conditions. It occurs in three stages: activation, initiation and outgrowth. 

Once activated, a spore will initiate germination if the environmental conditions are favourable. The spore loses its refractility and swells. 

The spore wall is shed and the germ cell appears by rupturing the spore coat. The germ cell elongates to form the vegetative bacterium.

Shape and Position of Spores

Bacterial Spores

Types of Bacterial Spore Formation: 1. Central, bulging, 2. Central, not bulging, 3. Subterminal, bulging, 4. Subterminal, not bulging, 5. Terminal, spherical, 6. Terminal, oval

The shape and position of the spore and its size relative to the parent cell are species characteristics. Spores may be central (equatorial), subterminal (close to one end), or terminal. 

The appearance may be spherical, ovoid or elongated, and being narrower than the cell, or broader and bulging it. 

The diameter of spore may be same or less than the width of bacteria (Bacillus), or may be wider than the bacillary body producing a distension or bulge in the cell (Clostridium).

Resistance

Resistance Bacterial spores constitute some of the most resistant forms of life. They may remain viable for centuries. 

Though some spores may resist boiling for prolonged periods, spores of all medically important species are destroyed by autoclaving at 120°C for 15 minutes. 

Methods of sterilization and disinfection should ensure that spores also are destroyed. Sporulation helps bacteria survive for long periods under unfavourable environments.

Endospore heat resistance probably is due to several factors: calcium-dipicolinate and acid-soluble protein stabilization of DNA, protoplast dehydration, the spore coat, DNA repair, the greater stability of the cell proteins in bacteria adapted to growth at high temperatures and others.

Demonstration Endospores

Demonstration Endospores can be examined with both light and electron microscope. In unstained preparations, the spore is recognized within the parent cell by its greater retractability.

Gram staining

Spores appear as an unstained refractile body within the cell. When mature, the spore resists coloration by simple stains, appearing as a clear space within the stained cell protoplasm.

Ziehl-Neelsen staining

Spores are slightly acid-fast and may be stained differentially by a modification of the Ziehl-Neelsen method. Ziehl-Neelsen staining with 0.25-0.5 percent sulfuric acid (instead of 20 percent sulfuric acid as used in conventional method) as decolouring agent is used for spore staining

Uses of Bacterial Spore Formation

1. Importance in Industry: Endospores are of great practical importance in food, industrial, and medical microbiology because of their resistance and the fact that several species of endospore forming bacteria are dangerous pathogens. Endospore often survives boiling for an hour or more. Therefore, autoclaves must be used to sterilize many materials. 

2. Sterilization control: For proper sterilization, spores of certain species of bacteria are employed as indicator, e.g. Bacillus stearothermophilus which is destroyed at a temperature of 121°C for 10 to 20 minutes (same temperature and time as used in autoclaving). Prior to its use, these spores may be kept in autoclave. Proper sterilization is indicated by the absence of the spores after autoclaving. 

3. Research: bacterial spore formation is well suited for research on the construction of complex biological structures. 

Further Readings

Reference