Some bacteria, notably those of the genera Bacillus and Clostridium, develop a highly resistant resting phase or endospore that does not grow or reproduce, and exhibits absolute dormancy.
A single vegetative bacterium forms a single spore by a process called sporulation. A single vegetative bacterium emerges from a spore during germination.
Sporulation
Sporulation is triggered by the onset of unfavorable environmental conditions e.g. depletion of nutrients, accumulation of metabolites or changes in the growth requirements (e.g. moisture, temperature, pH, or oxygen tension).
The cytoplasmic membrane invaginates enclosing a section of the cytoplasm that contains the bacterial chromosome, some ribosomes and other cytoplasmic materials that will be needed for germination. It acquires a thick cortex and a thin but tough outer spore coat.
Viability and resistance
Spores are much more resistant to disinfectants, drying and heating. Moist heat at 121°C for 10-20 minutes is needed to kill spores while 60°C suffices to kill vegetative forms. The marked resistance of the spores has been attributed to several factors:
- Thermal resistance is provided by their high content of Calcium and dipicolinic acid (a compound unique to endospores).
- The impermeability of their cortex and outer coat.
- Their low content of water.
- Their very low metabolic and enzymatic activity.
Germination
Endospores respond quickly to environmental changes returning to the vegetative state within 15 min. In the process of germination, the spores absorb water and swell, the protective coat disintegrates and a single vegetative cell emerges.
Morphology
- Staining: Using Gram's stain, the spore remains uncoloured and can be seen as a clear area within the stained cell. The spores can be stained using special procedures.
- The position (Fig. 1): In relation to the body of the bacillus, the spore may be central, terminal or subterminal.
- The shape: The spores may be oval or rounded.
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| Figure 1: Position of bacterial spores |
The position and shape of spores are characteristic of the species and may help in the microscopic identification of the bacterium.