Bacterial Spores: Structure, Importance and examples of spore forming bacteria
Bacterial spores are latent, highly resistant structures (that is, without metabolic activity) that form in response to adverse environmental conditions. They help the survival of organisms in adverse environmental conditions; they have no role in reproduction.
Spores of fungi have a reproductive role.
Spore formation (sporulation) occurs when nutrients, such as carbon and nitrogen sources, are depleted. Bacterial spores are highly resistant to
- Heat
- Dehydration
- Radiation and
- Chemicals.
An endospore is structurally and chemically more complex than the vegetative cell. It contains more layers than vegetative cells. Resistance of bacterial spores may be mediated by dipicolinic acid, a calcium ion chelator found only in spores. When favorable conditions prevail (i.e. availability of water, adequate nutrients), the germination of spores that form vegetative cells of pathogenic bacteria occurs.
The following factors / constituents play an important role in the resistance of bacterial spores:
- Calcium dipicolinate in core
- Keratin spore coat
- New enzymes (i.e., dipicolinic acid synthetase, heat-resistant catalase)
- Increases or decreases in other enzymes.
A mature endospore contains a complete set of vegetative cell genetic material (DNA), ribosomes, and specialized enzymes.
The shape and position of the spores vary in different species and can be useful for classification and identification purposes. Endospores can be located in the middle of the bacteria (central), at the end of the bacteria (terminal), and near the end of the bacteria (subterminal) and can be spherical or elliptical.
Spores may be:
- Central or equatorial, giving the bacillus a spindle shape (eg. Clostridium bifermentans)
- Sub-terminal, the bacillus appearing Club shaped (eg. Clostridium perfringens)
- Oval and terminal, resembling a tennis racket (eg. Clostridium tertium)
- Spherical and terminal, giving a drumstick appearance (Clostridium tetani)
The mature endospores are released from the vegetative cell to become free endospores. When free endospores are placed in an environment that encourages growth, the endospores will revert to being a vegetative cell in a process called germination. It should be noted that, unlike the binary fission process observed with vegetative cells, the formation of endospores is not a reproductive process but rather a differentiation process that provides bacteria with a survival mechanism.
Constituents of Bacterial Spores:
- Thick keratinlike coat
- Peptidoglycan
- Cell membrane
- A small amount of cytoplasm
- Very little water
- Bacterial DNA
Medical Importance of Bacterial Spores
| Important features of Spores | Medical Implications |
|---|---|
| Spores are highly resistant to heating; spores are not killed by boiling (100°C) but are killed at 121°C. | Medical supplies must be heated to 121°C for at least 15 minutes to be sterilized. |
| Spores are highly resistant to many chemicals, including most disinfectants. | Only solution designated as sporicidal will kill spores. |
| Spores can survive for many years in soil and other inanimate objects. | Wound contaminated with soils can be infected with spores and cause diseases such as tetanus, gas gangrene. |
| Spores do not exhibit measurable metabolic activity. | Antibiotics are ineffective against spores. |
| Spores formed only when nutrients are insufficient. | Spores are not often found at the site of infection because nutrients are not limiting. |
Examples of Spore forming Bacteria- Spores formed by only two genera of Gram-positive rods are of medical importance.
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