Bacterial Spores: Structure, Importance and examples of spore forming bacteria

Bacterial spores are highly resistant, dormant structures (i.e. no metabolic activity) formed in response to adverse environmental conditions. They help in the survival of the organisms during adverse environmental conditions; they do not have a role in reproduction.

Note: Spores of fungi have a reproductive role. 

Spore formation (sporulation) occurs when nutrients, such as sources of carbon and nitrogen are depleted.  Bacterial spores are highly resistant to

  • Heat
  • Dehydration
  • Radiation and
  • Chemicals.
Structure of Bacterial Spore
Structure of Bacterial Spore

An endospore is structurally and chemically more complex than the vegetative cell.  It contains more layers than vegetative cells. Resistance of Bacterial spore may be mediated by dipicolinic acid, a calcium ion chelator found only in spores.  When the favorable condition prevail, (i.e. availability of water, appropriate nutrients)  spores germination occurs which forms vegetative cells of pathogenic bacteria.

Following factors/constituents plays major role for the resistance of Bacterial Spore: 

  • Calcium dipicolinate in core
  • Keratin spore coat
  • New enzymes (i.e., dipicolinic acid synthetase, heat-resistant catalase)
  • Increases or decreases in other enzymes.

Spore of Clostridium botulinum

A mature endospore contains a complete set of the genetic material (DNA) from the vegetative cell, ribosomes and specialized enzymes.

The shape and the position of spores vary in different species and can be useful for classification and identification purposes. Endospores may be located in the middle of the bacterium (central), at the end of the bacterium (terminal) and near the end of the bacteria (subterminal) and may be spherical or elliptical.

Spores may be:

  1. Central or equatorial, giving the bacillus a spindle shape (eg. Clostridium bifermentans)
  2. Sub-terminal, the bacillus appearing Club shaped (eg. Clostridium perfringens)
  3. Oval and terminal, resembling a tennis racket (eg. Clostridium tertium)
  4. Spherical and terminal, giving a drumstick appearance (Clostridium tetani)

Mature endospores are released from the vegetative cell to become free endospores.  When the free endospores are placed in an environment that supports growth, the endospores will revert back to a vegetative cell in a process called germination.  It should be noted that unlike the process of binary fission observed with vegetative cells, endospore formation is not a reproductive process but a process of differentiation that provides the bacteria with a mechanism for survival.

Constituents of Bacterial Spores:

  1. Thick keratinlike coat
  2. Peptidoglycan
  3. Cell membrane
  4. A small amount of cytoplasm
  5. Very little water
  6. 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 (100OC) but are killed at 121OC. Medical supplies must be heated to 121oC for atleast 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.

  • Bacillus spp
  • Clostridium spp

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