Bacterial Spores: Structure and Spore-Forming Bacteria

By Acharya Tankeshwar •  Updated: 05/21/22 •  4 min read

Bacterial spores are highly resistant, dormant structures (i.e. no metabolic activity) formed in response to adverse environmental conditions. When vegetative cells of certain bacteria such as Bacillus spp and Clostridium spp are subjected to environmental stresses such as nutrient deprivation, they produce metabolically inactive or dormant form-endospore. The formation of endospores circumvents the problems associated with environmental stress and ensures the survival of the organisms.

Note: Spores of fungi have a reproductive role. 

During unfavorable conditions (especially when carbon and nitrogen become unavailable) spore-forming bacilli form endospores. The size, shape, and location of endospores are particularly useful for identifying Clostridium, Bacillus, and related species.


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

Structure of the Bacterial Spore

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 spores may be mediated by dipicolinic acid, a calcium ion chelator found only in spores.  When the favorable condition prevails, (i.e. availability of water, appropriate nutrients)  spores germination occurs which forms vegetative cells of pathogenic bacteria.

The following factors/constituents play major roles in the resistance of bacterial spore: 

Spore of Clostridium botulinum

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

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 cells 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 keratin-like 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 SporesMedical 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.The only solution designated as sporicidal will kill spores.
Spores can survive for many years in soil and other inanimate objects.Wounds contaminated with soils can be infected with spores and cause diseases such as tetanus, and gas gangrene.
Spores do not exhibit measurable metabolic activity.Antibiotics are ineffective against spores.
Spores are formed only when nutrients are insufficient.Spores are not often found at the site of infection because nutrients are not limited.

Examples of spore-forming bacteria

Most endospore-forming bacteria are found in soil or aquatic environments.  However, some species of Bacillus and Clostridium have medical significance. Clostridium perfringens, C. botulinum (a potential agent of bioterrorism), and C. tetani are the causative agents of gas gangrene, botulism, and tetanus, respectively.  Bacillus anthracis and Bacillus cereus are the causative agents of anthrax and self-limiting food poisoning, respectively.

Positions of Bacterial Spores

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

Spore location
  1. Central endospores are located within the middle of the vegetative cell.  
  2. Terminal endospores are located at the end of the vegetative cell.  
  3. Sub-terminal endospores are located between the middle and the end of the cell.


  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)

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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5 responses to “Elek Test: Principle, Procedure, Results”

  1. Anonymous says:

    thank you very much

  2. haliru says:

    Good day, i have a presentation tomorrow which will include elek’s precipitation test, i want to know the significance of the serum added to the molten medium. Your immediate reply will be appreciated. Thanks

  3. India Doc says:

    Hi Mr Acharya. I had my exam yesterday and a photograph of Elek’s test was kept as a spotter. One of the question asked was what other bacteria (other than corynebacterium diphtheria of course) can produce diphtheria toxin? Any answers? Thanks

  4. Yeva says:

    Please inform where can I buy elek test for detecting toxin of corynebacterium diphthetriae?

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