Bacterial Spores

Structure, Importance, Spore Formers

Last updated on June 21st, 2021

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. Formation of endospores circumvent 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

  • Heat
  • Dehydration
  • Radiation and
  • Chemicals.

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.

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.

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 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.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

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 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)

About Acharya Tankeshwar 476 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.