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Microbiology · Morphology and Classification of Microbes

Bacterial spores

Endospores

Bacterial spores are a special resting form, predominantly of Gram-positive microorganisms, designed to survive under extreme conditions. It is important to remember that sporulation in bacteria is not a method of reproduction: only one spore is formed from a single vegetative cell.

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FunctionPreservation of the species in an unfavorable environment, rather than reproduction.
SurvivalCapable of persisting in the soil for decades.
ThermotoleranceProvided by the presence of calcium dipicolinate.
StainingAcid-fast, staining red by the Ziehl–Neelsen method.
4 min readFor medical studentsUpdated 09/24/2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesReviewed by medical editor: Aulov A. A., general practitioner · 09/24/2026

Nature and purpose of bacterial spores

Spores are formed inside the mother cell (which is why they are called endospores) in response to adverse external factors. Desiccation, nutrient deficiency, or ultraviolet irradiation can serve as triggers for this process.

The main biological meaning of sporulation is species preservation. In the spore state, the microorganism enters cryptobiosis: metabolic processes slow down dramatically, and the water content drops to a minimum. Due to their multi-layered coat, spores can survive for decades, creating reservoirs of infection in the environment (for example, in soil for the causative agents of tetanus and anthrax).

Morphology: bacilli and clostridia

Based on the size of the spore relative to the vegetative cell and changes in its shape, bacteria are divided into two groups:

  • Bacilli (using the genus Bacillus as an example). The size of the forming spore does not exceed the width of the cell itself, so the shape of the bacterium does not change.
  • Clostridia. The diameter of the spore is greater than the thickness of the vegetative cell. Because of this, the cell deforms and becomes spindle-shaped (from Lat. Clostridium — spindle).

An important species characteristic that helps in identifying bacteria is the shape of the spore (oval or spherical) and its location inside the cell:

  1. Central — typical of the anthrax pathogen (Bacillus anthracis).
  2. Subterminal (closer to the end of the rod) — characteristic of the causative agents of gas gangrene and botulism.
  3. Terminal (at the very end of the rod) — a distinguishing feature of the tetanus pathogen (Clostridium tetani).
Test your knowledge on this topicAssignments from the Microbiology course bank — instant answers and explanations.

Stages of sporulation

Sporulation is a complex process of isolating genetic material and constructing protective coats.

  1. Initial stage. A part of the cytoplasm along with the chromosome is sequestered inside the cell. The cytoplasmic membrane invaginates, surrounding this region, and forms a forespore.
  2. Coating formation. The forespore is covered by two cytoplasmic membranes. The cortex — a thick layer of modified peptidoglycan — forms between them. On the inside, the cortex is adjacent to the cell wall of the future spore, and on the outside, it borders the inner coat. Then the vegetative cell forms the outer coat, and in some species, yet another external layer is formed — the exosporium.
  3. Biochemical rearrangement. The forespore actively accumulates calcium ions and dipicolinic acid, forming calcium dipicolinate. It is this compound that provides the phenomenal thermotolerance of bacterial spores.

Spore germination

When a spore enters favorable conditions, it returns to active life. Exactly one vegetative cell always emerges from a single spore. The germination process takes place in three stages:

  • Activation. The spore enters a state of readiness. Temperature shock (heating to 60–80 °C) often serves as the trigger.
  • Initiation. The direct start of germination, which takes only a few minutes.
  • Outgrowth. Rapid growth begins, the spore coat breaks down, and a fully formed vegetative bacterium (germling) emerges.

Briefly: answers to topical questions

What structural layers does a mature bacterial spore consist of?

A mature bacterial spore consists of several structural layers surrounding its internal contents (nucleoid, ribosomes, and enzymes).

  • Exosporium — an additional outer layer present in some species.
  • Outer coat — a layer formed by the vegetative cell.
  • Inner spore coat — a layer bordering the cortex on the outside.
  • Cortex — a thick layer of modified peptidoglycan formed between two cytoplasmic membranes.
  • Core wall (spore cell wall) — the inner layer bordering the cortex on the inside.
How to memorize

To remember spore locations, use associations: CENTRAL — anthrax (center of attention), SUBTERMINAL — Botulism and Gas gangrene (SBG), TERMINAL — tetanus (TETAnus).

Class and exam questions

Why is sporulation in bacteria not considered a method of reproduction?

Unlike fungi, whose spores serve to increase the number of individuals, in bacteria, exactly one spore is formed from a single mother cell. Upon germination, it also gives rise to only one new cell, so there is no increase in the number of bacteria.

How do clostridia differ from bacilli?

Clostridia form a spore whose diameter exceeds the width of the bacterium itself, causing the cell to swell and look like a spindle. In bacilli, the spore size is smaller than or equal to the cell diameter, so its shape is preserved.

What provides high resistance of spores to temperature?

The main factor of thermotolerance is the accumulation of calcium dipicolinate in the spore during sporulation. Survival is also promoted by the multi-layered coat, low water content, and the state of cryptobiosis.

What methods are used to stain spores for microscopy?

Spores are acid-fast, so they are detected using special methods — by Aujeszky or Ziehl–Neelsen. As a result, the spore stains red, and the vegetative part of the cell stains blue.

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What else to study on this topic

  • Structure and functions of the vegetative cell
  • Ziehl–Neelsen staining technique: steps and reagents
  • Biochemical role of dipicolinic acid in detail
  • Ecological features of clostridia and bacilli in soil
  • Mechanisms of germination initiation at the molecular level

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