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Short answer
Bacteria form spores through a developmental program called sporulation. In the best-studied case, Bacillus subtilis, a starving cell divides asymmetrically into a large mother cell and a small forespore. The mother cell engulfs the forespore, builds a cortex and a protein coat around it, and finally lyses to release a dehydrated, dormant endospore.
Definition and scope
A bacterial endospore is a metabolically dormant cell made inside another cell. It is not a way of multiplying: the spore stays dormant until conditions suit germination, and then vegetative growth resumes. The protective layers and the dehydrated core give the spore resistance to environmental challenges, including antibiotics, desiccation and radiation (Khanna et al., 2020).
The program is found in the Firmicutes. Engulfment genes are present in most sporulating Bacillus and Clostridium species sequenced so far. Almost everything below describes B. subtilis, the model organism, so treat the stage details as the reference case rather than a universal template. For the staining side of the same organism, see our guide to the Bacillus subtilis Gram stain, which covers Gram versus endospore staining and is not repeated here. Fungal spores are a different topic, introduced in What Is Mycology?
What triggers spore formation
The broad signal for entry is limited nutrient availability, although the ligands that activate the sensor histidine kinases have been hard to identify (Tan and Ramamurthi, 2014). Several histidine kinases feed a phosphorelay that phosphorylates the master regulator Spo0A. The level of phosphorylated Spo0A (Spo0A~P) steers the fate of the cell: lower levels favor biofilm matrix production, higher levels favor sporulation.
Stages of endospore formation
The table maps each stage to what happens and to the compartment that controls it. The two compartments run on different sigma factors, which are activated in a set order.
| Stage | What happens | Compartment and control |
|---|---|---|
| 0 to I | Cell commits to the program; duplicated chromosomes form an axial filament, anchored at the cell poles by RacA | Whole cell; Spo0A~P |
| II | Asymmetric septation gives a larger mother cell and a smaller forespore | σF active in the forespore, σE in the mother cell |
| III | The mother cell engulfs the forespore, which ends up enclosed in two membranes | σG active in the forespore only after engulfment; σK (made as a precursor and processed) in the mother cell |
| IV to V | A cortex of specialized peptidoglycan forms between the two forespore membranes, and a coat of about 70 different proteins assembles | Mother cell; σK upregulates cortex-synthesis (Mur) proteins |
| VI to VII | The spore matures into a dehydrated, dormant cell; the mother cell lyses and releases it | Mother cell lysis |
The commitment point
Sporulation is not irreversible at the start. Commitment becomes irreversible only after σF in the forespore and σE in the mother cell have been activated, which is why the biofilm versus sporulation choice at low and high Spo0A~P still matters up to that point.
What each layer protects against
Resistance is not one property. Tan and Ramamurthi attribute different protections to different structures:
- Coat: protects against enzymatic attack, with lysozyme named as the example.
- Cortex: protects against high temperature and helps maintain partial dehydration of the core.
- Dipicolinic acid (DPA) and mineralization: contribute to heat resistance and reduced water content. Maturation includes accumulation of calcium dipicolinate and partial dehydration of the forespore cytoplasm.
- Small acid-soluble proteins (SASPs): protect the spore DNA. Spores lacking them are more susceptible to UV irradiation and hydrogen peroxide.
Spore formation versus germination
Germination is the reverse trip. The cortex is hydrolyzed by the functionally redundant enzymes SleB and CwlJ, after which the spore resumes vegetative growth.
Where it matters for labs and clinics
The same properties that make a spore hard to kill explain why sterilization monitoring uses spores. The US CDC states that steam sterilization is checked with a biological indicator containing spores of Geobacillus stearothermophilus, a heat-resistant test organism (CDC, 2026). See Autoclave Validation: Using Biological and Chemical Indicators and the broader What Is an Autoclave? guide. The layer-by-layer view above also shows that the protections differ by layer: an enzyme, heat and a DNA-damaging agent each meet a different barrier. For how disinfection levels are classified, see High-Level Disinfection and the Spaulding Classification.
Common misunderstandings
- Spores are not reproduction. Sporulation produces a dormant cell, and growth resumes only after germination.
- It is not a generic stress response. The broad entry signal described in the literature is limited nutrient availability.
- Not all bacteria do it. The program is described in the Firmicutes, with Bacillus and Clostridium as well-studied sporulating groups.
- The mother cell does not survive. It lyses at the end of the program to release the spore.
FAQ
How do bacteria form spores?
Through sporulation: a starving cell divides asymmetrically into a mother cell and a forespore, the mother cell engulfs the forespore and builds a cortex and coat, and the mother cell then lyses to release the mature endospore (Tan and Ramamurthi, 2014).
What triggers spore formation in bacteria?
The broad signal is limited nutrient availability, relayed through a phosphorelay to the master regulator Spo0A (Tan and Ramamurthi, 2014).
Is sporulation a form of reproduction?
No. One cell makes one dormant spore, and the spore remains dormant until conditions are appropriate for germination, after which vegetative growth resumes (Khanna et al., 2020).
Which bacteria form endospores?
Endospore formation is described in the Firmicutes, including Bacillus and Clostridium species (Khanna et al., 2020).
Which layer protects a spore against what?
The coat protects against enzymes such as lysozyme, the cortex against high temperature, and small acid-soluble proteins protect the DNA (Tan and Ramamurthi, 2014).
Sources
- Tan IS, Ramamurthi KS. Spore formation in Bacillus subtilis. Environ Microbiol Rep, 2014. https://pmc.ncbi.nlm.nih.gov/articles/PMC4078662 (checked 2026-10-29).
- Khanna K, Lopez-Garrido J, Pogliano K. Shaping an Endospore: Architectural Transformations During Bacillus subtilis Sporulation. Annu Rev Microbiol, 2020. https://pmc.ncbi.nlm.nih.gov/articles/PMC7610358 (checked 2026-10-29).
- US Centers for Disease Control and Prevention. Steam Sterilization (Disinfection and Sterilization Guideline). https://www.cdc.gov/infection-control/hcp/disinfection-sterilization/steam-sterilization.html (checked 2026-10-29).
