endospore

An endospore is a dormant, highly resistant structure formed by some Gram-positive bacteria as a survival strategy. Endospores are not reproductive but enable the cell to endure extreme heat, desiccation, chemicals, radiation, and nutrient limitation. They are most notably produced by Bacillus and Clostridium species.

Structure: The core contains the bacterial DNA, ribosomes, and essential cellular components, and is unusually dehydrated. Surrounding the core are a peptidoglycan-rich cortex and a multilayered protein coat; some species have an outer layer called the exosporium. The core accumulates dipicolinic acid and calcium, which contribute to dehydration and heat resistance. Small acid-soluble proteins protect DNA during dormancy and stress.

Formation and germination: Sporulation is triggered by nutrient deprivation or environmental stress and involves asymmetric cell division to form a forespore that is engulfed by the mother cell. Layers such as the cortex and coat are assembled, followed by maturation and lysis of the mother cell to release the mature endospore. In favorable conditions, spores germinate: nutrients initiate rehydration, coat and cortex degrade, and the cell returns to vegetative growth.

Ecological and practical significance: Endospores play a key role in soil ecology and environmental persistence. Their extreme resistance makes them difficult to eradicate with standard disinfection or heating, which has implications for medical, food, and industrial settings. Endospores are used as biological indicators to validate sterilization processes. Pathogenic spore-formers, including certain Bacillus and Clostridium species, pose health and safety concerns due to their persistence and potential to cause disease.

Detection and study: In laboratories, endospores are identified by spore staining and microscopy; molecular and biochemical methods are employed to characterize sporulation stages and specific species.