Endospore

An endospore is a dormant, highly resistant form that certain bacteria make inside the cell when conditions turn harsh, especially when nutrients run out. Spore formers are chiefly members of Bacillus and Clostridium, two groups of Gram-positive bacteria. An endospore is not a way of reproducing, since one cell gives rise to one spore; instead it keeps the cell's DNA alive through hard times.

During sporulation, the cell copies its chromosome and walls one copy off in a smaller compartment, the forespore. The mother cell then wraps the forespore in layers: a second membrane, a thick cortex of peptidoglycan, and a tough protein coat. The core inside is very dry and rich in calcium dipicolinate, and special proteins bind and shield the DNA. With no measurable metabolism, the spore can survive boiling, drying, ultraviolet light and many disinfectants, sometimes for very long periods. When conditions improve, it germinates back into an active, growing cell.

Because endospores outlast boiling, killing them on lab and medical equipment usually takes pressurized steam in an autoclave. Spore-forming pathogens cause anthrax, tetanus, botulism and gas gangrene, and Clostridioides difficile, a common cause of hospital infections, also spreads as spores.