Endospores: surviving hard times

An endospore is not merely a thick-walled cell; it is a cell rebuilt into a package that can wait. The outermost layer is the coat, a set of tough protein shells rich in disulphide bonds and often wrapped in an additional loose layer called the exosporium. Beneath the coat lies the cortex, a thick layer of specialised peptidoglycan whose cross-links are deliberately looser than those of a normal wall, so that it acts as a cushion and helps withdraw water from the centre. The centre itself is the core, which holds the chromosome, the ribosomes and the enzymes but very little free water — a dehydrated gel rather than a living cytoplasm. Two molecules explain much of the resistance. Dipicolinic acid, usually written DPA, is made only in sporulating cells and is stored as calcium dipicolinate; it chelates calcium and replaces free water in the core, lowering the core water content and holding the spore’s contents in a stable, partially dehydrated state. Small acid-soluble proteins, or SASPs, bind the DNA itself and protect it against heat, desiccation and ultraviolet radiation. Because the core water content falls sharply, enzymes stop working, metabolism halts and the spore becomes metabolically dormant: resistant but not active. The cortex and coat together exclude disinfectants and the enzyme lysozyme, and the scarcity of water means heat cannot easily denature the proteins that remain. Germination reverses all of this within minutes once nutrients return.

Boiling at 100 °C at ordinary atmospheric pressure kills growing bacteria within minutes, which is why heat is such a dependable method of control for vegetative cells. It does not kill endospores, which can survive hours of boiling. The reason lies in the way moist heat destroys a cell and in what the spore has done to itself. Moist heat kills by coagulating proteins, and it works far better when water is present; a core with very little water, its DNA saturated by calcium dipicolinate and shielded by small acid-soluble proteins, resists exactly that damage. The practical answer is to raise the temperature above the boiling point of water, and that requires pressure. An autoclave is a pressure vessel: saturated steam at 121 °C corresponds to about 15 pounds per square inch, roughly one extra atmosphere, or 103 kilopascals, above ambient pressure. At 121 °C moist heat destroys even heavy spore contamination in about 15 to 20 minutes, so a typical cycle is 121 °C for 15 minutes at 15 psi, lengthened for dense loads, large volumes or bulky packs. Steam must actually reach the load; air trapped in a closed container or a tightly folded pack insulates the contents and the cycle silently fails. This is why packs are loosely wrapped, why containers are left open or perforated, and why autoclave tape, chemical indicators and biological indicators containing Geobacillus stearothermophilus spores are used to prove that the cycle worked. Dry heat is far slower and needs much higher temperatures: 160 °C for two hours, or 170 °C for one hour. For practice the conclusion is blunt: boiling an instrument disinfects it but cannot sterilise it.

Endospores matter clinically for two reasons: they survive in the environment long enough to reach a patient, and the genera that make them include some of the most dangerous bacteria in medicine. Clostridium species are anaerobic, so their spores lie dormant in soil, dust and faeces and germinate only where oxygen is absent. A deep puncture wound, a crush injury with dead muscle, a dirty abortion, a diabetic foot ulcer or a wound closed over contaminated material all create that oxygen-free niche. Once the spores germinate, the growing cells release toxins. Clostridium tetani causes tetanus: its round terminal spore gives the rod the drumstick appearance described earlier, and its tetanospasmin blocks the inhibitory neurons that normally restrain muscle contraction, so the patient develops lockjaw, rigidity and painful spasms. Clostridium perfringens causes gas gangrene in devitalised muscle, producing gas and rapidly spreading necrosis that may end in amputation. Clostridium botulinum grows in anaerobic food or in wounds and makes a neurotoxin that causes flaccid paralysis. Clostridium difficile forms spores that resist alcohol hand rub, persist on ward surfaces and cause antibiotic-associated colitis, which is why soap and water plus chlorine-based cleaning are required. Bacillus anthracis forms central spores that can persist in soil for decades and cause anthrax, and the notes correctly note the resistance of its dried spores. Nursing consequences follow directly: never rely on boiling for an instrument that will break the skin; irrigate, debride and drain contaminated wounds so that oxygen reaches the tissue; use contact precautions and sporicidal cleaning for a patient with Clostridium difficile; and check tetanus immunisation for every dirty or deep wound.