Question
Bacterial spore.
Answer
A bacterial spore is a dormant, highly resistant, metabolically inactive form produced by certain Gram-positive bacteria (chiefly Bacillus and Clostridium species) when exposed to adverse environmental conditions (nutrient depletion, desiccation, heat) — a survival mechanism, not a means of reproduction (one vegetative cell produces one spore, and one spore germinates back into one vegetative cell).
Structure (from outside in): exosporium (outermost, loose covering, not always present), spore coat (keratin-like protein, provides resistance to chemicals/enzymes), cortex (peptidoglycan layer, important in heat resistance and dehydration), core wall, and the core/protoplast (contains DNA, ribosomes, and a very low water content).
Basis of resistance: high concentration of calcium dipicolinate in the core stabilizes DNA and proteins and confers marked resistance to heat, radiation, and desiccation; the thick, impermeable, multilayered coat resists chemical disinfectants and enzymatic digestion; the low water content of the core reduces heat-mediated protein denaturation.
Clinical/practical significance: spores of Clostridium tetani, C. perfringens, C. botulinum, C. difficile, and Bacillus anthracis can survive in soil/environment for years and germinate under favourable (anaerobic/nutrient-rich) conditions to cause disease; spores are the target organism used to validate sterilization procedures (autoclaving must kill even bacterial spores, unlike boiling or pasteurization) — biological indicators use Geobacillus stearothermophilus spores.
Location of spore (central, subterminal, or terminal) and whether it bulges the cell wall are used as identifying/classifying features (e.g., C. tetani’s round terminal spore gives a “drumstick” appearance).