A dormant, tough structure produced by some Gram-positive bacteria to ensure their survival through periods of environmental stress. The presence of oxygen is one such stress for Clostridium. Spores are formed when the vegetative bacterium produces a thick internal wall enclosing its DNA and part of its cytoplasm. Spores are resistant to ultraviolet radiation, desiccation, starvation, and chemical disinfectants, putting them at a significant advantage for successful dispersion. Once in a favorable habitat, a spore germinates (transforms) into its more vulnerable vegetative form where it can multiply and produce toxins.