In response to nutrient deprivation (starvation) the rod-shaped soil bacterium Bacillus subtilis differentiates into a dormant, stress-resistant spore (Fig. 1). B. subtilis spores can withstand extreme heat, radiation, chemical assaults and time (spores can remain dormant for more than a million years).
A 1ml culture of wild-type B. subtilis produces ~5x108 spores. These spores can easily withstand 80°C for 20 minutes while vegetative (non-sporulated) cells are killed at this temperature. Thus, the study of sporulation is grounded in an assay that is quantitative over 8 logs.