Bacillus subtilis: a bacterium for all seasons.

Microbes inhabit ever-changing environments in which the conditions for propagation, whether they be favorable or unfavorable, are often transient. Plants and animals are able to cope with changing circumstances in their external environment by maintaining homeostasis for internal cells, tissues and organs. Microbes do not have this luxury. Instead, they have evolved mechanisms for adapting to change by being versatile. In some cases, these adaptations involve the expression of a limited set of genes that exploit the availability of a food source or provide protection against chemical, radiation or thermal stress. In other cases, however, microbes adapt in a more elaborate manner by entering a specialized state that enables them to exploit a particular niche or protects them against environmental extremes. Entry into such a state can occur as a direct response to an external cue or stochastically (by chance) as part of a bet-hedging strategy, or sometimes a combination of the two. An example of a versatile microbe that exhibits a wide variety of states is the bacterium Bacillus subtilis, the subject of this Primer. Many of the states exhibited by B. subtilis are similar to states observed in other bacteria. What is special about B. subtilis is the unusually rich repertoire of alternative states exhibited by one bacterium, enabling it to cope with a wide range of environmental challenges.