Click here to download a movie from the Elowitz
lab showing different behaviors within a clonal
population of Bacillus subtilis in relatively
homogeneous conditions. Here, the cells express a
green fluorescent protein when growing normally,
but turn on a red fluorescent protein when
differentiating into a transient competent state.
Some others differentiate into resilient spores
(white objects). Source.
Bacteria that are born genetically equal aren't necessarily the same. The same genome, residing in cells side-by-side in the same medium in the same flask, does not guarantee the same phenotype. One example that comes to mind is the persisters in E. coli populations—the small number of cells that spontaneously stop growing. If the population is hit by a β-lactam antibiotic, those cells escape death. Similarly, under lab conditions that trigger genetic competence in B. subtilis, only a small fraction of the cells make the switch to competence.
B. subtilis cells growing in a rich growth medium offer yet another example. Here genetically identical cells comprise two distinct types. Most are flagellated and actively swimming about as individuals, while a minority have no flagella and form long chains. The game is different in cells in the stationary phase where virtually all of the cells are found in long chains, bound together by an abundant matrix. Losick, Kolter, and colleagues have been working with this system for some years (for earlier papers, click here and here) seeking to determine how such bimodal cell populations are established and maintained in growing cultures.
Others have approached this question from a theoretical point of view asking what sort of a regulatory "circuit" would it take to produce robust bistability. In other words, how might a cell, using a simple system employing but a few regulatory genes, be able to operate in either of two distinct states. In 2000, Gardner et al. started with a model for a bistable network (Figure 1) and then attempted to create such a circuit in E. coli. (A simple system this may be, but conveying it in words is not so simple. I suggest you wrap your mind around the diagrams first.)