Bacterial Decision Making

Medical and biological physics sometimes appear on the cover of Physics Today. For instance, this month (February, 2014) the cover shows E coli. The caption for the cover picture states

Escherichia coli bacteria have served for decades as the “hydrogen atom” of cellular decision making. In that branch of biology, researchers strive to understand the origin of cellular individuality and how a cell decides whether or not to express a particular gene in its DNA. For some of the physics involved, turn to the article by Jané Kondev on page 31.

The article begins with a description of Jacques Monod’s work with the lac operon: a stretch of DNA that regulates the lac genes responsible for lactose digestion. (This story is told in detail in Horace Freeland Judson’s masterpiece The Eighth Day of Creation.) Kondev writes

The key question I’ll address in this article is, What is the molecular basis by which a cell decides to switch a gene on? Although all the cells in figure 1b are genetically identical and experience the same environment, only one appears to be making the protein. As we’ll see, that cellular individuality is a direct consequence of molecular noise that accompanies cellular decision making. The sources of the noise and its biological consequences are currently a hot topic of research. And statistical physics is proving to be an indispensable tool for producing mathematical models capable of explaining data from experiments that look at decisions made by individual cells.

The caption of Fig. 1b reads

In the presence of a lactose surrogate, individual cells can switch from a state in which they are unable to digest lactose to a state in which they are able to consume the secondary sugar. Yellow indicates the amount of a fluorescently labeled protein, lactose permease, which is one of the enzymes needed by the cell to digest lactose.

The article then draws on several physics concepts that Russ Hobbie and I discuss in the 4th edition of Intermediate Physics for Medicine and Biology: the Boltzmann factor, the Gibbs free energy, the Poisson probability distribution, and feedback. The last of these concepts is crucial.

Thanks to that positive feedback, E. coli cells exist in two different steady states—one in which there are many permeases in the cell (the yellow cell in figure 1b), the other in which the number of permeases is low (the dark cells in 1b). Stochastic fluctuations in the expression of the lac genes—fluctuations, for instance, between an on and an off state of the promoter—can flip the switch and turn a lactose noneater to a lactose eater.

The article concludes

Physics-based models are leading to more stringent tests of the molecular mechanisms responsible for gene expression than those provided by the qualitative model presented in biology textbooks. They also pave the way for the design of so-called synthetic genetic circuits, in which the proteins produced by the expression of one gene affect the expression of another. Such circuits hold the promise of bacterial cells capable of producing useful chemicals or combating diseased human cells, including cancerous cells. Whether this foray of physics into biology will lead to fundamentally new biological insights about gene expression remains to be seen.

Kondev’s review offers us one more example of the importance of physics in biology and medicine. And for those of you who think E. coli bacteria is not an appropriate topic for a Valentine’s Day blog post, I say bah humbug.