Relationship between cellular response and behavioral variability in bacterial chemotaxis

Original Entry: Tom Dimiduk APPHY, Fall 2010

Relationship between cellular response and behavioral variability in bacterial chemotaxis Thierry Emonet† and Philippe Cluzel Proceedings of the National Academy of Sciences

Soft Matter Keywords

Random Walk, Fluctuation Dissipation, Population Behavior

Summary

This article discusses how amplification of noise in an enzyme regulation system is used to drive the bacteria's stochastic response to nutrient gradients. The authors find that hypersensitivity to these fluctuations actually provides beneficial behaviors both for allowing single bacteria to progress rapidly up gradients and for allowing populations to explore a large area. Finally, the dynamics of bacteria in response to stimulus can be inferred from their spontaneous fluctuations in the absence of stimuli.

Soft Matter Discussion

This paper presents only computational results, though they point at a previous paper with supporting experimental work. Any reference to cells or mutants here actually refers to simulated cells and simulated cells expressing a simulated mutant phenotype.

Figure 3: "Power spectra of the fluctuations of the output signal (CheY-P) from nonstimulated cells. Shown are 1-fold (black), 2-fold (gray), and 4-fold (light gray) wild-type levels of CheR for a fixed wild-type level of [CheB]".

The first portion of their results is a lengthy discussion of enzyme dynamics which is of little interest from a soft matter perspective. The important results from this section are that increasing expression of the CheY-P kinase (1) reduces fluctuations in control loops (Figure 3), reducing the power spectrum of the cell's motion, and (2) decreases the relaxation time in response to stimulus.

Figure 4: "Relationship between relaxation time and chemotactic drift. (A) Temporal evolution of the kinase activity relative to steady state upon sudden deactivation of active receptor complexes for 1-fold (black), 2-fold (gray), and 4-fold (light gray) wild-type level of [CheR] and fixed wild-type level of [CheB]. ... (B) Role of the relaxation time in chemotaxis. Cells with longer relaxation time swim farther along the gradient of attractant (gray shade). ... (C) Effect of variations of [CheR] on the chemotactic response of a bacterial population of 400 cells. ... (D) Position of the cells from C with 1-fold (blue) and 4-fold (green) wild-type level of CheR after 12 min. Below the gray transparent plane (z ϭ Ϫ0.1 mm) there is no nutrient, and bacteria perform an unbiased random walk. Above the plane the random walk is biased upward the gradient of aspartate."

They observe that the decrease in relaxation time caused by increasing expression of CheY-P shortens the length of runs while going up a gradient (Figure 4a,b), reducing the rate at which the bacteria travel up a gradient (Figure 4c).

The large fluctuations in wild type bacteria cause more spread along directions perpendicular to the gradient than mutants with damped noise, as shown in Figure 4d. This spread is broadened by increased variability in behavior between bacteria within the population. They state that this is beneficial for a population because it allows a colony as a whole to explore larger spaces in the search for nutrients.