Present or Future Work

Parameters controlling swarm expansion

Extending PeiDa Guo's work, I would like to measure swarm colony expansion for different strains of Bacillus subtilis and under different geometries (e.g. inoculation in a line rather than a point; swarm propagation through a constriction).  We have a rough measure of the canonical expansion profile: this could be used to make a 3D simulation of nutrient profiles within the swarm.  We need to better understand the mechanical and diffusional properties of agar (a large-pore elastic mesh).  The basic experimental setup (illumination and time-lapse still camera within an incubator)  exists but should be optimized for higher contrast imaging. 

Chemotaxis with N flagella

Vikyath Rao predicts that cells with 5 or more flagella should be much worse at  chemotaxis than cells with only a few flagella.  This should be measurable in either (1) a heterogeneous population of cells whose flagella are fluorescently labeled so they can be counted or (2) different populations grown so that, on average, they have different numbers of flagella.  This will require developing protocols for controlling the number of flagella per cell (microbiology), construction of controllable chemotaxis gradients (microfluidics) and either (1) counting of individual cells (high-mag microscopy and hand counting) or (2) measurement of population movement (low-mag microscopy and image processing).

Past Work

Theses

swarm evolution
PeiDa Guo: 2007-2008

Swarm Expansion Rate of Bacillus Subtilis: Does Chemotaxis Play a Role?

Swarming is a rapid and coordinated form of motility of B. subtilis on wet and nutritious surfaces.  Some possible driving and limiting factors of swarming are chemotaxis, i.e. the bacteria’s tendency to move toward high nutrient gradient, population growth pressure, wetness on agar surface, and hardness of agar.  Chemotactic behavior is studied by modeling nutrient diffusion with nutrients consumption by bacteria in one-dimension.  Experiments were performed to measure swarm expansion under various initial nutrients configurations, such as different initial nutrients concentration and different depths of agar gel.  No difference in expansion rates were observed in each configuration beyond experimental error (click on attached figure for readable resolution).  Diffusion coefficient tests were performed to provide counter evidence to the common belief that limited diffusion of nutrients causes non-initiation of B. subtilis swarming.

 

Projects

Rao 2008 Figure 9
Vikyath Rao: 2008

The standard model of bacterial chemotaxis assumes than each motor  - and therefore each flagellum - operates independently.  This raises the question of whether a cell with many non-cooperating flagella would swim differently from a cell with fewer flagella.  We predicted the chemotactic performance of a cells with 1 to 6 flagella (see figure), assuming

  1. flagellar motors reverse independently
  2. motors obey the impulse response curve of Block, Segall and Berg [Cell 31:215-226 (1982)]
  3. reorientation due to reversal follows the data of Turner, Ryu and Berg [J. Bact. 182:2793–2801 (2000)]
  4. chemotactic drift velocity is described by the theory of Locsei [J. Math. Biol. 55:41–60 (2007) ]

This effect should be readily observed in a population of cells with varying numbers of flagella.