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The fractal patterns of bacterial colonies

The views expressed are those of the author and are not necessarily those of Scientific American.


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Bacteria may be single celled organisms but they very rarely exist as single cells on their own. Instead, bacteria form colonies made up of many cells, all growing and dividing together. These colonies are often ordered in shape and form and various physical systems have been created to model this self-organisation; for example small vibrating rods in close proximity which organise themselves into patterns. However these systems often miss out two of the most crucial factors of bacterial colony development: all the cells are both growing and dividing continuously.

A group of researchers at Cambridge university used synthetic biology techniques to observe the patterns seen in a growing colony of the bacteria E. coli. In order to observe the patterns of cell organisation the genes for either a red, green or blue florescent protein were introduced into the bacteria which were then allowed to grow and develop on a flat surface.

The patterns of growing bacteria: A, B, C and D shown at different magnifications, in the final one individual cells can be seen. Scale bars, 1 mm in A, 100 μm inB, 100 μm in C, 10 μm in D. Image by J. Haseloff and F. Federici.

There are many different reasons why bacteria should form patterns during growth and development. It might be due to signalling molecules moving between the bacteria, adhesive forces holding them together, or the physics of dividing shapes pushing against each other. The researchers used a computational modelling system called CellModeller to see if they could make a system of dividing rod-shapes that produced the patterns seen in the physical colonies.

The model was put together with a series of assumptions. Firstly, it contained only rigid elongating capsules, the same shape as the dividing E. coli. These capsules would grow to a certain length and then divide in half. Each capsule did not move under its own propulsion, but only when subjected to outside forces. Finally they constrained the growth with viscous drag forces caused by the interactions of cells pressing and growing against each other. The CellModeller model was then compared with the naturally growing E. coli, as shown below (the graph on the right shows the fractal dimension measurements of the bacteria and the model):

Comparison between the bacterial growth (labelled BW27783) and the model (CellModeller). Image by J. Haseloff and F. Federici.

As shown by the computational model, these patterns rely not on genetic or cellular features of the bacteria, but on the physical interactions between the growing rods and their surrounding physical environment. The forces of the rods pressing up against each other, and jostling for space as they grow, creates the patterns observed.

In order to explore this further, the researchers used a mutant of their E. coli strain that is round, rather than rod shaped (strain KJB24). These cells grow and divide in the same way as the rod shaped cells, but don’t form the same pretty spiky patterns. Instead smoothed domain boundaries are seen, and large disappointing block-shapes instead of the colourful peaks and troughs.

Comparison of the rod-shaped bacteria (left) and the circular bacteria (middle). The graph on the right shows the fractal dimension measurements of the two strains. Image by J. Haseloff and F. Federici.

This model uses physical interactions to create the growth patterns seen in single layer of E. coli cells. Despite the apparent simplicity of the system, emergent fractal patterns are still seen forming. The development of more complex models to include genetic and cellular factors seen in more sophisticated bacterial colonies, such as those exhibiting swarming, swimming or biofilm behaviour could provide a useful way of researching bacterial colony development and growth.

Reference: Cell Polarity-Driven Instability Generates Self-Organized, Fractal Patterning of Cell Layers, Timothy J. Rudge, Fernán Federici, Paul J. Steiner, Anton Kan, and Jim Haseloff, ACS Synthetic Biology 2013 In Press DOI: 10.1021/sb400030p

EDIT: This research has also been covered over at the Oscillator! http://blogs.scientificamerican.com/oscillator/2013/06/09/fractal-bacteria/

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. rloldershaw 9:11 pm 06/9/2013

    If anyone would like to see a brief review of about 80 examples of fractal self-similarity manifesting itself everywhere in nature from subatomic to galactic scales, go to:

    http://www3.amherst.edu/~rloldershaw

    and see Paper #14 on the “Selected Papers” page.

    Robert L. Oldershaw
    Discrete Scale Relativity/Fractal Cosmology

    Link to this
  2. 2. Andrew Planet 4:12 pm 06/10/2013

    I wonder if the fractals in bacteria have anything to do with cell shape determining the final organisation of many of them relative to each other?

    As a comparison, I quote from Wikipedia on the flower Aquilegia http://en.wikipedia.org/wiki/Aquilegia

    ”Interestingly, it was shown that this amazing spur length diversity is achieved solely through changing cell shape, not cell number or cell size. This suggests that a simple microscopic change can result in a dramatic evolutionarily relevant morphological change.”

    Perhaps an analogy can be drawn? I don’t know.

    Link to this
  3. 3. Andrew Planet 4:25 pm 06/10/2013

    Perhaps a good analogy can only be drawn if specifying that it is only the change of cell shape that affects overall morphology in Aquilegia flowers and not any of the inner features of the flower cells?

    Link to this
  4. 4. S.E. Gould in reply to S.E. Gould 6:33 am 06/12/2013

    @Andrew: That’s some really interesting work on the flowers. Bacteria tend to stay a single shape, so I don’t think in this case the E. coli fractals were connected to cell shape, move to the movement of the rods against each other. If you look at the final part of the analysis they did examine a shape-changing mutant and found that with circular bacteria the fractal shapes almost completely vanished.

    There are some bacteria that show differing forms within colonies, so it might be seen in other situations, and will certainly affect cell organisation where it occurs.

    Link to this
  5. 5. Andrew Planet 9:28 am 06/14/2013

    Thank you Shuna for your thorough reply which I’m trying to assimilate. I’ve questioned myself, without daring to make any conclusion (as an uninformed but fascinated non specialist) whether fractals in species generally might occur with the specialization of cells, be they unicellular but gregarious, or multicultural. The key to fractals in morphology of organisms might be, at a cellular level, spurred by some cells specializing to some degree relative to others whilst still operating together as a unit.

    Link to this
  6. 6. Andrew Planet 9:30 am 06/14/2013

    Apologies for my browser word correct function, should read ”Multicellular”

    Link to this
  7. 7. Andrew Planet 2:18 pm 06/14/2013

    I had to come back to this for a further think. I had a doubt that the fact that circular shaped bacteria produced hardly any fractals was evidence in itself that cell shape could indeed perhaps affect the ultimate layout of bacterial colonies, but being circular is not one of the shapes that will cause that to happen. Thank you for having the time to explain the above to someone who’s a layperson in this domain. That was mentally very enjoyable.

    Link to this

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