Patterns of Bacterial Growth

An agar plate demonstrating bacteria with strong chirality. Image courtesy of the laboratory of Prof. Eshel Ben-Jacob of Tel-Aviv University.

Introduction

In general, as environmental conditions become less favorable, the pattern of growth in a bacterial colony becomes more complex.

For the purposes of identifying patterns of growth, bacteria colonies can be through of as multicellular organisms. (info about communication)

Patterns of growth are a result of bacteria adapting their behavior to suit their environment.

Bacteria in the lab are grown specifically to isolate single colonies. Bacteria in the wild must cope with environmental conditions.

There are three main patterns of growth as described by Prof. Eshel Ben-Jacob of Tel-Aviv University: fine radial branches, branching patterns, and dense fingers.

Agar Density

P is peptone level. Nc is related to agar concentration.

The larger the concentration of agar, the harder the resulting gel.

As agar density increases, the width of branches found in the fractal pattern of bacteria growth is thinner.

If the concentration of agar is very high and the solution is very hard, then the bacteria cannot move. The colony grows in one spot, and grows radially when new bacteria are physically pushed from the center. (Lacasta) The resulting pattern of high agar concentration and high nutrient concentration is a colony with compact, concentric rings, and little to no branching. (Golding) If the nutrient level is low, then bacteria must rely on nutrients diffusing towards the colony. The resulting pattern of bacterial growth follows the diffusion-limited aggregation model.

Example of random walker movement and the first steps DLA lattice formation. Courtesy of Yale University.

Diffusion-Limited Aggregation (DLA) is one growth model used to predict bacterial growth. It creates complex, multi-branched forms, and can be applied to any system where diffusion is the main method of particle transportation. It can be observed in bacterial growth on agar plates, dendrites, dust balls, electrodeposition, and mineral deposits.

To form a DLA pattern, begin with a seed molecule at the origin of the lattice. A "random walker" molecule diffuses from far away in a random pattern of motion. It stops once it reaches a space adjacent to the seed molecule, and another random walker is launched. In a DLA lattice, a molecule that sticks out of a main branch will not be rounded or smoothed over by new growth, but rather emphasized. Nodes are more likely to catch wandering particles because they three facets available for growth, compared to a molecule in the branch, which only has one facet.

Nutrient Density

Bacterial growth patterns are compact at high nutrient densities, and become more fractals and peptone density decreases.

Intercellular Communication

Length of cells and handedness of flagella also affect how patterns form.

Chirality is a feature of bacteria which relates to symmetry. A bacteria with high chirality is not identical to its mirror image, and a cell with low chirality is very similar to its mirror image.

Conclusion

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