Shaped to Survive

Microbes form complex patterns that blur the boundary between life and nonlife, demonstrates TAU physicist Prof. Eshel Ben-Jacob and his colleagues.

A colony of bacteria self-organize into a snowflake-like shape.

In the past few decades, physicists have come to understand how some patterns, such as the shape of a snowflake, form. TAU condensed-matter physicist Eshel Ben-Jacob was surprised to discover, however, that he could predict and model the self-organization of living creatures, such as colonies of bacteria, according to similar physical principles.

Visualize a snow "fern" appearing on an icy window. Randomly migrating molecules of water on the glass pane occasionally collide with the fern, getting stuck. Because the molecules are more likely to hit the protruding parts of the fern and to attach there, any branch that sticks out tends to grow longer. In addition, the molecules are more likely to stick in certain positions because of the orientation of ice molecules already there. In this way - and in response to certain environmental conditions - an intricate, crystalline structure spontaneously grows.

In extremely hard agar, Paenibacillus vortex bacteria congregate into tight, rotating vortices that cut through the growth medium like a circular saw. The cooperative behavior allows the colony to grow outward; the saw's "teeth" are the dark dots at the end of each branch.

Living creatures such as bacterial colonies also pattern their growth in response to environmental conditions, in ways that startlingly resemble inanimate objects. For example, microorganisms can create branches according to the same principles governing a snow fern. If the agar (culture gel) is too hard, the bacteria cannot move toward the nutrients. Molecules of food can, however, bounce randomly through the gel, tending to reach those bacteria that are protruding from the colony, just like water molecules attaching to the spikes of an ice crystal. The bacteria then eat, grow, and split into two, and the protrusion slowly grows into a branch.

An extra ingredient: life

Unlike the snow fern, however, bacteria can sense their environment and clearly "choose" certain designs over others to ensure their survival. Colonies can take up more complex forms because of an extra ingredient - life.

Curls form when Paenibacillus dendritiformis are placed on soft agar. The organisms mutate to a longer form that moves fast but tends to veer to one side, giving rise to the curls.

Prof. Ben-Jacob found that if the bacteria are cultured on softer agar, the initially branched pattern changes spontaneously to a curling one that spreads much faster. Under a microscope, each bacterium is seen to become significantly longer when the pattern begins to curl. Although the transformation obviously helps to make the bacteria more mobile - and thus to improve their access to food - no one knows exactly how it is triggered. Ben-Jacob is collaborating with Prof. David Gutnick of the TAU Department of Molecular Microbiology and Biotechnology to try and solve this conundrum.

Bacteria not only sense their environment, maintains Ben-Jacob, but they can also influence it, as well as one another: colonies of E. Coli can aggregate by secreting attractant chemicals. The bacteria respond to stresses by communicating with each other and generating new variations of themselves.

At present, biologists rely mostly on biochemical and genetic techniques to analyze how the motion of cells is controlled by external signals. Their task involves unraveling the microscopic rules of the game. But physicists have learned that getting from such microscopic laws to macroscopic patterns is an exceedingly challenging problem, one which the artistry of microbes may help resolve.

This article was adapted from the cover story of Scientific American, October 1998, written by Prof. Ben-Jacob and an American colleague, Prof. Herbert Levine, with their permission. Prof. Ben-Jacob is a member of TAU's School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences. Levine is a professor of physics at the University of California, San Diego.

Photos this item: Amikam Shoob