Number 238 (Story #1), September 1, 1995 by Phillip F. Schewe and Ben Stein GEOMETRIC PATTERNS IN BACTERIAL COLONIES are being studied by physicists in an effort to elucidate universal mechanisms for pattern formation in nature. For example, placing a drop of e. coli bacteria on a nutrient-rich surface causes them to multiply and spread out and, under certain conditions, form visually striking patterns such as organized spots or stripes. Lev Tsimring and Herbert Levine (619-534-4844) of the University of California at San Diego and colleagues in Israel and the United States have developed a model which describes the pattern formation as a complex interplay between the rate at which the bacteria spread out on the surface, the amount of nutrient available, and the level at which the bacteria respond to a chemical attractant emitted by other bacteria. The spots or stripes form at regions at which there is a higher-than-average buildup of chemical attractant. Whether spots or stripes form, according to the model, depends on the level of response of the bacteria to the chemical attractant. When the bacteria run out of nutrients, they enter a dormant "non-motile" state, locking the pattern into place. The model contains similarities to the "reaction-diffusion model" introduced by mathematician Alan Turing in the 1950s to explain the patterns in animal coats. In Turing's model, the presence of two chemical "morphogens" diffusing through animal cells at different rates leads to spatially varied concentrations of the chemicals, providing a template for patterns such as leopard spots and tiger stripes. In the bacterial colony model, the patterns form through a similarly unequal competition between the random component of the bacteria motion and their motion towards the chemical attractant. (Lev Tsimring et al., Physical Review Letters, 28 August.)