Number 238, 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.) AN AREAL DATA STORAGE DENSITY OF 8 GBIT/SQ.IN. has been demonstrated at the University of Oregon (Thomas Mossberg, 503-346-4779). By the encoding of a signal not directly in the form of bits but in the form of an ensemble of laser lightwaves at slightly different frequencies (essentially the Fourier transform of the signal), data can be stored as patterns of excited atoms in a frequency-sensitive medium. The Oregon physicists were able to write the equivalent of 2000 bits of data onto a single spot with an area of about 200 sq. microns. The resulting data storage density is bigger by a factor of 10 than that achieved for standard optical or magnetic recording methods. Another important figure of merit is the density-bandwidth (how much data and how fast): the Oregon figure is 1.5 x 10**17 bits/sq.in./sec., a factor of 3 to 10 larger than previously published reports for non-parallel systems. The material used in the Oregon work---thulium atoms lodged in a crystal---was chosen for its compatibility with the common diode laser (the same used in compact disk players) used to write the data. The researchers feel that even higher data storage densities are possible and that the use of other materials---retaining data for longer periods and at more forgiving temperatures (their current work is at 4-6 K)---will eventually make this new form of data storage highly efficient, fast, and practical. (H. Lin et al., Optics Letters, 1 August 1995.) CONCERNING UPCOMING ARTICLES IN PHYSICAL REVIEW LETTERS: Owing to our limited resources, we can send (by fax) text and figures from articles in Physical Review Letters only to science journalists, that is, writers who work for newspapers or magazines intended for a general readership. We regret that we cannot send the articles to physicists who, we hope, can get the journal through regular channels.