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James Shapiro and Clara Hsu, Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, Ill., USA

　　Escherichia coli microcolonies were produced after 11.5 h of growth at 37C on an agar slide. This mass of tightly packed individual cells arises from a single cell through binary division.

　　The emergence of a second layer of cells can be seen in the central region. The appearance of this layer (and of successive additional layers) is a regular feature of early colony development, which shows that the colony center remains an active zone of cell growth. With longer times of incubation, additional layers form near the perimeters of older microcolonies to generate a multilayered structure.