We have studied the morphological diversity and change in bacterial colonies, using the bacterial species Escherichia coli, as a function of both agar concentration C a and nutrient concentration C n . We observed various colony patterns, classified them into four types by pattern characteristics and established a morphological diagram by dividing it into four regions. They are regions A [diffusionlimited aggregation (DLA)-like], B (Eden-like), C (concentric-ring), and D (fluid-spreading). In particular, we have observed a concentric-ring colony growth for E. coli. We focused on the periodic growth in region C and obtained the following results: (i) A colony grows cyclically with the growing front repeating an advance (migration phase) and a momentary rest (consolidation phase) alternately. (ii) The growth width L and the bulge width W in one cycle decrease asymptotically to certain values, when C a is increased. (iii) L does not depend on C n , while W is an increasing function of C n . Plausible mechanisms are proposed to explain the experimental results, by comparing them with those obtained for other bacterial species such as Proteus mirabilis and Bacillus subtilis.

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