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Proteus mirabilis forms a concentric-ring colony by undergoing periodic swarming. A colony in the process of such synchronized expansion was examined for its internal population structure. In alternating phases, i.e., swarming (active migration) and consolidation (growth without colony perimeter expansion), phase-specific distribution of cells differing in length, in situ mobility, and migration ability on an agar medium were recognized. In the consolidation phase, the distribution of mobile cells was restricted to the inner part of a new ring and a previous terrace. Cells composing the outer part of the ring were immobile in spite of their ordinary swimming ability in a viscous solution. A sectorial cell population having such an internal structure was replica printed on fresh agar medium. After printing, a transplant which was in the swarming phase continued its ongoing swarming while a transplanted consolidation front continued its scheduled consolidation. This shows that cessation of migration during the consolidation phase was not due to substances present in the underlying agar medium. The ongoing swarming schedule was modifiable by separative cutting of the swarming front or disruption of the ring pattern by random mixing of the pattern-forming cell population. The structured cell population seemed to play a role in characteristic colony growth. However, separation of a narrow consolidation front from a backward area did not induce disturbance in the ongoing swarming schedule. Thus, cells at the frontal part of consolidation area were independent of the internal cell population and destined to exert consolidation and swarming with the ongoing ordinary schedule.There are various kinds of surfaces in nature. Most of these surfaces have fractal properties and are able to provide an enormous amount of places for microbial habitation (17). For efficient and flexible colonization on these surfaces, microbes seem to have evolved special strategies in each special situation, e.g., the flagellar and twitching motility of Pseudomonas aeruginosa in biofilm formation (21). Surface translocation by flagellated bacteria may be one of these examples. Many species of bacteria have been reported to have such a surface swarming ability (2, 4, 13). Among these, Proteus mirabilis is well known for forming a giant colony with a concentric-ring pattern and cellular dimorphism (differentiation to extremely elongated and hyperflagellated swarmer cells).The growth process of a P. mirabilis colony is composed of cyclic repetition of a swarming (expansion) phase and a consolidation (no-expansion) phase. During the consolidation phase, swarmer cells dedifferentiate into vegetative cells for intensive cell multiplication. These vegetative cells will become the source of future swarmer cells after differentiation (2). Since many types of mutants which show abnormal patterns of colony growth have been isolated (3,5,29), it is evident that P. mirabilis has a special genetic program for such characteristic swarming. Although mutational a...

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