The mode transition of the bacterial colony

The colony patterns of Bacillus circulans on agar medium were experimentally investigated to study about the growth mode transition. From the optical microscopic observation, the bacteria distribute inside the medium, that is, the colony grows three-dimensionally (3D) in the soft agar condition (CA<0.9 wt%). On the other hand, the bacteria distribute on the surface, that is, the colony grows two-dimensionally (2D) in the hard agar condition (CA 0.9 wt%). It indicates that the mode transition occurs by changing the hardness of the medium. The mode transition is confirmed by the discontinuous change in the pattern diagram and in the colony growth rate, which is defined as the expanding speed of the colony. Under the softer agar condition in the range of the 2D mode (0.9 CA 1.2 wt%), the induction time exists. Before the induction time, the colony grows in 3D mode. The length of the induction time decreases with increasing the bacterial number density. In addition, bacterial aggregation plays a key role on 2D growth. These results suggest that the increasing bacterial number density causes bacterial aggregation resulting in the mode transition