Initial growth and division of three neighboring cells: A, B, and C. The three starting cells and two daughters of cell A are labeled. Numbers in the upper left-hand corner give the time of each frame.

In the first four frames, the interaction between daughter cells from B and C shows cell elongation at the outside poles, producing an aligned cell pair involving the progeny of two different bacteria. In the next four frames, the progeny of A show a striking example of differential cell elongation. At 15:03 (135 min after inoculation), cell A1 had divided twice to give four progeny, but cell A2 had only elongated. In the next 12 min, A2 divided, and one of its daughters preferentially elongated towards the merged progeny of B and C, reaching them in another 6 min. The final frame, taken 217 min after inoculation, illustrates how the progeny of these three independent cells were unified into a single microcolony which would have been difficult to distinguish from a single clone of cells.

This image can be used to allow students to visualize how a microcolony of bacteria is generated.

To initiate microcolony development, saturated cultures of Escherichia coli K-12 were diluted in TYE broth (tryptone, yeast extract) and about 20 µl of the diluted cell suspension was placed on a thin agar layer on a microscope slide. After excess liquid was blotted, a cover slip was placed on the inoculated agar, and the slide was incubated under an air curtain on the microscope stage.