Bacterial colony

A bacterial colony is a spatially localized population of bacteria growing together. On a suitable solid culture medium, growth can produce a visible patch containing many cells. The patch, its constituent cells and the lineage that produced them are different things to measure.

For an original ideal lineage calculation, assume one founder, exactly one binary fission per generation, no cell loss and no merging with another population. After $n$ generations the population is $2^n$; after 20 generations it is 1,048,576 cells. This explains how one localized growth origin can produce many cells. The founding-cell assumption is not established simply by observing the final patch's outline.

A colony also has physical organization. In a 2008 experiment, Escherichia coli grew in a microfluidics-based chemostat. As the population became dense and expanded, rod-shaped cells developed orientational alignment with the flow. Imaging and mechanical models connected cell division and contact forces with a collective pattern.^[ref]

Colony size, cell number and cell arrangement therefore answer different questions. Two equally populated colonies could occupy different areas if they differ in packing or grow in different numbers of layers. Conversely, two equal visible areas need not contain equal numbers of viable cells. A defined counting method and a defined spatial measurement are needed to make either comparison.