Pelletier, J. et al. Proc. Natl. Acad. Sci. USA 109, E2649–E2656 (2012).

The single, circular chromosome of Escherichia coli occupies only a fraction of the space in a bacterial cell. As it segregates during cell division, the chromosome's morphology changes, but to date little is known about the micromechanical properties of the chromosome. Pelletier et al. now present a combination of microfluidics, imaging and single-molecule manipulation tools to answer questions such as “How much force is needed to keep the chromosome in its compacted state in vivo?” The researchers extracted the chromosome into a microchannel the width of a bacterial cell and visualized the expansion dynamics with mCherry directed against a histone-like associated protein. Using optical-trap micropistons, they then measured the compression force needed to fold the chromosome back to its in vivo size. Surprisingly, the force and free energy needed to compact the chromosome is an order of magnitude lower than the turgor pressure inside the cell.