A bacterium that cracks under pressure

It takes only about a millisecond—literally less than the blink of an eye—for a single Staphylococcus cell to separate into two distinct daughter cells. That swift split, known as “popping,” occurs on a time scale too fast to be explained by the enzyme-mediated processes that govern conventional cell division. Now Julie Theriot and coworkers at Stanford University have made the case that popping is driven not by biochemistry but by mechanics. As a Staphylococcus cell prepares to divide, it constructs a double-layered septum that partitions the spherical parent cell into two hemispheres—the soon-to-be daughters. Theriot and her colleagues hypothesized that when sufficient stress accumulates along the perimeter where the septum meets the cell wall, a crack forms. As that crack grows, the daughter cells tear away from each other. Based on crack-propagation speeds in similar biological materials, such a rupture could easily account for popping’s short time scale. That the team could induce popping by osmotically manipulating the pressure on the cell wall served as further indication of the phenomenon’s mechanical origins. Crucially, the rupture model predicts that newly formed daughter cells should remain connected at a hinge, like a clam shell. That prediction was borne out in images such as the scanning electron micrograph shown here, which captures the immediate aftermath of a cell division. Indeed, the hinging behavior may help to explain another of the bacterium’s morphological mysteries—its tendency to colonize not in neat arrays of cells but in irregular clusters resembling bunches of grapes. (X. Zhou et al., Science 348, 574, 2015 .)