Instead of a sphere, maybe you would like to make your cell cylindrical, like this Cupriavidus necator. Rod-shaped cells (cylinders with hemispherical caps) are a very common form for bacteria and archaea, likely because they make efficient swimmers and swarmers (more on that in Chapter 6). Starting from a sphere, imagine that you had a construction contractor who could direct where workers lay in new cell wall. Instead of random insertion, you could, say, direct them to work around a single plane. As the workers laid in more and more hoops of peptidoglycan in this region, a cylinder would form with the same diameter as the initial sphere (which would now serve as the structure’s end caps).
The contractor for most rod-shaped bacterial cells is a cytoskeletal protein named MreB, which is a homolog of the eukaryotic cytoskeletal protein actin. It remains unclear exactly how it works, but small patches of MreB seem to shuttle rapidly around the circumference of the cell, directing where new peptidoglycan is added to the sacculus. MreB’s circuit is restricted to the cylindrical portion of the cell, expanding the rod without affecting the ends. Not all rod-shaped bacteria use MreB, and we are still figuring out how the shape forms in many species (⇩). For rod-shaped archaea (⇩), the surface layer plays an important role in determining the shape.