A mechanical finger-trap mechanism controls cell width in Bacillus subtilis*

The Gram-positive bacteria B.subtilis retains its rod shape during growth, meaning that the cell elongates as maintaining the same width. In this organism, the cytoplasmatic turgor pressure promotes cell elongation by enlarging the cell membrane, thus, the cell wall. Despite the force generated by the turgor pressure, cell width remains constant. In bacteria, the cell wall determines cell shape. This structure is formed of circumferentially oriented stiff glycans crosslinked by soft peptides. It is known that the constitutive properties of the cell wall are fundamental to control the width. However, we observed that during the hydrolysis of the cell wall, which is essential for growth, the cell length and width increase in response to hydrolases cleavage, indicating that an alternative mechanism to counteract the turgor pressure must exist. By studying the mechanical properties of the cell wall using osmotic shocks, we found that the cell responded to an increase in turgor pressure with longitudinal stretching and circumferential compaction. Vice versa, when the pressure decreased, the cell shrank and swelled in diameter, similar to a finger trap. These data suggested that the compaction forces to counteract the turgor pressure are generated in the cell wall. Accordingly, when we genetically reduced the synthesis of new peptidoglycan, the cell could not compact in response to increasing pressure. We conclude that the synthesis of new peptidoglycan prevents swelling of the diameter during growth.