Untwisting the mystery of supercoiling: Mbl configuration in growing bacterial filaments

\textit{Bacillus subtilis}, a commonly studied prokaryote form long filaments, or chains of cells, when the cells fail to separate upon replication. These mutants undergo supercoiling where the bacterial filament buckles and wraps about itself like an over-twisted phone cord. It has long been supposed that twisting stress is generated in the cell wall during growth and causes this coiling. But, the twisting mechanism has remained an enigma. A recently discovered actin-like protein, Mbl, forms helical structures under the cell wall and controls cell morphogenesis in \textit{B. subtilis}. Based on these findings, a new model suggests how these helical structures could lead to supercoiling. We report here experiments connecting growth, Mbl structure, and supercoiling. We have studied the helical pitch of the Mbl under regular growth conditions, various concentrations of xylose, and under the influence of different concentrations of ammonium and magnesium. These experiments demonstrate how growth effects the configuration of the Mbl cables and suggest that growth induced deformation of the Mbl cables generate twist in the filaments, which eventually leads to supercoiling in bacterial filaments.