Magnesium modulates Bacillus subtilis cell division frequency

By chance, we discovered a window of extracellular magnesium (Mg²⁺) availability that modulates Bacillus subtilis division frequency without affecting growth rate. In this window, cells grown with excess Mg²⁺ produce shorter cells than those grown in unsupplemented medium. The Mg²⁺-responsive adjustment in cell length occurs in both rich and minimal media and in domesticated and undomesticated strains. Of other divalent cations tested, manganese (Mn²⁺) and zinc (Zn²⁺) also resulted in cell shortening, but only at concentrations that affected growth. Cell length decreased proportionally with increasing Mg²⁺ from 0.2 mM to 2.0 mM, with little or no detectable change in labile, intracellular Mg²⁺ based on a riboswitch reporter. Cells grown in excess Mg²⁺ had fewer nucleoids and possessed more FtsZ-rings per unit cell length, consistent with increased division frequency. Remarkably, when shifting cells from unsupplemented to supplemented medium, more than half of the cell length decrease occurred in the first 10 min, consistent with rapid division onset. Relative to unsupplemented cells, cells growing at steady-state with excess Mg²⁺ showed enhanced expression of a large number of SigB-regulated genes and activation of the Fur, MntR, and Zur regulons. Thus, by manipulating the availability of one nutrient, we were able to uncouple growth rate from division frequency and identify transcriptional changes suggesting cell division is accompanied by oxidative stress and an enhanced demand to sequester and/or increase uptake of iron, Mn²⁺, and Zn²⁺.