A metabolite morphogen coordinates multicellular development in Bacillus subtilis
The transition from unicellular to multicellular growth requires diversification of cellular functions within genetically identical populations. In Bacillus subtilis, biofilm formation is historically viewed as a developmental precursor to sporulation along a linear pathway. Here, we show that biofilm formation and sporulation instead diverge along a branched pathway. A subpopulation that first initiates sporulation catabolizes lipoteichoic acid through the sequential action of the enzymes ShfP (Sporulation heterogeneity factor Poison) and PhoA (alkaline phosphatase A), leading to the release of millimolar concentrations of glycerol. This glycerol impedes sporulation by disrupting cell wall synthesis and cytoplasmic pH, necessitating counteraction by another protein, ShfA (Sporulation heterogeneity factor Antidote). The extracellular glycerol, however, acts as a morphogen that directs neighboring cells to initiate biofilm formation, which we directly visualize in developing populations of cells. Thus, B. subtilis multicellularity emerges through a branched developmental program in which sporulating cells generate the cue that creates the biofilm-producing lineage via cell-cell communication through repurposing of a canonical intracellular metabolite.