Abstract
Spore formation in the Gram-positive bacterium Bacillus subtilis has been classically viewed as an example of unicellular differentiation that occurs in response to nutritional starvation. We present evidence that B. subtilis produces an extracellular factor(s) that is required, in addition to starvation conditions, for efficient sporulation. This factor is secreted and accumulates in a cell density-dependent fashion such that cells at a low density sporulate poorly under conditions in which cells at a high density sporulate efficiently. Conditioned medium (sterile filtrate) from cells grown to a high density contains this extracellular differentiation factor (EDF-A) and stimulates spore formation of cells at low density under normal starvation conditions. EDF-A is heat-resistant, protease-sensitive, and dialyzable, indicating that it is at least in part an oligopeptide. Production of EDF-A is reduced or eliminated in spoOA and spoOB mutants, which are defective in many processes associated with the end of vegetative growth. Mutations in abrB, which suppress many of the pleiotropic phenotypes of spoOA mutants, restore production of EDF-A.