Cereulide, produced by certain Bacillus cereus strains, is a lipophilic cyclic peptide of 1152 Da that binds K+ ions with high specificity and affinity. It is toxic to humans, but its role for the producer organism is not known. We report here that cereulide operates for B. cereus to scavenge potassium when the environment is growth limiting for this ion. Cereulide-producing B. cereus showed higher maximal growth rates (mu(max)) than cereulide non-producing B. cereus in K+-deficient medium (K+ concentration similar to 1 mM). The cereulide-producing strains grew faster in K+-deficient than in K+-rich medium with or without added cereulide. Cereulide non-producing B. cereus neither increased mu(max) in K+-deficient medium compared with K+-rich medium, nor benefited from added cereulide. Cereulide-producing strains outcompeted GFP-labelled Bacillus thuringiensis in potassium-deficient (K+ concentration similar to 1 mM) but not in potassium-rich (K+ concentration similar to 30 mM) medium. Exposure to 2 mu M cereulide in potassium-free medium lacking an energy source caused, within seconds, a major efflux of cellular K+ from B. cereus not producing cereulide as well as from Bacillus subtilis. Cereulide depleted the cereulide non-producing B. cereus and B. subtilis cells of a major part of their K+ stores, but did not affect cereulide-producing B. cereus strains. Externally added 6-10 mu M cereulide triggered the generation of biofilms and pellicles by B. cereus. The results indicate that both endogenous and externally accessible cereulide supports the fitness of cereulide-producing B. cereus in environments where the potassium concentration is low.