Psychrotolerant Paenibacillus tundrae Isolates from Barley Grains Produce New Cereulide-Like Depsipeptides (Paenilide and Homopaenilide) That Are Highly Toxic to Mammalian Cells
ABSTRACTPaenilide is a novel, heat-stable peptide toxin fromPaenibacillus tundrae, which colonizes barley.P. tundraeproduced 20 to 50 ng of the toxin mg−1of cells (wet weight) throughout a range of growth temperatures from +5°C to +28°C. Paenilide consisted of two substances of 1,152 Da and 1,166 Da, with masses and tandem mass spectra identical to those of cereulide and a cereulide homolog, respectively, produced byBacillus cereusNS-58. The two components of paenilide were separated from those of cereulide by high-performance liquid chromatography (HPLC), showing a structural difference suggesting the replacement ofO-Leu (cereulide) byO-Ile (paenilide). The exposure of porcine spermatozoa and kidney tubular epithelial (PK-15) cells to subnanomolar concentrations of paenilide resulted in inhibited motility, the depolarization of mitochondria, excessive glucose consumption, and metabolic acidosis. Paenilide was similar to cereulide in eight different toxicity endpoints with porcine and murine cells. In isolated rat liver mitochondria, nanomolar concentrations of paenilide collapsed respiratory control, zeroed the mitochondrial membrane potential, and induced swelling. The toxic effect of paenilide depended on its high lipophilicity and activity as a high-affinity potassium ion carrier. Similar to cereulide, paenilide formed lipocations, i.e., lipophilic cationic compounds, with K+ions already at 4 mM [K+], rendering lipid membranes electroconductive. Paenilide-producingP. tundraewas negative in a PCR assay with primers specific for thecesBgene, indicating that paenilide was not a product of plasmid pCER270, encoding the biosynthesis of cereulide inB. cereus. Paenilide represents the first potassium ionophoric compound described forPaenibacillus. The findings in this paper indicate that paenilide fromP. tundraeis a potential food-poisoning agent.