Cereulide
Cereulide is a highly stable, cyclic dodecadepsipeptide toxin produced by emetic strains of the Gram-positive, spore-forming bacterium Bacillus cereus, serving as the primary causative agent of the emetic type of B. cereus foodborne illness.[1] This pre-formed toxin, typically generated in contaminated starchy foods such as rice or pasta during improper storage, induces rapid-onset vomiting (0.5–5 hours post-ingestion) due to its role as a potassium ionophore that disrupts mitochondrial function and cellular energy production.[2][1] Structurally, cereulide consists of three repeating units of the tetradepsipeptide sequence [D-O-Leu–D-Ala–L-O-Val–L-Val], forming a 36-membered ring with a molecular weight of approximately 1,153 Da, which confers its exceptional resistance to heat (surviving 121°C for 2 hours), extreme pH (2–12), and proteolytic degradation.[2][1][3] It is biosynthesized via a non-ribosomal peptide synthetase (NRPS) encoded by the ces gene operon, located on a 270 kb megaplasmid (pCER270) in producing strains, with optimal production occurring at temperatures between 12–37°C, neutral pH (6–7), and high water activity (>0.953).[1][2] The toxin's cytotoxicity primarily targets mitochondria by facilitating potassium influx, leading to uncoupling of oxidative phosphorylation, depletion of ATP, and subsequent cell death, particularly in hepatic and pancreatic tissues.[1] In humans, the estimated emetic dose ranges from 0.02–1.83 µg/kg body weight, though severe cases can result in liver failure, acute encephalopathy, or multi-organ dysfunction, with higher risks to children and the elderly.[1][2] Unlike the diarrheal toxins of B. cereus, cereulide is not neutralized by gastric acid and persists through cooking, underscoring the importance of rapid food refrigeration to prevent its accumulation.[4] Detection methods, such as mass spectrometry (e.g., MALDI-TOF MS targeting m/z 1,175 or 1,191), enable quantification in food products at low levels (limit of detection ~30 pg/mL), aiding in outbreak investigations.[2][5]Chemical Properties
Structure
Cereulide is a cyclic dodecadepsipeptide toxin, consisting of a 36-membered ring formed by three repeating units of the tetrapeptide sequence D-O-Leu–D-Ala–L-O-Val–L-Val, where O-Leu and O-Val denote α-hydroxy acid derivatives of leucine and valine, respectively.[6] This structure features alternating ester and amide bonds, with the esters linking the carboxylic acid of an amino acid residue to the hydroxyl group of the adjacent hydroxy acid, and the amides connecting the amino group of an amino acid to the carboxylic acid of the previous residue.[6] The molecular formula of cereulide is C57H96N6O18, with a molecular weight of 1153 Da.[3] The specific stereochemistry of cereulide includes D-configurations at the O-Leu and Ala residues and L-configurations at the O-Val and Val residues within each repeating unit, contributing to its overall chiral architecture.[6] This arrangement creates a rigid, bracelet-like cyclic structure that resembles the ionophore valinomycin, enabling selective binding of alkali metal ions such as K+ and Rb+.[6] The ring can be represented textually as:In this conformation, the ion complexation sites are primarily the six amide carbonyl oxygen atoms—three from the L-Val residues forming one coordination plane and three from the D-Ala residues forming the opposite plane—positioning the metal ion centrally within the cavity through ion-dipole interactions.[6]cyclo[(D-O-Leu–D-Ala–L-O-Val–L-Val)<sub>3</sub>]cyclo[(D-O-Leu–D-Ala–L-O-Val–L-Val)<sub>3</sub>]