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Background  Cereulide is a cyclic dodecadepsipeptide ionophore, produced via non-ribosomal peptide synthetases (NRPS), which in rare cases can lead to human death. Early studies had shown that emetic toxin formation belongs to a homogeneous group of  Bacillus cereus sensu stricto  and the genetic determinants of cereulide (a 24-kb gene cluster of  cesHPTABCD ) are located on a 270-kb plasmid related to the  Bacillus anthracis  virulence plasmid pXO1.    Results  The whole genome sequences from seven emetic isolates, including two  B. cereus sensu stricto  and five  Bacillus weihenstephanensis  strains, were compared, and their inside and adjacent DNA sequences of the cereulide biosynthesis gene clusters were analyzed. The sequence diversity was observed, which classified the seven emetic isolates into three clades. Different genomic locations of the cereulide biosynthesis gene clusters, plasmid-borne and chromosome-borne, were also found. Potential mobile genetic elements (MGEs) were identified in the flanking sequences of the  ces  gene cluster in all three types. The most striking observation was the identification of a putative composite transposon, Tn ces , consisting of two copies of IS ces  element (belonging to IS 6  family) in opposite orientations flanking the  ces  gene cluster in emetic  B. weihenstephanensis . The mobility of this element was tested by replacing the  ces  gene cluster by a Km R  gene marker and performing mating-out transposition assays in  Escherichia coli . The results showed that Tn ces::km  transposes efficiently (1.04 × 10 -3  T/R) and produces 8-bp direct repeat (DR) at the insertion sites .Conclusions  Cereulide biosynthesis gene clusters display sequence diversity, different genomic locations and association with MGEs, in which the transposition capacity of a resistant derivative of the composite transposon Tn ces  in  E. coli  was demonstrated. Further study is needed to look for appropriate genetic tools to analysis the transposition of Tn ces  in  Bacillus  spp. and the dynamics of other MGEs flanking the  ces  gene clusters.

The genetic diversity of cereulide biosynthesis gene cluster indicates a composite transposon Tnces in emetic Bacillus weihenstephanensis