Abstract
BackgroundCereulide 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.ResultsThe 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, Tnces, consisting of two copies of ISces element (belonging to IS6 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 KmR gene marker and performing mating-out transposition assays in Escherichia coli. The results showed that Tnces::km transposes efficiently (1.04 × 10-3 T/R) and produces 8-bp direct repeat (DR) at the insertion sites.ConclusionsCereulide 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 Tnces in E. coli was demonstrated. Further study is needed to look for appropriate genetic tools to analysis the transposition of Tnces in Bacillus spp. and the dynamics of other MGEs flanking the ces gene clusters.
Highlights
Cereulide is a cyclic dodecadepsipeptide ionophore, produced via non-ribosomal peptide synthetases (NRPS), which in rare cases can lead to human death
The Bacillus cereus group consists of B. cereus sensu stricto, Bacillus thuringiensis, Bacillus anthracis, Bacillus weihenstephanensis, Bacillus mycoides, Bacillus pseudomycoides and Bacillus cytotoxicus, which share close genetic and biochemical relatedness
Most virulence genes of B. cereus are located on the chromosome [5,6] with the exception of the cereulide genetic determinants [7,8]
Summary
Cereulide is a cyclic dodecadepsipeptide ionophore, produced via non-ribosomal peptide synthetases (NRPS), which in rare cases can lead to human death. The Bacillus cereus group consists of B. cereus sensu stricto, Bacillus thuringiensis, Bacillus anthracis, Bacillus weihenstephanensis, Bacillus mycoides, Bacillus pseudomycoides and Bacillus cytotoxicus, which share close genetic and biochemical relatedness They have traditionally been classified as different species based on their distinct virulence characteristics or phenotypes [1,2], the formers are mostly directly associated with. The remaining members of the group, B. mycoides, B. pseudomycoides and B. weihenstephanensis, are mainly distinguished on the basis of their morphology (rhizoidal growth) and physiology (psychrotolerance), respectively [9,10], but may have enteropathogenic potential [11,12] In this respect, two B. weihenstephanensis isolates were found to produce a higher amount of cereulide than the reference B. cereus AH187 quantified by liquid chromatography mass spectrometry [13,14]. Severe and even lethal cases have been reported after the ingestion of food contaminated with high amounts of cereulide [21,22,23,24]
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