Bacillus cereus sensu lato is composed of: Bacillus cereus sensu stricto, Bacillus thuringiensis, Bacillus anthracis, Bacillus mycoides, Bacillus pseudomycoides, and the recently described Bacillus weihenstephanensis. Most of these have a great impact on human activity. B. cereus and B. anthracis are well-known pathogens of mammals (including humans); B. thuringiensis is a commonly used insecticide, while B. mycoides improves plants growth. The psychotropic B. weihenstephanensis is a serious problem in food cold-storing. B. cereus s.s. produces one emetic toxin causing emesis and at least six different enterotoxins such as: hemolytic enterotoxin (HBL), non-hemolytic enterotoxin (NHE), enterotoxin T (BcET), hemolysin II, cytotoxin K (CytK), and enterotoxin FM (EntFM). HBL, NHE, and CytK have been involved in food poisoning. The other bacilli of the B. cereus group are also reported to produce enterotoxins which may lead to serious outbreaks of illness. Thus, the consequences of the above study in the area of food safety need to be seriously evaluated.
Bacillus thuringiensis subsp. thuringiensis IS5056, a strain highly toxic to Trichoplusia ni larvae, produces the newly described Cry1Ab21 δ-endotoxin encoded by a gene located in the 63.8 kb pIS56-63 plasmid. In this report we present the structure and functional similarity of this plasmid to other B. thuringiensis large toxigenic plasmids with particular interest focused on its modular architecture. The 61 open reading frames (ORFs) of the plasmid made four functional modules: (i) M1-mic, the mobile insertion cassette harboring cry1Ab21;(ii) M2-tra, the putative conjugative element; (iii) M3-reg, regulation sequence; and (iv) M4-rep, the ori44 replicon. These modules display similarity to corresponding sequences in distinct B. thuringiensis plasmids, but, in general, not to plasmid of other Bacillus cereus sensu lato. The nucleotide sequence and organization of genes in pIS56-63 were highly similar (80–100%) to those in pHT73 of B. thuringiensis HT73, and in p03 of B. thuringiensis HD771, particularly within the M3-reg and M4-rep modules, and slightly less in M2-tra, the latter of which is composed of two segments exhibiting homology to sequences in pBMB28, pAH187_45, pCT83, and pIS56-85 or to pCT72, pBMB67, p04, and pIS56-68. The tetrapartite structure of the toxigenic pIS56-63 plasmid strongly suggests that its hybrid nature is a result of recombination of various genetic elements originating from different extrachromosomal and chromosomal sources in B. thuringiensis. The presence of cry1Ab21 in the mobile cassette suggests that its occurrence on pIS56-63 resulted from recombination and transposition events during the evolution of the plasmid.
Cereulide produced by Bacillus cereus sensu stricto and valinomycin synthesized mainly by Streptomyces spp. are natural dodecadepsipeptide ionophores that act as potassium transporters. Moreover, they comprise three repetitions of similar tetrapeptide motifs synthesized by non-ribosomal peptide synthesis complexes. Resemblances in their structure find their reflections in the same way of action. The toxicity of valinomycin and cereulide is an effect of the disturbance of ionic equilibrium and transmembrane potential that may influence the whole organism and then cause fatal consequences. The vim and ces operons encoding valinomycin and cereulide are both composed of two large, similar synthetase genes, one thioestrase gene and four other ORFs with unknown activities. In spite of the characterization of valinomycin and cereulide, genetic determinants encoding their biosynthesis have not yet been clarified.