Bacillus cereus cytotoxins Hbl, Nhe and CytK are secreted via the Sec translocation pathway.
BACKGROUND
Bacillus cereus and the closely related Bacillus thuringiensis are Gram positive opportunistic pathogens that may cause food poisoning, and the three secreted pore-forming cytotoxins Hbl, Nhe and CytK have been implicated as the causative agents of diarrhoeal disease. It has been proposed that the Hbl toxin is secreted using the flagellar export apparatus (FEA) despite the presence of Sec-type signal peptides. As protein secretion is of key importance in virulence of a microorganism, the mechanisms by which these toxins are secreted were further investigated.


RESULTS
Sec-type signal peptides were identified in all toxin components, and secretion of Hbl component B was shown to be dependent on an intact Sec-type signal peptide sequence. Further indication that secretion of Hbl, Nhe and CytK is dependent on the Sec translocation pathway, the main pathway on which bacterial secretion relies, was suggested by the observed intracellular accumulation and reduced secretion of the toxins in cultures supplemented with the SecA inhibitor sodium azide. Although a FEA deficient strain (a flhA mutant) showed reduced toxin expression and reduced cytotoxicity, it readily secreted overexpressed Hbl B, showing that the FEA is not required for Hbl secretion. Thus, the concurrent lack of flagella and reduced toxin secretion in the FEA deficient strain may point towards the presence of a regulatory link between motility and virulence genes, rather than FEA-dependent toxin secretion.


CONCLUSIONS
The Hbl, Nhe and CytK toxins appear to be secreted using the Sec pathway, and the reduced Hbl expression of a FEA deficient strain was shown not to be due to a secretion defect.
Norwegian School of Veterinary Science, Department of Food Safety and Infection Biology, PO Box 8146 Dep, N-0033 Oslo, Norway. toril.lindback@nvh.no
Annette
A
Fagerlund
Toril
T
Lindbäck
Per Einar
PE
Granum
D000595:Amino Acid Sequence; D000818:Animals; D001409:Bacillus cereus / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001427:Bacterial Toxins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002522:Chlorocebus aethiops; D003603:Cytotoxins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004768:Enterotoxins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006460:Hemolysin Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D021382:Protein Sorting Signals; D021381:Protein Transport; D055571:Secretory Pathway; D016415:Sequence Alignment; D014709:Vero Cells
D001426:Bacterial Proteins; D001427:Bacterial Toxins; D003603:Cytotoxins; D004768:Enterotoxins; D006460:Hemolysin Proteins; D021382:Protein Sorting Signals; C083961:hemolysin BL protein, Bacillus