Bacillus cereus Nhe is a pore-forming toxin with structural and functional properties similar to the ClyA (HlyE, SheA) family of haemolysins, able to induce osmotic lysis in epithelia.
The mechanism by which Bacillus cereus causes diarrhoea is unknown. Three putative enterotoxins have been proposed, haemolysin BL (Hbl), cytotoxin K and non-haemolytic enterotoxin (Nhe). Both Hbl and Nhe are three-component cytotoxins and maximal cytotoxicity of Nhe against epithelia is dependent on all three components. However, little is known of the mechanism of cytotoxicity. Markers of plasma membrane disruption, namely propidium iodide uptake, loss of cellular ATP and release of lactate dehydrogenase (LDH), were observed in epithelia exposed to Nhe from culture supernatants of B. cereus, but not in those exposed to supernatants from a mutant strain lacking NheB and NheC. Consistent with an exogenous cause of membrane damage, purified Nhe components combined to form large conductance pores in planar lipid bilayers. The inhibition of LDH release by osmotic protectants and the increase in cell size caused by Nhe indicate that epithelia lyse following osmotic swelling. Nhe and Hbl show sequence homology, and Hbl component B has remarkable structural similarities to cytolysin A (ClyA), with both structures possessing an alpha-helix bundle and a unique subdomain containing a hydrophobic beta-hairpin. Correspondingly, we show that Nhe has haemolytic activity against erythrocytes from a variety of species. We propose that the common structural and functional properties indicate that the Hbl/Nhe and ClyA families of toxins constitute a superfamily of pore-forming cytotoxins.
Fagerlund
Annette
A
Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, PO Box 8146 Dep., N-0033 Oslo, Norway.
Lindbäck
Toril
T
Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, PO Box 8146 Dep., N-0033 Oslo, Norway.
Storset
Anne K
AK
Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, PO Box 8146 Dep., N-0033 Oslo, Norway.
Granum
Per Einar
PE
Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, PO Box 8146 Dep., N-0033 Oslo, Norway.
Hardy
Simon P
SP
Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, PO Box 8146 Dep., N-0033 Oslo, Norway.
D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001409:Bacillus cereus / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics / Q000302:isolation & purification / Q000633:toxicity*; D018938:Caco-2 Cells; D048429:Cell Size; D002522:Chlorocebus aethiops; D003603:Cytotoxins / Q000235:genetics / Q000302:isolation & purification / Q000633:toxicity*; D004847:Epithelial Cells / Q000382:microbiology*; D017353:Gene Deletion; D006461:Hemolysis; D006801:Humans; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D008051:Lipid Bilayers / Q000378:metabolism; D008969:Molecular Sequence Data; D009995:Osmosis; D052899:Pore Forming Cytotoxic Proteins / Q000235:genetics / Q000302:isolation & purification / Q000633:toxicity*; D011419:Propidium / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D014709:Vero Cells
D001426:Bacterial Proteins; D003603:Cytotoxins; D008051:Lipid Bilayers; D052899:Pore Forming Cytotoxic Proteins; D011419:Propidium; D000255:Adenosine Triphosphate; D007770:L-Lactate Dehydrogenase