The acute diarrheal disease cholera remains a significant public health problem, due to its ability to spread rapidly and kill a high proportion of those affected. Vibrio cholerae, the etiologic agent of cholera, is a highly motile non-invasive Gram-negative bacterium which colonizes the small intestine and produces and secretes a potent enterotoxin called cholera toxin (CT), the major virulence determinant primarily responsible for the disease cholera
Cholera toxin, composed of one A and five B subunits, are synthesized in the cytosol in unfolded form and translocated across the inner membrane via Sec dependent pathway to the periplasmic space where appropriate folding occurs leading to 3D-structure of CT. CT then translocates across the outer membrane of V. cholerae via type-II secretion system consisting of multi-protein complexes [2] and [3]. Biological action of CT initiates by binding of the pentameric B-subunit to the intestinal epithelial cells through GM1 receptor. A conformational change is induced when A subunit enters the cell via receptor mediated endocytosis which in activates the adenylate cyclase in the cell membrane and increases the cAMP production in intestinal epithelial cells [4].
Outer membrane vesicles or OMVs are discrete, closed outer membrane blebs produced naturally by growing cells and do not arise due to cell lysis or cell death [5]. Electron microscopic studies reveal that vesicles emerge as spherical bodies having a bilayer membrane and electron-dense luminal content. Biochemical analysis indicated that OMVs consist only of outer membrane (OM) and periplasmic components and do not contain inner membrane (IM) and cytoplasmic fractions [5]. OMVs has been found in a variety of Gram-negative bacteria [6] and [7] and several studies demonstrated that OMV play a role as protective transport vesicles, delivering toxins, enzymes and DNA to eukaryotic cells [5]. As early as 1967, formations of OMVs were demonstrated in growing cells of V. cholerae. The thin-section electron micrographs of V. cholerae cells revealed the bulging out and pinching off the cell wall in the form of vesicles encapsulating soluble periplasmic cargo [8] and [9]. Recently mucosal immunization by V. cholerae OMVs has been shown to induce long term protective immune response against this pathogen in a mice model [10] .
So far no reports of association of cholera toxin (CT) with OMVs are available. The present study is the first report which demonstrates that CT is associated with OMVs in a physiologically active form and binding and internalization of V. cholerae OMVs by intestinal epithelial cells is mediated by CT.