The aim of the present work was to evaluate the effect of spent culture supernatants of different strains of lactobacilli on giardia trophozoites. The growth of Giardia intestinalis strain WB, as well as the attachment to the human intestinal epithelial cell line Caco-2, was evaluated by using proliferation and adhesion assays with radiolabeled parasites. In addition, scanning electron microscopy and flow cytometric analysis were performed. The effect of spent culture supernatants from lactobacilli was strain dependent. Lactobacillus johnsonii La1 significantly inhibited the proliferation of G. intestinalis trophozoites. Although the effect was strongly pH dependent, it was not simply due to lactic acid. According to flow cytometric analysis, trophozoites were arrested in G 1 phase but neither significant necrosis nor apoptosis could be detected. Bacterial cells or their spent culture supernatants were unable to modify trophozoite attachment to Caco-2 cells. However, trophozoites treated with spent culture supernatants had little, if any, proliferative capacity. These results suggest that La1 produces some substance(s) able to inhibit proliferation of Giardia trophozoites. Partial characterization of the factors involved in the antigiardiasic action showed that they have a low molecular mass and are inactivated by heating. On this basis, it seems worthwhile to explore how colonization of the proximal small bowel with these lactic acid bacteria could interfere with giardiasis in vivo.

In the present study, the role of direct procaryote-eucaryote interactions in the virulence of Bacillus cereus was investigated. As a model of human enterocytes, differentiated Caco-2 cells were used. Infection of fully differentiated Caco-2 cells with B. cereus in the exponential phase of growth, in order to minimize the concentration of spores or sporulating microorganisms, shows that a strain-dependent cytopathic effect develops. Interestingly, addition of 3-h-old cultures of some strains resulted in complete detachment of the cultured cells after a 3-h infection whereas no such effect was found after a 3-h infection with 16-h-old cultures. Infection of enterocyte-like cells with B. cereus leads to disruption of the F-actin network and necrosis. Even though the effect of secreted factors cannot be ruled out, direct eucaryote-procaryote interaction seems to be necessary. In addition, we observed that some B. cereus strains were able to be internalized in Caco-2 cells. Our findings add a new insight into the mechanisms of virulence of B. cereus in the context of intestinal infection.Bacillus cereus is a spore-forming microorganism responsible for different pathological processes. In the context of intestinal infections, this microorganism is associated with emetic and diarrheic syndromes, and its role in extraintestinal infections has been also reported (14,15). So far, the virulence of B. cereus has been ascribed to the production of exocellular factors such as (i) cereolysin O, a thiol-activated cholesterol binding cytolysin (3, 25); (ii) phospholipases (8, 10, 23); (iii) emetic toxin (cereulide), a heat-stable cyclic dodecadepsipeptide with a molecular mass of around 1.2 kDa (1, 2, 37); (iv) hemolysin BL, a tripartite enterotoxin that requires all the three components (B, L1, and L2) for maximal activity (5,7,8,9,10,15,35); (v) the nonhemolytic enterotoxin complex, a complex of three subunits (34); (vi) hemolysin IV (Hly-IV), which has a strong effect on plasma membranes with a wide range of compositions (8); and (vii) the cytotoxic protein CytK, associated with necrotic enteritis (26,34).A cellular response triggered by the interaction between microorganisms and intestinal epithelial cells contributes to the virulence of several pathogens. These biological effects are related to the binding of bacterial surface structures and/or bacterial toxins to cell receptors coupled with cellular signaling pathways. Attack of the host cell cytoskeleton is the main mechanism by which pathogenic microorganisms structurally and functionally alter the host cells, particularly the intestinal epithelial barrier. This interference with the organization of the cytoskeleton can be mediated by enzymatic modification by soluble toxins (12) or by upregulation of depolimerization cascades by direct prokaryote-eucaryote interactions (11,24,39). The ability of microorganisms of the genus Bacillus to interact with Caco-2 and HeLa cells has been reported, but no further insight into the role of this interaction in the virulence of this...

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