Abstract:
The present invention relates to the use of a supernatant of lactic acid bacteria or of Bifidobacteria capable of preventing colonization of intestinal cells by  Giardia intestinalis , for the preparation of an ingestible carrier for the treatment and/or prophylaxis of disorders associated with the colonization of the gut by  Giardia intestinalis . The present invention also pertains to specific strains of Bifidobacterium having the above traits and to the ingestible carrier, such as a food or pharmaceutical composition, containing such supernatant or the microorganisms.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to lactic acid bacteria capable of preventing colonization of intestinal cells by  Giardia intestinalis , or to a culture supernatant thereof, respectively, for use in the treatment and/or prophylaxis of disorders associated with the colonization of the gut by  Giardia intestinalis . The present invention also pertains to strains of Bifidobacterium having the above traits and to the use of lactic acid bacteria for the preparation of an ingestible carrier, such as a food or pharmaceutical composition, for the treatment and/or prophylaxis of an infestation of the intestine by  Giardia intestinalis .  
         [0002]     The present invention relates to lactic acid bacteria capable of preventing colonization of intestinal cells by  Giardia intestinalis , or to a culture supernatant thereof, respectively, for use in the treatment and/or prophylaxis of disorders associated with the colonization of the gut by  Giardia intestinalis . The present invention also pertains to strains of Bifidobacterium having the above traits and to the use of lactic acid bacteria for the preparation of an ingestible carrier, such as a food or pharmaceutical composition, for the treatment and/or prophylaxis of an infestation of the intestine by  Giardia intestinalis.    
         [0003]     Cysts are dormant, quadrinucleate and ovoid forms responsible for transmission of giardiasis. After ingestion of cysts by an individual, excystation is triggered by exposure to gastric acid and/or digestive enzymes. The parasite thus emerging, which is termed trophozoite, is binucleate and half-pear shaped having a size of around 10 μm with a broad anterior and a narrow posterior side. The ventral side is by and large covered by the ventral disk which is deemed to at least in part account for an attachment of the trophozoite to the intestinal surface.  
         [0004]     After excystation trophozoites may persist in the small intestine of the infected individuum for a long time amounting even to years. If trophozoites are carried downstream by the flow of intestinal fluid they again start to encyst to adapt to a new environment.  
         [0005]     Cysts are excreted with the faeces and may withstand a variety of extreme environmental conditions, including different temperature, pH and tonicity conditions. An infected individual may secrete about 9×10 8  cysts per day, with doses to as low as 10 cysts proved to be sufficient to produce infection in another individual. Transmission of  Giardia intestinalis  may be accomplished in a variety of different ways with the main routes of transmission being waterborne and foodborne. Person to person spread is supported by some daycare and nosocomial outbreaks. In addition, wild animals are found to be infectious reservoirs of  Giardia intestinalis  and may contribute to a spread of the pathogen.  
         [0006]     Giardiasis, like other intestinal diseases, is more severe in infants and children. Infection is normally associated with diarrhea and malabsorption resulting in an impaired growth and development of the child and may eventually also lead to the death of newbornes.  
         [0007]     Around one half of infected people are, however, asymptomatic and contribute to the spread of the pathogen, since due to the absence of any perceptible symptoms no corresponding regimen is applied.  
         [0008]     Though a great deal of scientific effort has been invested on the investigation of  Giardia intestinalis  pathogenesis of giardiasis could not be explained by means of a single virulence factor alone and no toxic compound has been isolated so far.  
         [0009]     The light microscopic appearance of a gut colonized by  Giardia  has been found to be rather variable ranging from a normal mucosal structure to a subtotal villous atrophy. Results of electron microscopy investigations revealed ultrastructural changes such as shortening and disruption of microvilli (Chavez et al., Experimental Parasitol. 80 (1995), 133-138).  
         [0010]     Such structural abnormalities have been found to be accompanied by a reduction in lactase, sucrase and maltase activities in the microvillus membrane as well as to an impaired intestinal transport (Buret et al, Gastroenterology 103 (1992), 506-513; Roberts-Thomson et al, Gastroenterology. 71 (1976), 57-61).  
         [0011]     In addition to the two forms illustrated above  Giardia intestinalis  has developed an extremely changing surface structure and has evolved a family of protective proteins that cover all exposed surfaces (Aley et al., Infect. Agents Dis. 4 (1995), 161-166; Muller, et al., Infect. Immun. 64 (1996), 1385-1390; Nash et al., J. Euk. Microbiol. 42 (1995), 604-609). These variable surface proteins (VSPs) protect trophozoites from both immunological and environmental factors, such that  Giardia  may evade the host&#39;s defenses and may survive in a highly degradative environment such as is prevailing in the intestinal tract of mammals. A modification in the VSPs occurs at about every 6th to 13 th  generation making it difficult or nearly impossible for the host&#39;s immune system to develop a specific response against the pathogen. Immune and environmental stress can select different VSP phenotypes. Furthermore, antigen switching of VSPs after excystation has also been reported (Gillin et al., Annu. Rev. Microbiol. 50 (1996), 679-705).  
         [0012]     Human giardiasis is associated with an increase in the number of lamina propria and intraepithelial lymphocytes suggesting that T-cell activation could be responsible for micro-villous damage. However, induced immunosuppression in mice resulted in a more profound effect on microvillous associated enzymes as compared to non-immunosupressed animals, indicating that during an infection by  Giardia intestinalis  epithelial damage seems not be dependent on immune function alone.  
         [0013]     A common therapy of giardiasis is the administration of antibiotics, such as those belonging to the class of nitroimidazoles. Yet, in endemic areas the response to therapy has been shown to be rather inconsistent (Katelaris et al., Aliment. Pharmacol. Ther. 8 (1994), 187-192). Moreover, due to the at least in part destruction of the intestine&#39;s natural microflora, the administration of such antibiotics is always accompanied by severe side effects such as extended diarrhea or even an expanded infestation of the gut by other detrimental microorganisms, such as yeast.  
         [0014]     Hence, there is a need in the art for additional options to treat giardiasis or means to prevent infestation of an individual by the pathogen.  
         [0015]     The problem of the present invention therefore resides in providing additional means for the treatment or prophylaxis of an infection by  Giardia intestinalis.    
       SUMMARY OF THE INVENTION  
       [0016]     This problem has been solved by providing the use of a culture supernatant of a Lactic acid bacterium Lactic acid bacterium or a Bifidobacterium capable of preventing adhesion of  Giardia intestinalis  to intestinal cells for the preparation of an ingestible carrier for the treatment and/or prophylaxis of disorders associated with the colonization of the gut by  Giardia intestinalis.    
         [0017]     According to a preferred embodiment the present invention provides novel Lactic acid bacteria and Bifidobacteria, respectively, capable to prevent adhesion of  Giardia intestinalis  to intestinal cells, which are selected from the group consisting of NCC 90 (1-2332), NCC 189 (I-2333) or NCC 200 (I-2334). These microorganisms have been deposited according to the Budapest Treaty with the Institute Pasteur on November 30 th , 1999 and received the above mentioned deposit numbers. The present invention pertains also to food and pharmaceutical compositions containing such microorganisms.  
         [0018]     The microorganisms may be included in the carrier in an amount of from about 10 6  to about 10 12  pfu (plaque forming units). They may be included as such or optionally after essentially purifying them from the cultivating medium. Alternatively, the supernatant of a culture of such microorganisms may be included in the carrier, which prior to its inclusion may preferably be concentrated by means well known in the art.  
         [0019]     The ingestible carrier may be a food or a pharmaceutical composition such as milk, yogurt, curd, cheese, fermented milks, milk based fermented products, ice-creams, fermented cereal based products, milk based powders, infant formulae or any sort of pet food. Such carriers may e.g. be easily manufactured by using a microorganisms having the corresponding traits for fermentation of the starting materials itself. Alternatively, the microorganisms or an optionally concentrated culture supernatant thereof may be added to the respective carrier in liquid or dry form. A pharmaceutical composition, such as a tablet, a liquid bacterial suspension, a dried or wet oral supplement, a dry or a wet tube feeding may be prepared using standard techniques while including the microorganisms and/or an optionally concentrated culture supernatant thereof into the carrier. Depending on the mode of administration the skilled person will select the formulation deemed to be appropriate.  
         [0020]     Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0021]      FIG. 1  shows adhesion of Portland-1 strain to Caco-2 cells when 10 5  trophozoites/cm 2  were added;  
         [0022]      FIG. 2  shows the results of adhesion assays performed with undifferentiated HT-29 cells;  
         [0023]      FIG. 3  shows a scanning electron micrography of strain UNO attached to differentiated Caco-2 cells;  
         [0024]      FIG. 4  shows the microvilli damage at the epithelial cell brush border by  Giardia intestinalis.    
         [0025]      FIG. 5A  shows the results of coincubation of giardias, lactic acid bacteria and Caco-2 cells;  FIG. 5B  shows the results of preincubation of Caco-2 monolayers with lactic acid bacteria.  
         [0026]      FIG. 6A  shows the results of an adhesion experiment, wherein Portland-1 and WB were contacted with a neutralized supernatant of the lactic acid bacteria of the present invention;  
         [0027]      FIG. 6B  shows the results of an adhesion experiment, wherein WB and CIDCA were contacted with a neutralized supernatant of the lactic acid bacteria of the present invention;  
         [0028]      FIG. 7A  shows the results obtained by incubating the strain WB with acid supernatants (pH 4) (DMEM:supernatant=1:1) during 1 hr.  
         [0029]      FIG. 7B  shows that neutralization of supernatants (pH 7) abolishes inhibitory effect.  
         [0030]      FIG. 8A  shows the results obtained by incubating WB strain with supernatants of La10 strain adjusted to different pH.  
         [0031]      FIG. 8B  shows the results of diluting acid supernatants with MRS.  
         [0032]      FIG. 9  shows a low cytometric analysis of suspensions of strain WB 1 incubated during 1 h at 37° with MRS acidified to pH 5 (DMEM:MRS=1:1);  
         [0033]      FIG. 10  shows the results of an inoculation of parasites preincubated at pH 4 or 7 in TYI-S-33 medium with 100 μCi of  3 H adenine.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     During the studies leading to the present invention the inventors have hypothesized that an ecological approach in the treatment and/or prevention of giardiasis could be a colonization of the intestine with bacteria that might be able to antagonize with the parasite.  
         [0035]     Attachment of  Giardia intestinalis  trophozoites to epithelial cells is deemed to be a key step for infection in humans and animals. Although the mechanisms for  Giardia  adhesion are not fully understood, evidence supports the ventral disk, trophozoite contractile elements, hydrodynamic and mechanical forces and lectin mediated binding to be involved.  
         [0036]     Taking into account that lactic acid is an important factor in  Giardia&#39;s  life cycle the interaction between  Giardia intestinalis  and lactic acid bacteria has been studied using enterocyte like cells in culture.  
         [0037]     To this end, the following microbial parasite strains and culture media have been used:  
         [0038]     Microbial Strains:  
         [0039]     Bacterial strains La1 ( Lactobacillus johnsonii ) (I-1225) and La10 ( Lactobacillus acidophilus ) (I-2332) were from Nestec collection (Lausanne, Switzerland). Strains CIDCA 536 ( Bifidobacterium bifidum ) and CIDCA 538 ( Bifidobacterium infantis ) were from the collection of Centro de Investigacion y Desarrollo en Criotecnologia de Alimentos (Universidad Nacional de La Plata, Argentina) and have been deposited according to the Budapest Treaty receiving the deposit nos. 12333 and i-2334, respectively.  
         [0040]      Giardia intestinalis  strains Portland-1 (ATCC 30888), UNO/04/87/1 (ATCC 50184), New Orleans-1 (ATCC 50137) and WB (ATCC 30957) were purchased from American Type Culture Collection (Rockville, USA).  
         [0041]     Bacteria were grown for 24 hours at 37° C. in MRS broth supplemented with 0,05% cysteine hydrochloride (Sigma). Incubations were performed under anaerobic conditions (BBL GasPak Plus).  
         [0042]     Protozoa were grown in Keister&#39;s modified TYI-S-33 medium containing (per liter): casein digest (Difco), 20 g; yeast extract (BBL), 10 g; dextrose (Merck), 10 g; bovine bile (Difco), 0.75 g; NaCl (Merck), 2 g; L-cystein. HCl (Sigma), 2 g; ascorbic acid sodium salt (Fluka), 0.2 g; K 2 HPO 4  (Merck), 1 g; KH 2 PO 4  (Merck), 0.6 g; ferric ammonium citrate (Sigma), 22.8 mg, adult bovine serum (Sigma), 100 ml; penicillin/streptomycin (Gibco, 1000 IU/ml, 1000 μg/ml), 15 ml. Horse serum (Gibco) instead of bovine serum was also assayed. The pH was adjusted to 6.9 with 1 N NaOH prior filter sterilization (022 μm pore size). Keister&#39;s modified medium supplemented with 10% horse serum did not support growth of strain Portland-1 although incubation was extended during 11 days and despite the fact that trophozoites were initially attached to the tube walls. During the first 5 days of incubation, an high proportion of motile parasites was observed. After this time agglutination of trophozoites and a dramatic decrease of viable cells were observed. Addition of bovine serum (10%) to these damaged cells allowed for growth after 24 hrs. To increase the surface available for  Giardia  attachment, 25 cm 2  tissue culture bottles were used (plastic bottles). Culture medium was added up to the bottleneck to maintain anaerobic conditions. This procedure resulted in yields of around 10 7  trophozoites/bottle within 2-3 days of incubation and parasites formed a confluent monolayer on the bottle walls.  
         [0043]     Subcultures were made by chilling cultures in an ice bath (5-10 min) to detach adhering trophozoites and inoculating 0.2 ml of the resulting suspension into fresh medium. Incubations were performed at 37° C. for 72 hours and different recipients (glass or plastic) were used.  
         [0044]     Storage of  Giardia intestinalis  Strains:  
         [0045]     Trophozoites were detached as indicated above and suspended in TYI-S-33 medium. Two fold concentrated cryoprotectant solution (DMSO, sucrose) prepared in TYI-S-33 medium, was added to the suspension in three equal aliquots at 2-minute intervals. Final concentrations of cryoprotectants were 12% (v/v) DMSO, 4% (w/v) sucrose. Suspensions (around 1×10 6  trophozoites/ml) were allowed to equilibrate during 20 minutes at room temperature before start of the cooling cycle.  
         [0046]     Vials were thermally isolated with polystyrene (4 cm wall thickness) and placed at −80° C.  
         [0047]     Reactivation of Frozen Throphozoites:  
         [0048]     Vials were thawn in a water bath at 37° C. and immediately inoculated in TYI-S-33 medium at a ratio parasite suspension/fresh medium=0.5/7. Cultures were incubated at 37° C. in the dark.  
         [0049]     The present invention will now be illustrated by way of examples.  
         [0050]     As a model for intestinal cells the cell lines Caco-2 and HT29 were used. Adhesion to said enterocyte like cells was tested as follows:  
       EXAMPLE 1  
     Radiolabelling of Trophozoites  
       [0051]     Parasites were inoculated in TYI-S-33 and 2.6 to 7.1 μCi/ml of either 2- 3 H adenine (21 Ci/mmol, 1 mCi/ml, Amersham Life Science) or methyl  3 H thymidine (2 Ci/mmol, 1 mCi/ml, Amersham Life Science) were added. Incubations were performed at 37° C. for 48-72 hrs.  
         [0052]     Growth of trophozoites with 7.1 μCi/ml of either  3 H-adenine or  3 H-thymidine gave very different DPM/parasite ratios (Table 1). Although thymidine concentration was 10-fold higher as compared to that of adenine, incorporation was very low with the result that 0.08 and 2.8  
         [0053]     DPM/parasite were obtained, respectively.  
                                 TABLE 1                           Incorporation of  3 H by  Giardia  strain Portland-1       adding 7 μCi/ml of radiolabeled bases                Radiolabeled substance   Mmols/ml   DPM/parasite                       2- 3 H adenine   3 × 10 −7     2.8 ± 0.10           Methyl- 3 H thymidine   35 × 10 −7     0.08 ± 0.00                       
 
         [0054]     Table 2 shows the results of the adenine incorporation by  Giardia intestinalis  strains. Suitable radiolabelling was obtained by using adenine concentrations as low as 2.5 μCi/ml. In these conditions, DPM/parasite ranged from 0.3 for Portland-I strain to 1.6 for UNO strain.  
                                               TABLE 2                           Incorporation of 2- 3 H adenine by different strains of  Giardia  Results       are averages of at least two determinations.                DPM per parasite                Strain   7 μCi/ml   5 μCi/ml   2.5 μCi/ml                       Portland-1   2.8 ± 0.1   0.9 ± 0.1   0.3 ± 0.0           UNO   ND   1.2 ± 0.2   1.6 ± 0.1           WB   ND   1.0 ± 0.2   1.4 ± 0.1           New Orleans-1   ND   2.3 ± 0.8   0.6 ± 0.1                      
 
       EXAMPLE 2  
     Cell Cultivation  
       [0055]     Caco-2 cells were grown in DMEM supplemented with non essential amino acids, penicillin (12 IU/ml), streptomycin (12 μg/ml), gentamicine (47 μg/ml) and inactivated fetal calf serum (20% v/v). Monolayers were prepared in 6-well tissue culture plates by seeding 2×10 5  cells per well. Incubations were performed at 37° C. in a 10% CO 2 /90% air atmosphere. The culture medium was changed each 2 nd  day. Adhesion assays were performed with cells between passages 49 and 51 and only monolayers in late postconfluence stage were used (at least two weeks in culture).  
         [0056]     Undifferentiated HT-29 cells (American Type Culture Collection, Rockville, Md.) were grown in DMEM containing 4.5 g/l glucosa (Seromed, Biochrom KG, Berlin, Germany) supplemented with Glutamax 1 (0.01% v/v; L-alanyl-Lglutamine 200 mM, GIBCO, Basel, Switzerland), gentamycin (0.57 mg/ml, GIBCO, Basel, Switzerland) and fetal calf serum (10% v/v, GIBCO, Basel, Switzerland). Cells were used at passage 52.  
       EXAMPLE 3  
     Adhesion Assays  
       [0057]     When confluent monolayers of trophozoites were obtained, the culture medium was discarded, thus eliminating any parasites not attached to a surface. Attached parasites were harvested as described and washed 3 times with DMEM (Gibco) containing 11.4 mM cysteine HCl. Dilutions (1:2) of parasites in paraformaldehyde 1% were enumerated in an hemocytometer.  
         [0058]     Radiolabelled trophozoites suspended in DMEM-CYS (DMEM+11.4 mM cysteine HCl) were added to monolayers of Caco-2 cells and incubated for 1 h at 37° C. in a 5% CO 2 /95% air atmosphere. Monolayers were then washed 3 times with DMEM-CYS at 37° C. to detach unbound trophozoites.  
         [0059]     After washing 1 ml of 1 N NaOH was added per well and the plates were incubated during 1 hr at room temperature. Cell lysates were placed in scintillation vials and a further wash of the wells with PBS was performed. Radioactivity was measured with a β-counter (RackBeta Spectral, LKB, Wallac).  
       EXAMPLE 4  
     Preincubation Assays  
       [0060]     Bacterial cultures were washed 3 times with PBS. Suspensions, that had been adjusted to 1×10 8  CFU/ml in DMEM, were added to Caco-2 monolayers and plates were incubated for 1 hr at 37° C. After adhesion of the bacterial suspensions radiolabelled trophozoites, adjusted to 1×10 5  parasites/ml, were added. Adhesions assays were performed as indicated above.  
       EXAMPLE 5  
     Coincubation Assays  
       [0061]     1 ml aliquots of parasitic suspensions containing 1×10 5  radiolabelled  Giardia  per ml were mixed with 1 ml of bacterial suspensions (1×10 8  CFU/ml) in 6-well tissue culture plates containing Caco-2 monolayers. Adhesion assay were performed as indicated above.  
       EXAMPLE 6  
     Action of Lactic Acid Bacteria Supernatants on Adhesion of  Giardia intestinalis    
       [0062]     Cultures of lactic acid bacteria were centrifuged at 900 g during 1.5 min and supernatants were neutralized (pH 6.9-7.4) with 4 N NaOH. 1 ml of a suspension containing 10 5  radiolabelled trophozoites/ml in DMEM-CYS was mixed with 1 ml of neutralized supernatant in 6-well tissue culture plates containing Caco-2 monolayers. Adhesion assays were performed as indicated above. Appropriate controls with MRS or MRS acidified to pH 4.5 with lactic acid and then neutralized to pH 7 were included in the assays.  
         [0063]     Preincubation assays were performed by incubating parasitic suspensions (DMEM-cys) with an equal volume of supernatant for 1 h at 37° C. In another series of experiments, parasites were suspended in pure supernatants or supernatants diluted with MRS. After incubation, giardias were suspended in DMEM and added to Caco-2 cells.  
         [0064]     Assessment of growth was performed by incubating parasites in TYI-S-33 medium with 100 μCi of  3 H adenine in 40 ml of culture. Flow cytometric analysis was done using a blue-green excitation light (FACScan™).  
       EXAMPLE 7  
       [0065]     Scanning electron microscopy Caco-2 cells were grown on round glass slides (diameter, 10 mm) into 24-well tissue culture plates and adhesion of trophozoites was done as indicated. Specimens were fixed by the addition of 25% glutaraldehyde in PBS for 16 h at 4° C. Post-fixation was performed with 2% osmium tetroxide at room temperature for 2 hrs and then smears were dehydrated in a graded series of ethanol solutions (solutions of ethanol in water of 30, 50, 70, 90, 100%). Finally, samples were critical point dried using CO 2 , gold coated and examined using a Philips SEM 505 at an accelerating voltage of 30 KV.  
         [0066]     EXAMPLE 8  
         [0067]     Adhesion of  Giardia intestinalis  trophozoites to Caco-2 cells A total of 10 5  trophozoites were added to a caco-2 culture prepared as detailed in example 2. The results of the adhesion assays are shown in  FIG. 1 . Adhesions of around 5% were obtained despite the age of the monolayers. Attachment to plastic surfaces was very high (21.8±2.4%).  
         [0068]     The results of adhesion assays performed with undifferentiated HT-29 cells are shown in  FIG. 2 . Values of 7.0±1.2% and 5.5±0.6% were obtained for strains Portland-1 and WB respectively.  
         [0069]     Scanning electron micrography of strain UNO attached to differentiated Caco-2 cells is shown in  FIG. 3 . Most  Giardia  trophozoites are situated with the ventral side attached to the monolayer. Some parasites were also seen with their dorsal surface facing Caco-2 cells (arrow). As may be clearly derived from  FIG. 4 , attachment of trophozoites produces an imprint on the brush border of enterocyte like cells.  
         [0070]     Coincubation of  giardias , lactic acid bacteria and Caco-2 cells did not change attachment ( FIG. 5A ) Furthermore, preincubation of Caco-2 monolayers with lactic acid bacteria did not interfere with parasite adhesion ( FIG. 5B ).  
       EXAMPLE 9  
     Influence of Supernatants of Lactic Acid bacteria Cultures on  Giardia intestinalis  Adhesion to Caco-2 Cells  
       [0071]     Coincubation of the strains Portland-1 and WB with neutralized supernatants of the lactic acid bacteria to be used according to the present invention produced a reduction of adhesion to Caco-2 cells ranging from 19% (La1 strain) to 40% (LalO strain) for Portland-1 strain ( FIG. 6 A ). For strain WB, values ranged from 20% for strain CIDCA 536 to 40% for strain La10 ( FIG. 6B ). Although differences between the strains could not be established, strain La10 showed significant differences as compared to the MRS control for both, the Portland-1 (P=0,06) and the WB strain (P=0.04).  
         [0072]     Taking into account that the range of pH compatible with the coincubation experiments with Caco-2 cells is narrow, assays were performed by preincubating parasites with supernatants and then by resuspending them in DMEM-cysteine before the adhesion assays. Incubation of strain WB with acid supernatants (pH 4) (DMEM: supernatant=1:1) during 1 hr produced a reduction of adhesion from 15% for MRS to around 3% for all the strains under study ( FIG. 7A ). Neutralization of supernatants (pH 7) eliminated the inhibitory effect and an increase in adhesion was found ( FIG. 7B ).  
         [0073]     The results obtained by incubating WB strain with supernatants of La10 strain adjusted to different pH are shown in  FIG. 8A . No differences as compared to the MRS control at pH 4 and 5 were found but adhesion was found to be higher at a pH of 6 and 7.  
         [0074]     Dilution of acid supernatants with MRS produced an increase in pH and adhesion was reduced with supernatants diluted 1/8 (relative concentration 0.125, pH 4.07,  FIG. 8B ).  
         [0075]     Flow cytometric analysis of suspensions of strain WB 1 incubated for 1 hr at 37° with MRS acidified to pH 5 (DMEM:MRS=1:1) showed differences in size (FSC) and cytoplasmatic complexity (SSC) ( FIG. 9 ). Inoculation of parasites preincubated at pH 4 or 7 in TYI-S-33 medium with 100 μCi of  3 H adenine showed differences in incorporation ( FIG. 10 ). Low adenine incorporation was detected at pH 4 after 28 hrs of incubation in both MRS and supernatants samples. Although some parasites were attached to the bottle walls, they were not motile. MRS broth, acidified to pH 4.05 and neutralized, showed values of DPM representing less than 20% of the control. For neutralized supernatants of strain La10 values were around 80% of the control and differences were significant at 0.09 level. Big aggregates of parasites were found at pH 7 in both MRS and supernatants.  
         [0076]     The results for  Giardia  adhesion to different cell lines ( FIGS. 1 and 2 ) suggests that non specific attachment seems to be an important feature of this system, since no significant differences were found between differentiated cells (Caco-2 with different times in culture) and undifferentiated cells (HT-29). Furthermore, adhesion to plastic surfaces was very high. Adhesion of trophozoites to Caco-2 monolayers obviously produced microvilli damage ( FIG. 4 ).  
         [0077]     Experiments of exclusion of parasites with a washing step after bacterial adhesion led to a damage of the monolayer resulting in exclusion values that did not seem to be authentic. The degree of damage depended on the washing solution ranging from effacing of microvilli with DMEM-cys (scanning electron microscopy) to cell detachment with PBS. Taking into account the above findings, assays were performed, wherein monolayers were preincubated with bacteria suspended in DMEM-cys but without a washing step prior to the addition of the parasites. Another series of experiments was performed by coincubating parasites and bacteria with Caco-2 cells. The results obtained ( FIG. 5 ) are compatible with “active” attachment of trophozoites mediated by flagella motility and contractile elements. Lactic acid bacteria, being non motile, could not compete with parasites for attachment.  
         [0078]     Concerning the action of supernatants of lactic acid bacteria cultures, they have been shown to inhibit  Giardia  adhesion. Without to be bound to any theory it is deemed that this inhibitory effect may be attributed mainly to particular organic acids secreted by the lactic acid bacteria.  
         [0079]     A preincubation of parasites with MRS acidified with lactic acid showed dramatic changes in cellular size and cytoplasmatic complexity and adhesion to Caco-2 cells was reduced both for supernatants and MRS controls. In these conditions, the viability of parasites is highly affected, as is shown in experiments wherein treated parasites are inoculated in fresh TYI-S-33.  
         [0080]     After contact with lactate at pH 4 growth of parasites essentially stopped although some parasites were able to attach to the bottle walls. It seemed that under these conditions lactic acid is lethal to  Giardia . At pH 7, the inhibitory effect of lactate is still present and differences between acidified MRS and supernatant of La10 strain could be explained by assuming differences in lactate concentration due to the differences in buffer capability between fresh MRS and spent supernatants.  
         [0081]      Giardia  aggregates were formed during preincubation of parasites with supernatants and they might account for the high values of adhesion founded at pH 6 and 7 ( FIG. 8 ). As stated before, the main mechanisms for  Giardia  adhesion requires parasite viability, though adhesion of death protozoa could be shown after treatment of trophozoites with supernatants at pH 4. These parasites were not able to multiply and thus will not be effective in producing parasitosis. Furthermore, neutralized supernatants of cultures of lactic acid bacteria retarded  Giardia  growth and produced aggregates unable to attach.  
         [0082]     In view of the above experimental results it becomes clear that the lactic acid bacteria to be used according to the present invention may be successfully applied for the treatment or prophylaxis of giardiasis.  
         [0083]     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.