Gutiérrez Ramírez, Luz AdrianaAcosta Otálvaro, Elly Vanesa2012-07-052012-07-0520081794-4449http://hdl.handle.net/10567/492Introduction. Probiotics are living microorganisms with a great capacity against other microorganisms, this is why there is an interest in using them as natural bio-preservatives, reducing partially the use of chemical preservatives in food. Objective. To determine the bactericide effect of the raw extract of native L. cassei on the in Vitro growth of pathogenic bacteria, such as E.coli. Materials and methods. The raw extract was obtained by cultivating the microorganism in 20 ml of MRS breeding ground at 37 ºC Turing 72 h, anaerobically. The experiments were done in three periods of time: 5, 10 and 15 days. The method used to detect the activity of the extract was the experiment of diffusion in wells. Those that represented an inhibition halo above 2mm were considered as active. The results found were compared with those from a strain of Lactobacillus plantarum of for commercial use with a bactericide activity against Escherichia coli. Results. L.cassei represented hostile activity against E.coli, because the inhibition zones read as response variable were higher than 2mm until getting the 10-3 dilution in the three experiments made. However, on day 15 the bactericide activity of the extract was considerably reduced when compared to that from day 5. The analysis of activity units showed that the extract from this isolated is active even in high dilutions, which means that the extracts are active even at very low concentrations. Conclusion. The native isolated of L.cassei produces antimicrobials against Gram negative bacteria, such as E.coli, being available for their possible use as natural bio preservatives in food.esCorporación Universitaria LasallistaProbióticosE.ColiLactobacillus casseiPrebióticosBactericidaArticle