It is evident that the results of preliminary experiments with 5 different yeast killer proteins did not show emphatic cytotoxicity or any adverse effect on any mammalian and embryo-cells. Moreover, they are likely to be harmless to animals? and humans? tissue cells. Therefore, they could be used to explain the pre-therapeutic effect on mammalian cells (mostly animals) in the case of infections by strains Escherichia coli, called EHEC. It was found that the yeast killer toxin from Williopsis mrakii can protect mammalian cells such as HeLa and Vero cells against the challenge by Shiga-Like-Toxins (derived from cultures of pathogenic strains of Escherichia coli). The final activities of tested mammalian cells are better when they are pre-treated by the killer protein, i.e. before the challenge with Shiga-Like-Toxins. It appears that this prophylactic effect could be very interesting for veterinary, which has been proved on a big population (about 2000) of healthy and ill (with diarrhoea, i.e. haemorrhagic colitis) pigs (manuscript ? confidential data). We can conclude that the yeast killer strains are probiotic, i.e. could reduce or eliminate fecal shedding of EHEC strains in pigs prior treated with the developed yeast toxins.