An extract based on modified bacterial proteins includes a mixture of acid bacterial polyanions having a molecular weight in the range from 10,000 to 1,000,000 and an isoelectric point in the range from 2.5 to 5.5, and in which the added weights of the constituent amino acids amount to at least 50% of the extract. The preparation or this protein extract includes a cultivation of bacteria in an aqueous medium and then the alkaline extraction of this bacterial suspension and the purification of the protein extract. The alkaline extraction is carried out in the presence of a dilute aqueous source of OH.sup.- ions and at a stable pH in the range from 11 to 13, the decrease of this pH during the acid extraction not exceeding 0.4. The protein extract thus obtained can be used as an active ingredient in a pharmaceutical composition.