Abstract: A method for preparing liquid probiotic composition, comprising bacteria having at least a basal biologic activity, wherein said bacteria have been selected according to at least one selection pressure, wherein the composition preferably includes an autolysate (complete substances for maintaining bacteria) and wherein the composition is substantially free from substances suitable for bacterial growth but not similarly suitable for mammals, and particularly not suitable for human beings. Peptone and buffering salts, particularly phosphates, may not be harmful in small doses, but they are not specifically suitable for human beings and free from substances generated by bacteria.