Method for the selective enrichment and/or isolation of microbial nucleic acids from samples that contain a mixture of microbial cells, mammalian cells and freely circulating nucleic acids, in particular extracellular microbial nucleic acids, in a liquid, comprising the following steps: 1. Essentially separating the liquid components from the sample, including the substances dissolved in this liquid component such as the freely circulating nucleic acids, in particular the extracellular microbial nucleic acids, from the solid components of the sample, in particular from the cellular components, 2. Selective lysis of the mammalian cells contained in the solid components of the sample without, at the same time, lysis of the microbial cells, wherein the selective lysis of the mammalian cells is carried out in a mixture (buffer 1), comprising 1. at least one chaotropic substance selected from the group containing guanidine thiocyanate, guanidine hydrochloride (GuHCl), guanidine isothiocyanate, alkali perchlorates, alkali iodides, urea or mixtures thereof, 2. at least one non-ionic surfactant, selected from the group containing sorbitan esters of fatty acids, polyoxyethylene sorbitan esters of fatty acids, polyoxyalkylene ethers of fatty acid alcohols, polyoxyalkylene ethers of alkyl phenols, poloxamers or mixtures thereof, and 3. at least one buffer substance, preferably selected from the group containing Tris, MES, MOPS, HEPES or mixtures thereof, in such a way that the concentration of these substances in the sample, after adding the buffer, consist of 0.1 to 2 M of chaotrope, 0.5 to 5 % (w/v) of non-ionic surfactant and 2 to 50 mM of buffer substance, 3. Essentially separating the solid components, including the microbial cells, from the liquid component, including the substances dissolved in this liquid component, of the lysate obtained in step 2, 4. Resuspension of the solid components obtained in step 3, 5. Lysis of the microbial cells which are present in the suspension obtained in step 4, 6. Purification and/or concentration of the freely circulating, in particular extracellular, microbial nucleic acids from the liquid component obtained in step 1 and/or the intracellular microbial nucleic acids from the lysate obtained in step 5. 1. Essentially separating the liquid components from the sample, including the substances dissolved in this liquid component such as the freely circulating nucleic acids, in particular the extracellular microbial nucleic acids, from the solid components of the sample, in particular from the cellular components, 2. Selective lysis of the mammalian cells contained in the solid components of the sample without, at the same time, lysis of the microbial cells, wherein the selective lysis of the mammalian cells is carried out in a mixture (buffer 1), comprising 1. at least one chaotropic substance selected from the group containing guanidine thiocyanate, guanidine hydrochloride (GuHCl), guanidine isothiocyanate, alkali perchlorates, alkali iodides, urea or mixtures thereof, 2. at least one non-ionic surfactant, selected from the group containing sorbitan esters of fatty acids, polyoxyethylene sorbitan esters of fatty acids, polyoxyalkylene ethers of fatty acid alcohols, polyoxyalkylene ethers of alkyl phenols, poloxamers or mixtures thereof, and 3. at least one buffer substance, preferably selected from the group containing Tris, MES, MOPS, HEPES or mixtures thereof, in such a way that the concentration of these substances in the sample, after adding the buffer, consist of 0.1 to 2 M of chaotrope, 0.5 to 5 % (w/v) of non-ionic surfactant and 2 to 50 mM of buffer substance, 1. at least one chaotropic substance selected from the group containing guanidine thiocyanate, guanidine hydrochloride (GuHCl), guanidine isothiocyanate, alkali perchlorates, alkali iodides, urea or mixtures thereof, 2. at least one non-ionic surfactant, selected from the group containing sorbitan esters of fatty acids, polyoxyethylene sorbitan esters of fatty acids, polyoxyalkylene ethers of fatty acid alcohols, polyoxyalkylene ethers of alkyl phenols, poloxamers or mixtures thereof, and 3. at least one buffer substance, preferably selected from the group containing Tris, MES, MOPS, HEPES or mixtures thereof, in such a way that the concentration of these substances in the sample, after adding the buffer, consist of 0.1 to 2 M of chaotrope, 0.5 to 5 % (w/v) of non-ionic surfactant and 2 to 50 mM of buffer substance, 3. Essentially separating the solid components, including the microbial cells, from the liquid component, including the substances dissolved in this liquid component, of the lysate obtained in step 2, 4. Resuspension of the solid components obtained in step 3, 5. Lysis of the microbial cells which are present in the suspension obtained in step 4, 6. Purification and/or concentration of the freely circulating, in particular extracellular, microbial nucleic acids from the liquid component obtained in step 1 and/or the intracellular microbial nucleic acids from the lysate obtained in step 5.