Method for the specific isolation of nucleic acids of interest

A method and diagnostic kit for selective isolation of microorganisms of interest and/or nucleic acids of interest including: a) bringing a liquid biological sample into contact with a saponin formulation to destabilize untargeted elements within the liquid biological sample, b) inducing osmotic shock of the untargeted elements to specifically lyse the untargeted elements, c) adding a solution of at least one enzyme capable of lysing free nucleic acids derived from the untargeted elements lysed in solution in the sample, and d) selectively obtaining the microorganisms of interest or the nucleic acids of interest. A precipitant of the unlyzed microorganisms of interest may be optionally added in solution. The microorganisms of interest have cell membranes or capsids that do not contain cholesterol. The untargeted elements may include untargeted cells having cell membranes containing cholesterol, and optionally viruses with envelopes containing cholesterol, mycoplasmas containing cholesterol, and cell debris.