The preparation unit comprises a body (2) within which are fixed a first (3) and a second (4) membrane, said first membrane (3) having a first predetermined pore diameter and said second membrane (4) a second predetermined pore diameter smaller than said first predetermined pore diameter of said first membrane (3), said body also comprising means (16, 20) for retrieving said sample collected on said second membrane (4).The method of preparing such a sample comprises the step of procuring such a preparation unit (1), the step of passing a predetermined volume of said liquid through said first membrane and through said second membrane (4) in order to collect a sample on said second membrane (4); and the step of retrieving said sample so collected on said second membrane (4).