Abstract: The present invention relates to a method for treating liquid samples with a view to detecting any possible pathogenic microorganisms in a very small amount. More specifically, this method comprises a generic step of capturing and concentrating microorganisms on an ion exchange surface, followed by an in situ lysis treatment carried out on the microorganisms and capture of the nucleic acids released during the lysis. The implementation of this method enables an extremely concentrated and purified solution of nucleic acids to be obtained. This method is suitable for a continuous treatment of liquid samples. The invention also relates to a device for analysing liquid samples for biology, health or the environment, which is suitable for the implementation of the method according to the invention.