A new device and method for detecting the presence of living microorganisms in test samples are described. The device comprises a container with at least one section transparent to light, a growth zone located in said container containing a mixture of growth media capable of supporting growth of the microorganisms, and at least one indicator substrate that changes its optical properties due to growth of the microorganisms. A detection zone is located in the container adjacent to the transparent section, and a barrier layer comprising porous solid material separates the two zones, allowing diffusion of molecules and ions of metabolic by-products of the organisms, while preventing microorganisms and particulate matter of the test sample from penetrating into the detection zone.