The present invention is directed to a method and an apparatus for detecting micro-colonies growing on a membrane or an agarose medium of a sample in a closed device. According to the invention the sample is irradiated with a light incident at an angle (β) with respect to the normal to the membrane or the surface of the agarose medium from outside the device. An incident area of the light on the membrane or the surface of the agarose medium is imaged by means of a light receiving element using an imaging angle (α) different from angle (β) with respect to the normal to the membrane or the surface of the agarose medium from outside the device. The light reflected, scattered and/or diffused from the membrane or the surface of the agarose medium and/or the micro-colonies on the membrane and/or the micro-colonies on the agarose medium is detected.