Method for observing a sample, the sample comprising colonies of microorganisms bathing in a non-transparent culture medium (10 m), the culture medium favouring the development of the microorganisms, the sample being disposed between a light source and an image sensor, the method comprising: a) illumination of the sample by the light source, the light source emitting an incident light wave being propagated along an axis of propagation; b) acquisition of an image of the sample by the image sensor at an initial instant and at a measurement instant; c) from the acquired image, detection of the microorganisms; the method being characterized in that: - the culture medium (10 m) extends, parallel to the axis of propagation, to a thickness less than 500 μm; - the culture medium extends between a top face (10 sup) and a bottom face (10 inf), at right angles, or at right angles to within 20°, to the axis of propagation; - under the effect of their development, the microorganisms form colonies, at least one colony forming a light channel (10 c) extending from the top face (10 sup) to the bottom face (10 inf) through the culture medium; in such a way: - that at least one colony forms a light spot, appearing on the image acquired by the image sensor between the initial instant and the measurement instant, each light spot corresponding to an intense isolated zone; - that the step c) comprises a detection of the light spots on the image acquired by the image sensor at the measurement instant. a) illumination of the sample by the light source, the light source emitting an incident light wave being propagated along an axis of propagation; b) acquisition of an image of the sample by the image sensor at an initial instant and at a measurement instant; c) from the acquired image, detection of the microorganisms; the method being characterized in that: - the culture medium (10 m) extends, parallel to the axis of propagation, to a thickness less than 500 μm; - the culture medium extends between a top face (10 sup) and a bottom face (10 inf), at right angles, or at right angles to within 20°, to the axis of propagation; - under the effect of their development, the microorganisms form colonies, at least one colony forming a light channel (10 c) extending from the top face (10 sup) to the bottom face (10 inf) through the culture medium; in such a way: - that at least one colony forms a light spot, appearing on the image acquired by the image sensor between the initial instant and the measurement instant, each light spot corresponding to an intense isolated zone; - that the step c) comprises a detection of the light spots on the image acquired by the image sensor at the measurement instant. the method being characterized in that: - the culture medium (10 m) extends, parallel to the axis of propagation, to a thickness less than 500 μm; - the culture medium extends between a top face (10 sup) and a bottom face (10 inf), at right angles, or at right angles to within 20°, to the axis of propagation; - under the effect of their development, the microorganisms form colonies, at least one colony forming a light channel (10 c) extending from the top face (10 sup) to the bottom face (10 inf) through the culture medium; - the culture medium (10 m) extends, parallel to the axis of propagation, to a thickness less than 500 μm; - the culture medium extends between a top face (10 sup) and a bottom face (10 inf), at right angles, or at right angles to within 20°, to the axis of propagation; - under the effect of their development, the microorganisms form colonies, at least one colony forming a light channel (10 c) extending from the top face (10 sup) to the bottom face (10 inf) through the culture medium; in such a way: - that at least one colony forms a light spot, appearing on the image acquired by the image sensor between the initial instant and the measurement instant, each light spot corresponding to an intense isolated zone; - that the step c) comprises a detection of the light spots on the image acquired by the image sensor at the measurement instant. - that at least one colony forms a light spot, appearing on the image acquired by the image sensor between the initial instant and the measurement instant, each light spot corresponding to an intense isolated zone; - that the step c) comprises a detection of the light spots on the image acquired by the image sensor at the measurement instant.