A method for observation of a sample, the sample comprising microorganisms immersed in a nontransparent culture medium, the culture medium being favorable to the development of the microorganisms, and the sample being arranged between a light source and an image sensor, includes illuminating the sample with the light source, the light source emitting light propagating along an axis of propagation; acquiring an image of the sample by the image sensor; and, from the image acquired, characterizing the microorganisms. Light travels along the microorganisms through the culture medium to the sensor.