The invention relates to a device for detecting signs of bacterial infection of teeth, comprising a light source, a receiving unit, an evaluation unit, coupled to the receiving unit, at least one emission fiber, coupled to the light source, and at least one detection fiber, coupled to the receiving unit. The invention is characterized in that the common distal front face of the at least one emission fiber and the at least one detection fiber is connected to a front face of a flexible plastic optical wave guide, the diameter of the plastic optical wave guide being less than 400 μm, preferably less than 300 μm.