The device includes means for holding a support (19) adapted to retain microorganisms in a predetermined position in which a first face (10) of said support (19) is in a first predetermined location and a second face (11) of said support (19) is in a second predetermined location, a station (2) for spraying onto said support (19) a reagent for revealing the presence of ATP by luminescence, said station (2) facing said first location, and a station (4) for measuring said luminescence opposite the spraying station and facing said second location. The method includes the step of procuring a device of the above kind, the step of disposing said support (19) at said predetermined location, the step of spraying said reagent onto said support (19) and, simultaneously with the spraying step, the step of measuring the quantity of light emitted in response to said reagent.