The device for spraying a reagent onto a support (81) adapted to retain microorganisms on a predetermined surface (82), comprises a spraying bell (3) as well as a nozzle (71) for emitting a jet of droplets of said reagent into a spraying chamber (34) comprised by said bell (3), said device also comprising an absorbent pad (5) mounted against said bell (3) transversely to said jet and closing said chamber (34) from the opposite side to said nozzle (71) with the exception of a circular central opening (51) provided in said pad (5), the diameter of said central opening (51) being adapted to enable a portion of said jet, when said device faces said support (81) and is at a predetermined distance therefrom, to pass through said central opening (51) over its entire area and be deposited on the whole of said predetermined surface (82) of said support (81).