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