The invention relates to the use of a compound having formula (I) as an enzymatic substrate for the detection of a nitroreductase activity, wherein: W1, W2, W3 and W4 are independently H, Br, Cl, F, I, alkyl, alkoxy, thiomethyl, perfluoroalkyl, nitro, cyano, carboxyl (including the esters or amides thereof) or any combination of same; n=0, 1 or 2; X is NR, CZ5Z6, S or O, R being H, alkyl, aralkyl, aryl, alkanoic or alkylsulphonic, Z5 and Z6 being an alkyl; Y is N or N+R, R being alkyl, aralkyl, aryl, alkanoic or alkylsulphonic; Z1, Z2, Z3 and Z4 are independently H, Br, Cl, F, I, alkyl, aryl, alkoxy, perfluoroalkyl, nitro, cyano, carboxyl, sulphonyl, including the sulphonyl or carboxyl amides or esters thereof, and the salts of same.