A compound of formula (I) for use in the prevention or treatment of a bacterial infection wherein RP1 is either methyl, ethyl, isopropyl, cyclohexyl or phenyl； RP2 is selected from methyl, ethyl, isopropyl, cyclohexyl and phenyl； RP3 is either ethyl, isopropyl, cyclohexyl, phenyl or pyridyl； A is either S or Se； RA is selected from wherein each of Y1, Y2, Y3, Y4 and Y9 is independently selected from CH or N, wherein at least three of Y1, Y2, Y3, Y4 and Y9 are CH； V is selected from O, CH-ORO1, N-CO2-RC2 or N-RN2； one of Y5, Y6, Y7 and Y8 is selected from CH and N, and the others are CH； X is selected from NH, S or O； RC1 is selected from O-RO2 or NHRN1； RO1 is selected from H and C1-3 unbranched alkyl； RO2 is C1-3 unbranched alkyl； RN1 is selected from H and C1-3 unbranched alkyl； RN2 is C1-3 unbranched alkyl； RC2 is either C1-3 unbranched alkyl or C3-4 branched alkyl； RC3 is selected from C1-3 unbranched alkyl and C2H4CO2H； RC4 is either H or Me； RC5 is either H or Me； RC6 represents one or two optional methyl substituents； and n is an integer from 2 to 8.
