A compound of formula (I): for use in the prevention or treatment of a bacterial infection wherein: 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 is CH； V is selected from O, CH-OR01, 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.
