The invention relates to antibacterial compounds of formula I (Fornula I) (I) wherein R1a represents H or carboxy and R1b represents H, or R1a and R1b represent together either the group *-C(0)-NH-S-# or the group *-C(OH)=N-S-# wherein "*" represents the point of attachment of R1a and "#" represents the point of attachment of R1b； R2 represents H, (C1-C3)alkyl, hydroxy-(C1C3)alkyl, benzyl or (C3-C5)cycloalkyl； R3 represents H or halogen； U represents N or CR4； wherein R4 is H or (C1-C3)alkoxy； A represents CH, B represents NH and m represents 1 or 2 and n represents 1 or 2； or A represents N, B is absent, m represents 2 and n represents 2； Y represents CH or N； and Q represents O or S； and salts of such compounds.
