The invention relates to antibacterial compounds of formula I wherein R1a represents H or carboxy and R1b represents H, or R1a and R1b represent together either the group *&mdash；C(O)&mdash；NH&mdash；S&mdash；# or the group *&mdash；C(OH)&boxH；N&mdash；S&mdash；# wherein &ldquo；*&rdquo； represents the point of attachment of R1a and &ldquo；#&rdquo； represents the point of attachment of R1b； R2 represents H, (C1-C3)alkyl, hydroxy-(C1-C3)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.
