Compounds of the formula (II) ##STR1## or a salt, N-oxide or acyl derivative thereof, wherein Y is a group ##STR2## which is optionaly substituted and which optionally contain a nitrogen atom at one of positions A, B, C, D or E, in which the dotted line represents aromatic rings unless one of the rings contains a nitrogen atom in which case this ring is either aromatic or partially saturated, have antimicrobial properties. Processes for making these compounds, pharmaceutical compositions containing them and the medical use of the compounds are also disclosed.