The invention relates to antibacterial compounds of formula I wherein X represents sulphur or CH=CH R1 represents H, PO3H2, SO3H, phosphonooxymethyl or the group -CO-R2 wherein R2 is as defined in the claims M is one of the groups MA and MB represented below wherein A represents a bond or C=C and R1A, R2A, R3A and R1B are as defined in the claims and to salts thereof.