The invention relates to antibacterial compounds of formula I wherein M is one of the groups MA, MB and MC represented below and either R1 represents H and R2 represents a cleavable group as defined in the claims or R2 represents H and R1 represents a cleavable group as defined in the claims; and salts thereof.本發明係關於式I之抗菌化合物其中M為下文表示之基團MA、MB及MC中之一者且任一R1表示H且R2表示如申請專利範圍中所定義之可裂解基團,或R2表示H且R1表示如申請專利範圍中所定義之可裂解基團;及其鹽。