The present invention relates to novel, low molecular weight broad spectrum compounds in particular to a class of 4-substituted or unsubstituted-7-hydro-1,4-thiazepine-7-[bicyclic or tricyclic heteroaryl] substituted-3,6-dicarboxylic acid or their derivatives which have .beta.-lactamase inhibitory and antibacterial properties. The compounds are therefore useful in the treatment of antibacterial infections in humans or animals, either alone or in combination with other antibiotics.