There is disclosed a novel beta -lactam antibiotics represented by the formula: IMAGE (I) wherein A is a group represented by the formulae -NHCO-, -NHCONHCO-, -NHCOCH=CH- or IMAGE where R6 is a hydrogen atom or a lower alkyl group; R1 and R2 are independently a hydrogen atom or a protective group; R3 is a hydrogen atom or a methoxy group; X is a hydrogen atom, a hydroxyl group, a protected hydroxyl group, a halogen atom, a lower alkoxy group or a nitro group; n is an integer of 1 or 2; Y is a group represented by the formulae: IMAGE provided that a carbon atom which is bonded by a carboxyl group being bonded to nitrogen atom; M is a hydrogen atom, a protective group or an easily hydrolyzable group in a human body; R7 is a hydrogen atom, a methyl group a lower alkoxy-methyl group or a group represented by the formula: -CH2-T where T is an acyloxy group, a carbamoyloxy group, a quaternary ammonium, a substituted or unsubstituted heterocyclic ring or a formula: -S-R8 where R8 is an acyl group or a substituted or unsubstituted heterocyclic ring, R4 and R5 are each hydrogen atoms or combined with each other to form additional direct bond; Z is a direct bond or a carbonyl group when R4 and R5 are hydrogen atoms, or a formula: -O-B- where the oxygen atom is bonded to nitrogen atom and B is a straight, branched or cyclic alkylene group when R4 and R5 are combined with each other to form additional direct bond, or its pharmaceutically acceptable salt.
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