Cephalosporin antibiotics

Cephalosporin antibiotics of general formula: (I) (wherein Ra and Rb, which may be the same or different, each represent a C1-4 alkyl group or Ra and Rb together with the carbon atom to which they are attached form a C3-7 cycloalkylidene group, and Y(+) represents a C-linked 5- or 6-membered heterocyclic ring containing at least one C1-4 alkyl-substituted quaternary nitrogen atom, which ring may also contain one or more sulphur atoms) exhibit broad spectrum antibiotic activity, the activity being unusually high against gram-negative organisms such as strains of Pseudomonas organisms. A particular antibiotic compound of formula (I) possessing excellent antibacterial activity against strains of Pseudomonas organisms, as well as other valuable therapeutic properties, is (6R,7R)-7-[(Z)-2-(2-aminothiazol-4-yl)-2-(1-carboxycyclobut-1-oxyimino )acetamido]-3-[(1-methylpyridinium-4-yl)-thiomethyl]ceph-3-em-4-carbox ylate. The invention also includes the non-toxic salts and non-toxic metabolically labile esters of compounds of formula (I). Also described are compositions containing the antibiotics of the invention and processes for the preparation of the antibiotics.