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 together with the carbon atom to which they are attached form a C3-7 cycloalkylidene group). These compounds exhibit broad spectrum antibiotic activity, the activity being unusually high against gram-negative organisms such as strains of Pseudomonas organisms. Particularly effective compounds of formula (I) are (6R,7R)-7-[(Z)-2-(2-aminothiazol-4-yl)-2-(2-carboxyprop-2-oxyimino)ace tamio]-3-(pyridazinium-1-ylmethyl)ceph-3-em-4-carboxylate and (6R,7R)-7-[(Z)-2-(2-aminothiazol-4-yl)-2-(t-carboxycyclobut-1-oxyimino )acetamido]-3-(pyridazinium-1-ylmethyl)ceph-3-em-4-carboxylate. The invention also includes the non-toxic salts and non-toxic metabolically labile esters of compounds of formula (I), compositions containing the antibiotic compounds of the invention and a method of combatting bacterial infection utilizing the antibiotics.