Cephalosporin antibiotics of general formula ##STR1## (wherein R.sup.a and R.sup.b, which may be the same or different, each represent a C.sub.1-4 alkyl group, or together with the carbon atom to which they are attached form a C.sub.3-7 cycloalkylidene group and R.sup.1 represents a C.sub.1-4 alkyl group) 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)acetami do]-3-(2-methylpyrazolium-1-ylmethyl)ceph-3-em-4-carboxylate and (6R,7R)-7-[(Z)-2-(2-aminothiazol-4-yl)-2-[(1-carboxycyclobut-1-oxyimino)ac etamido]-3-(2-methylpyrazolium-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 methods for combatting bacterial infections utilizing the antibiotics.