The invention provides compounds of the general formula I ##STR1## wherein R.sub.1 is a hydrogen atom, a negative charge or an ester residue; R is:a) --(CH.sub.2).sub.n --A--CO.sub.2 H, --(CH.sub.2)n--A--SO.sub.3 H or --(CH.sub.2).sub.n --A--PO.sub.3 H.sub.2, wherein n is either zero, one or two and A is a group --CH.dbd.CH-- (either E or Z), --OCH.sub.2 --, --SCH.sub.2 -- or --CHOH--;b) --(CH.sub.2).sub.n --PO.sub.3 H.sub.2, --(CH.sub.2).sub.n SO.sub.2 NHCN, --(CH.sub.2).sub.n NHSO.sub.3 H, --(CH.sub.2).sub.n CONHSO.sub.2 CH.sub.3 or --(CH.sub.2).sub.n CONHSO.sub.2 CF.sub.3, wherein n is as defined above;c) --(CH.sub.2 S).sub.m --W--(CH.sub.2).sub.n Z, wherein W is an arylene group or a heterocyclediyl group, m is 0 or 1, n is as above defined, and Z represents CO.sub.2 H, PO.sub.3 H.sub.2, SO.sub.2 NHCN, NHSO.sub.3 H, CONHSO.sub.2 CH.sub.3 or CONHSO.sub.2 CF.sub.3 ;d) ##STR2## wherein Y is O or NH and X is NH, N--OH or N--O--(CH.sub.2).sub.n+1 COOH wherein n is as defined above; ore) --(CH.sub.2 S).sub.m --W', wherein W' is a heterocyclyl group convertible into an anion at physiological pH and m is as defined above.Compounds I, their salts and ester prodrugs have antibacterial activity.