A compound selected from the group consisting of a syn isomer of a compound of the formula ##STR1## in the R or S form or in the form of an R, S mixture wherein R us selected from the group consisting of ##STR2## R and R' are individually selected from the group consisting of hydrogen, alkyl of 1 to 4 carbon atoms, alkoxy of 1 to 4 carbon atoms, halogen, CO.sub.2 --Q, ##STR3## CH.sub.2 --SQ, Q and Q' are individually hydrogen or alkyl of 1 to 4 carbon atoms, P, P' and P" are individually alkyl of 1 to 4 carbon atoms optionally substituted by one of the substituents R and R' the dotted line indicating that P and P' can optionally form with the nitrogen atom to which they are linked a heterocycle with 5 or 6 links, R.sub.b and R.sub.c are individually selected from the group consisting of an equivalent of an alkali metal, an alkaline earth metal, magnesium, ammonium and an amine organic base or A and A' are the remainder of an easily cleavable ester group or CO.sub.2 A is CO.sub.2 --and the wavy line indicates that CH.sub.2 R.sub.1 is in the E or Z position and their non-toxic, pharmaceutically acceptable acid addition salts having anti-bacterial activity.