A compound according to the formula: wherein: A is -S- or -O-; R 1 is -P(=O)(OR v) 2; each R v is independently selected from the group consisting of -H and -(C 1 -C 7)hydrocarbyl; each R a is independently selected from the group consisting of -H, -(C 1 -C 6)alkyl, -(CH 2) 3 -NH-C(NH 2)(=NH), -CH 2 C(=O)NH 2, -CH 2 COOH, -(CH 2) 2 COOH, substituted and unsubstituted aryl, aryl(C 1 -C 3)alkyl wherein the aryl group is optionally substituted, substituted and unsubstituted heterocyclyl, and substituted and unsubstituted heterocyclyl(C 1 -C 3)alkyl; each R n is independently selected from the group consisting of -OR v, -NR v 2, and an N -terminally linked peptidyl residue containing from 1 to 3 amino acids in which the terminal carboxyl group of the peptidyl residue is present as a functional group selected from the group consisting of -CO 2 R v and -C(=O)NR v 2; each R 2 and R 3 is independently selected from the group consisting of halogen, -(C 1 -C 7)hydrocarbyl, -C(=O)R v, -NR v 2, -NHC(=O)R v, -NHSO 2 R v, -NHR a, -NHCR v R a C(=O)R n, -NHSO 2 R v, -C(=O)OR v, -C(=O)NHR v, -NO 2, -CN, -OR v, -P(=O)(OR v) 2, -C(=NH)NH 2, dimethylamino(C 2 -C 6)alkoxy, -NHC(=NR v)NHR v, -(C 1 -C 6)haloalkyl, and -(C 1 -C 6)haloalkoxy; the two R v groups on -P(=O)(OR v) 2 and -NR v 2 may optionally form a five- or six-membered heterocyclic ring, which may further optionally be fused to an aryl or carbocyclic ring; a is 0, 1, 2 or 3; b is 0, 1, 2 or 3; Y is -S(=O)- or -SO 2 -; R x is selected from the group consisting of -H, -(C 1 -C 6)alkyl, and -C(=O)(C 1 -C 6)alkyl; and * indicates that, when R x is other than -H, the configuration of the substituents on the designated carbon atom is -, - or any mixture of -and -, or a salt of such a compound; wherein the exocyclic carbon-carbon double bond is in the -configuration; wherein the term alkyl by itself or as part of another radical means a saturated hydrocarbon radical and includes straight, branched chain and cyclic groups; the term alkoxy, alone or in combination with another term, means an alkyl group connected to the rest of the molecule to which it is attached via an oxygen atom; and the term "heterocyclyl" or "heterocyclic" by itself or as part of another substituent means an unsubstituted or substituted, stable, mono- or multicyclic heterocyclic ring system which consists of carbon atoms and at least one heteroatom selected from the group consisting of N, O, and S, and wherein the nitrogen and sulfur heteroatoms may be optionally oxidized, and the nitrogen atom may be optionally quaternized. A is -S- or -O-; R 1 is -P(=O)(OR v) 2; each R v is independently selected from the group consisting of -H and -(C 1 -C 7)hydrocarbyl; each R a is independently selected from the group consisting of -H, -(C 1 -C 6)alkyl, -(CH 2) 3 -NH-C(NH 2)(=NH), -CH 2 C(=O)NH 2, -CH 2 COOH, -(CH 2) 2 COOH, substituted and unsubstituted aryl, aryl(C 1 -C 3)alkyl wherein the aryl group is optionally substituted, substituted and unsubstituted heterocyclyl, and substituted and unsubstituted heterocyclyl(C 1 -C 3)alkyl; each R n is independently selected from the group consisting of -OR v, -NR v 2, and an N -terminally linked peptidyl residue containing from 1 to 3 amino acids in which the terminal carboxyl group of the peptidyl residue is present as a functional group selected from the group consisting of -CO 2 R v and -C(=O)NR v 2; each R 2 and R 3 is independently selected from the group consisting of halogen, -(C 1 -C 7)hydrocarbyl, -C(=O)R v, -NR v 2, -NHC(=O)R v, -NHSO 2 R v, -NHR a, -NHCR v R a C(=O)R n, -NHSO 2 R v, -C(=O)OR v, -C(=O)NHR v, -NO 2, -CN, -OR v, -P(=O)(OR v) 2, -C(=NH)NH 2, dimethylamino(C 2 -C 6)alkoxy, -NHC(=NR v)NHR v, -(C 1 -C 6)haloalkyl, and -(C 1 -C 6)haloalkoxy; the two R v groups on -P(=O)(OR v) 2 and -NR v 2 may optionally form a five- or six-membered heterocyclic ring, which may further optionally be fused to an aryl or carbocyclic ring; a is 0, 1, 2 or 3; b is 0, 1, 2 or 3; Y is -S(=O)- or -SO 2 -; R x is selected from the group consisting of -H, -(C 1 -C 6)alkyl, and -C(=O)(C 1 -C 6)alkyl; and * indicates that, when R x is other than -H, the configuration of the substituents on the designated carbon atom is -, - or any mixture of -and -, or a salt of such a compound;