A compound according to general structural formula (I), (II), (III), or (IV), or a salt thereof, for use in treating a bacterial infection: formula (I), where X, Y, and Z are each independently aryl that is optionally substituted with one or more groups independently selected from C1-C5 alkyl, C1-C5 acyl, C1-C5-O-acyl, C1-C5-NH-acyl, carboxy, C1-C5 ester, C1-C5 amide, C1-C5 alkoxy, C1-C5-monoalkylamino, C1-C5 dialkylamino, amino, hydroxy, halogen, -C(=O)-O-C 1 -C 4 phenyl, and -O-C(=O)-phenyl; provided that, where X and Y are unsubstituted phenyl, Z is not 2-methyl-phenyl; formula (II): where X', Y', and Z' are each independently aryl that is optionally substituted with one or more groups independently selected from C1-C5 alkyl, C1-C5 acyl, C1-C5 -O-acyl, C1-C5-NH-acyl, carboxy, C1-C5 ester, C1-C5 amide, C1-C5 alkoxy, C1-C5-monoalkylamino, C1-C5 dialkylamino, amino, hydroxy, halogen, -C(=O)-O-C 1 -C 4 phenyl, and -O-C(=O)-phenyl; and where the configuration of the Cα atom of the amino acid moiety containing Y' is D; provided that, where X' and Y' are unsubstituted phenyl, Z' is not 2-methyl-phenyl; formula (III): where X", Y", and Z" are each independently phenyl or naphthyl, each optionally substituted with C1-C5 alkyl, C1-C5 acyl, C1-C5-O-acyl, C1-C5-NH-acyl, carboxy, C1-C5 ester, C1-C5 amide, C1-C5 alkoxy, C1-C5-monoalkylamino, C1-C5 dialkylamino, amino, hydroxy, or halogen; provided that, where X" and Y" are unsubstituted phenyl, Z" is not 2-methyl-phenyl; formula (IV): where X′′′, Y′′′, and Z′′′ each independently is one of phenyl or naphthyl, each optionally substituted with C1-C5 alkyl, C1-C5 acyl, C1-C5-O-acyl, C1-C5-NH-acyl, carboxy, C1-C5 ester, C1-C5 amide, C1-C5 alkoxy, C1-C5-monoalkylamino, C1-C5 dialkylamino, amino, hydroxy, or halogen; and where the configuration of the Cα atom of the amino acid moiety containing Y′′′ is D; provided that, where X′′′ and Y′′′ are unsubstituted phenyl, Z′′′ is not 2-methyl-phenyl. where X, Y, and Z are each independently aryl that is optionally substituted with one or more groups independently selected from C1-C5 alkyl, C1-C5 acyl, C1-C5-O-acyl, C1-C5-NH-acyl, carboxy, C1-C5 ester, C1-C5 amide, C1-C5 alkoxy, C1-C5-monoalkylamino, C1-C5 dialkylamino, amino, hydroxy, halogen, -C(=O)-O-C 1 -C 4 phenyl, and -O-C(=O)-phenyl; provided that, where X and Y are unsubstituted phenyl, Z is not 2-methyl-phenyl; where X", Y", and Z" are each independently phenyl or naphthyl, each optionally substituted with C1-C5 alkyl, C1-C5 acyl, C1-C5-O-acyl, C1-C5-NH-acyl, carboxy, C1-C5 ester, C1-C5 amide, C1-C5 alkoxy, C1-C5-monoalkylamino, C1-C5 dialkylamino, amino, hydroxy, or halogen; provided that, where X" and Y" are unsubstituted phenyl, Z" is not 2-methyl-phenyl; where X′′′, Y′′′, and Z′′′ each independently is one of phenyl or naphthyl, each optionally substituted with C1-C5 alkyl, C1-C5 acyl, C1-C5-O-acyl, C1-C5-NH-acyl, carboxy, C1-C5 ester, C1-C5 amide, C1-C5 alkoxy, C1-C5-monoalkylamino, C1-C5 dialkylamino, amino, hydroxy, or halogen; and where the configuration of the Cα atom of the amino acid moiety containing Y′′′ is D; provided that, where X′′′ and Y′′′ are unsubstituted phenyl, Z′′′ is not 2-methyl-phenyl. <EOS>