This invention features new compositions and methods that are useful in treating a host with a Gram-negative bacterial infection. Combination therapies comprising an aminocoumarin compound and a polymyxin compound are disclosed, including certain combinations that exhibit synergistic effects. Furthermore, aminocoumarin compounds are described having altered inhibition of DNA gyrase in Gram-negative bacteria and/or the ability to target the transport proteins responsible for assembling lipopolysaccharide in the outer membrane of Gram-negative bacteria.