Active transport of siderophore-mimicking antibacterials across the outer membrane

The outer membrane of gram-negative bacteria forms a permeability barrier that usually reduces antibiotic access to intracellular targets and renders gram-negative bacteria less susceptible to antibiotics than gram-positive bacteria, which lack an outer membrane. However, gramnegative bacteria become highly susceptible to antibiotics that are actively transported across the outer membrane. Some antibiotics use active transport systems of substrates with which they share structural features. Examples are naturally occurring sideromycins and synthetic derivatives of Fe3+-siderophores, which are taken up across the outer membrane by transport systems for Fe3+-siderophores. A well-studied example is albomycin, which has structural similarities to the natural substrate ferrichrome; albomycin and ferrichrome are both transported by the FhuA protein. A semisynthetic rifamycin derivative, CGP 4832, is also taken up by the FhuA transport protein, although its structure is completely different from that of ferrichrome. The crystal structures of FhuA with bound ferrichrome, albomycin, or rifamycin CGP 4832 reveal that the three compounds occupy the same site on FhuA; this site is accessible from the growth medium by a surface cavity that accommodates the antibiotic moieties. There is a rather strict stereochemical requirement for the portion that fits into the active site of FhuA, but a rather large tolerance regarding the portion that is located in the cavity. These data provide precise structural information for the design of highly active antibiotics composed of an antibiotically active moiety connected by a linker to a transported carrier. A number of Fe3+-siderophore carriers of the hydroxamate and catechol type linked to antibiotics have been isolated from microbes and synthesized; their superior efficacy has been demonstrated in vitro and in mice. Although none have been therapeutically employed, it is proposed that this alternative method of synthesizing useful antibiotics should be tested in light of the increasing problem of resistant pathogens. (C) 1999 Harcourt Publishers Ltd.