Active transport of antibiotics across the outer membrane of gram-negative bacteria and its implications in the development of new antibiotics

The outer membrane of gram-negative bacteria forms a permeability barrier that usually reduces the access of antibiotics to intracellular targets and renders gram-negative bacteria less susceptible to antibiotics than gram-positive bacteria, which lack an outer membrane. However, gram-negative bacteria become highly susceptible to antibiotics if the antibiotics are actively transported across the outer membrane. Some antibiotics can use active transport systems of substrates with which they share structural features. Examples are naturally occurring sideromycins and synthetic derivatives of Fe– siderophores, which are taken up across the outer membrane by transport systems of Fe-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 than 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 of 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 bound by a linker to a transported carrier. A number of Fe–siderophore carriers of the hydroxamate and catecholate type linked to antibiotics have been isolated from microbes and synthesized; their superior efficacy has been demonstrated in vitro and in mice. It is proposed that natural sideromycins and synthetic sideromycins should be developped to obtain new antibiotics in light of the increasing problem of resistant pathogens. PDF created with FinePrint pdfFactory Pro trial version http://www.fineprint.com