Iron transport-mediated antibacterial activity of and development of resistance to hydroxamate and catechol siderophore-carbacephalosporin conjugates

Peptides containing residues of N5-acetyl-N5-hydroxy-L-ornithine were evaluated as potential artificial siderophores of beta-lactam-hypersusceptible Escherichia coli X580. Only those peptides which were capable of forming a hexadentate complex around ferric iron, which is analogous to the natural siderophore ferrichrome, were able to reverse the growth inhibition effects of the ferric iron chelator ethylenediamine di(o-hydroxyphenylacetic acid). A synthetic bis(catechol) spermidine derivative, similar to the natural siderophores enterobactin and agrobactin, also exhibited siderophore activity with this strain. Conjugation of the N5-acetyl-N5-hydroxy-L-ornithine tripeptide and the bis(catechol) siderophore to the potent carbacephalosporin loracarbef and closely related analogs provided compounds which exhibited antibacterial activity against E. coli X580. As was observed with the naturally occurring albomycins, the initial bactericidal effect was followed by the appearance of survivors that were resistant to the test compound. An enhanced killing effect was observed when the parent was incubated simultaneously with hydroxamate and catechol siderophore-antibiotic conjugates. Natural and synthetic siderophore growth promotion experiments with survivors resistant to the conjugates strongly suggested that disabled ferrichrome and enterobactin-catechol assimilation mechanisms may be responsible for the observed resistance. One isolated survivor was postulated to be a tonB mutant. The antibacterial activities of the described siderophore-carbacephalosporin conjugates appear to be related to an iron transport assimilation mechanism and would not have been detected during routine MIC testing procedures.