The marine bivalve molluscs pathogen Vibrio neptunius produces the siderophore amphibactin, which is widespread in molluscs microbiota.

Amphiphilic siderophores, including amphibactins, are the most abundant siderophores in oceans. Genes putatively encoding the amphibactin system were proposed in some bacteria and homologues of these genes are particularly abundant in multiple bacterial lineages inhabitant of low-iron seawater. However, since no defective mutant strains in any of these genes were studied to date, their role in amphibactin synthesis or uptake was not demonstrated. In this work, an in silico analysis of the genome of the mollusc pathogen Vibrio neptunius leads us to identify a gene cluster (denoted absABDEF) that is predicted to encode an amphibactin-like siderophore and several mutant strains unable to synthesize or use siderophores were constructed. The results showed that genes absABDEF are required for amphibactin synthesis. A comparative chemical analysis of V. neptunius wild type and biosynthesis mutants allowed us to identify a mixture of nine amphibactin forms produced by this bacterium. In addition, the gene abtA is predicted to encode the ferri-amphibactin outer membrane transporter. The prevalence of the amphibactin system in bivalve hemolymph microbiota was also studied. We found that the amphibactin system is widespread in hemolymph microbiota including both commensal and pathogenic bacterial species. Thus, its contribution to bacterial fitness must be more related to environmental persistence than to pathogenicity.