Aeromonas salmonicida subsp. salmonicida, the etiological agent of
furunculosis in fish, produces a
catechol-type
siderophore under
iron-limiting conditions. In this study, the Fur titration assay (FURTA) was used to identify a cluster of six genes, asbG, asbF, asbD, asbC, asbB, and asbI, encoding
proteins similar to components of the
siderophore biosynthetic machinery in other bacteria.
Reverse transcriptase PCR analyses showed that this cluster consists of four
iron-regulated transcriptional units. Mutants with deletions in either asbD (encoding a multidomain
nonribosomal peptide synthetase), asbG (encoding a
histidine decarboxylase), or asbC (encoding a predicted
histamine monooxygenase) did not grow under
iron-limiting conditions and did not produce
siderophores. Growth of the DeltaasbG strain under
iron starvation conditions was restored by addition of
histamine, suggesting that the
siderophore in this species could contain a
histamine-derived moiety. None of the mutants could grow in the presence of
transferrin, indicating that A. salmonicida uses the
catechol-type
siderophore for removal of
iron from
transferrin rather than relying on a receptor for this
iron-binding protein. All 18 A. salmonicida strains analyzed by
DNA probe hybridization were positive in tests for the presence of the asbD gene, and all of them promoted the growth of asbD, asbG, and asbC mutants, suggesting that this
siderophore-mediated
iron uptake system is conserved among A. salmonicida isolates. This study provides the first description of
siderophore biosynthesis genes in this fish pathogen, and the results demonstrate that the asbD, asbG, and asbC genes are necessary for the production of a catecholate
siderophore that is essential for the growth of A. salmonicida under
iron limitation conditions.