Involved in the biosynthesis of petrobactin, a catecholate siderophore that functions in both iron acquisition and virulence (PubMed:16875672, PubMed:17189355, PubMed:22408253). Catalyzes the ATP-dependent condensation of citric acid and spermidine to form N(8)-citryl-spermidine (PubMed:17579415, PubMed:19259597, PubMed:22408253). It can also catalyze the condensation of several di- and triamine analogs of spermidine with citric acid and the condensation of the citric acid analog tricarballylic acid with spermidine (PubMed:19259597). Required for growth in iron-depleted medium and for full virulence in a mouse model of infection (PubMed:14756782). ATP + citrate + spermidine = AMP + diphosphate + H(+) + N(8)-citryl-spermidine kcat is 0.89 sec(-1). Siderophore biosynthesis; petrobactin biosynthesis. Deletion of the gene leads to a decrease in siderophore production during iron-depleted growth and attenuates growth (PubMed:14756782). The deletion mutant cannot produce petrobactin on iron-depleted medium (PubMed:16875672, PubMed:17189355). Mutant produces the petrobactin precursor 3,4-dihydroxybenzoyl (3,4-DHB) and bacillibactin (PubMed:16875672). The deletion mutant is severely attenuated for growth in macrophages and virulence in mice (PubMed:14756782). In vitro analysis show that mutants grow to a very limited extent as vegetative cells in iron-depleted medium but are not able to outgrow from spores under the same culture conditions (PubMed:17189355). The ability of the enzyme to catalyze condensation of citric acid with several di- and triamine analogs of spermidine suggests that it may be a versatile biocatalyst for the preparation of highly enantiomerically-enriched citrate derivatives via a novel type of desymmetrisation process. Belongs to the IucA/IucC family.