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Acinetobacter baumannii  causes severe infections in compromised patients, who present an iron-limited environment that controls bacterial growth. This pathogen has responded to this restriction by expressing high-affinity iron acquisition systems including that mediated by the siderophore acinetobactin. Gene cloning, functional assays and biochemical tests showed that the  A. baumannii  genome contains a single functional copy of an  entA  ortholog. This gene, which is essential for the biosynthesis of the acinetobactin precursor 2,3-dihydroxybenzoic acid (DHBA), locates outside of the acinetobactin gene cluster, which otherwise harbors all genes needed for acinetobactin biosynthesis, export and transport.  In silico  analyses and genetic complementation tests showed that  entA  locates next to an  entB  ortholog, which codes for a putative protein that contains the isochorismatase lyase domain, which is needed for DHBA biosynthesis from isochorismic acid, but lacks the aryl carrier protein domain, which is needed for tethering activated DHBA and completion of siderophore biosynthesis. Thus,  basF , which locates within the acinetobactin gene cluster, is the only fully functional  entB  ortholog present in ATCC 19606 T . The differences in amino acid length and sequences between these two EntB orthologs and the differences in the genetic context within which the  entA  and  entB  genes are found in different  A. baumannii  isolates indicate that they were acquired from different sources by horizontal transfer. Interestingly, the AYE strain proved to be a natural  entA  mutant capable of acquiring iron via an uncharacterized siderophore-mediated system, an observation that underlines the ability of different  A. baumannii  isolates to acquire iron using different systems. Finally, experimental infections using  in vivo  and  ex vivo  models demonstrate the role of DHBA and acinetobactin intermediates in the virulence of the ATCC 19606 T  cells, although to a lesser extent when compared to the responses obtained with bacteria producing and using fully matured acinetobactin to acquire iron.

The Acinetobacter baumannii entA Gene Located Outside the Acinetobactin Cluster Is Critical for Siderophore Production, Iron Acquisition and Virulence