Structural insights and therapeutic targets in Acinetobacter baumannii capsule biosynthesis.

Gram-negative bacteria produce nonulosonic acids, a family of nine-carbon sugars that mimic host sialic acids and may contribute to bacterial virulence. The nonulosonic acid C8-epimeric 5,7-di-N-acetyllegionaminic acid (8-epi-Leg5,7Ac2) is a component of the K-locus 49 (KL49) capsule of hypervirulent Acinetobacter baumannii, and its epimerization is proposed to be mediated by ElaA, ElaB, and ElaC. We identified a KL49 strain that induces lethal sepsis in mice and found deletion of elaA, elaB, or elaC abolished lethality and markedly reduced capsule production. Crystal structures of ElaB, ElaA, and ElaC revealed conserved N-acetyl recognition underlying CMP-Leg5,7Ac2 synthesis and regioselective C8 epimerization. Together, these genetic, structural, and in vivo findings identify ElaABC as potential antivirulence targets in emerging hypervirulent KL49 A. baumannii strains.