Acinetobacter baumannii is a major hospital-acquired pathogen, and responsible for a range of opportunistic infections. Over half of A. baumannii isolates are resistant to nearly all antibiotics, costing >$700million per year. Multidrug resistance, and a dearth of new antibiotic development, mean novel ways to treat infections are needed. Although many A. baumannii virulence factors have been studied, very little is known about the host receptors A. baumannii adhesins interact with. Nearly all proteins and cells are decorated by glycans (sugars), and the significance of glycans in microbial:host interactions is emerging. Recent studies have identified critical roles for glycan-binding adhesins in many pathogens including NTHi and UPEC. Many A. baumannii adhesins bind fibronectin and collagen, found throughout the body, and both are heavily glycosylated. Therefore, by defining the host glycans bound by A. baumannii, i.e., the ‘glycointeractome’, we can develop new ways to block host cell interactions.
As a proof of concept, we over-expressed the A. baumannii adhesin Ata in E. coli. This allowed us to determine the glycan binding specificity using glycan arrays, and determine affinity using surface plasmon resonance (SPR). Over-expression of the Ata ‘head domain’ allowed us to confirm that this is the receptor binding region of Ata. Glycan arrays identified a subset of ~20 glycans bound by Ata, with these glycans all containing a galactose (Gal) linked to N-acetyl-glucosamine (GlcNAc) or an N-acetyl-galactosamine (GalNAc). SPR using these glycans showed a preference for Gal-GlcNAc. These structures are common glycan decorations on both fibronectin and collagen. Carrying out SPR with the Ata-head domain using fibronectin that had been treated with glycosidases showed a complete loss of binding compared to untreated fibronectin.
This approach has shown that a key A. baumannii adhesin, Ata, specifically interacts with host glycans, and that this interaction is required for high-affinity binding to the human protein fibronectin. In future, we will characterise additional A. baumannii adhesins to define the complete glycointeractome, and use this knowledge to develop ways to block interactions with host cells. This will allow us to augment traditional antibiotics, providing a much-needed alternative to treat a major drug resistant pathogen.