Two novel single-chain variable fragments exert bactericidal activity against Acinetobacter baumannii through destabilization of the outer membrane

Abstract Background Acinetobacter baumannii is notorious for its high levels of resistance and the development of clinically-effective antimicrobial agents seems to be an urgent medical challenge. Bactericidal single-chain variable fragments (scFvs) are novel antibacterial agents capable of inhibiting the growth of pathogens (e.g. Staphylococcus aureus , Pseudomonas aeruginosa , A. baumannii ) independently of the host immune system. We previously found that two fully human scFvs, EB211 and EB279, showed direct growth inhibitory activity against A. baumannii strains. In the current study, the antibacterial activity of EB211 and EB279 against A. baumannii , Klebsiella pneumonia , and P. aeruginosa strains was appraised in the presence of a high concentration of magnesium (Mg 2+ ) (20 mM) to find the bactericidal mechanism of these two scFvs. Moreover, epitope mapping and immunoblotting were done to identify A. baumannii proteins targeted by EB211 and EB279. Results EB211 and EB279 similar to colistin sulfate, lost their activity in the presence of Mg 2+ , demonstrating that EB211 and EB279 exerted growth inhibitory activity by displacing Mg 2+ and interrupting the integrity of the outer membrane. The immunoblotting results also exhibited that EB211 and EB279 bound OprD and TonB family C-terminal domain, respectively. Conclusions EB211 and EB279 by disruption of the outer membrane A. baumannii elicit direct growth inhibitory activity without the need for immune cells or complements which could be helpful for use in immunocompromised patients.