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

Abstract

Abstract 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 inhibition against A. baumannii strains in vitro and demonstrated therapeutic effectiveness in immunocompromised mice with pneumonia caused by an extensively drug-resistant A. baumannii strain. 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 (Mg2+; 20 mM) to find the bactericidal mechanism of these two scFvs. Furthermore, epitope mapping was performed to identify A. baumanniiproteins that might be bound by EB211 and EB279. It was found that EB211 and EB279, similar to colistin sulfate, lost their activity in the presence of Mg2+. Indeed, both EB211 and EB279 exerted their growth inhibitory activity through displacing Mg2+ and interrupting the integrity of the outer membrane. Taken together, EB211 and EB279 by disruption of the outer membrane A. baumannii elicit their main direct growth inhibitory activity against A. baumannii without the need for immune cells or complements.