Objectives. The increasing prevalence of antibiotic-resistant Staphylococcus aureus, besides the inadequate numbers of effective antibiotics, emphasises the need to find new therapeutic agents against this lethal pathogen. Methods. In this study, to obtain antibody fragments against S. aureus, a human single-chain fragment variable (scFv) library was enriched against living methicillin-resistant S. aureus (MRSA) cells, grown in three different conditions, that is human peripheral blood mononuclear cells with plasma, whole blood and biofilm. The antibacterial activity of scFvs was evaluated by the growth inhibition assay in vitro. Furthermore, the therapeutic efficacy of anti-S. aureus scFvs was appraised in a mouse model of bacteraemia. Results. Three scFv antibodies, that is MEH63, MEH158 and MEH183, with unique sequences, were found, which exhibited significant binding to S. aureus and reduced the viability of S. aureus in in vitro inhibition assays. Based on the results, MEH63, MEH158 and MEH183, in addition to their combination, could prolong the survival rate, reduce the bacterial burden in the blood and prevent inflammation and tissue destruction in the kidneys and spleen of mice with MRSA bacteraemia compared with the vehicle group (treated with normal saline). Conclusion. The combination therapy with anti-S. aureus scFvs and conventional antibiotics might shed light on the treatment of patients with S. aureus infections.
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 (Mg2+) (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 Mg2+, demonstrating that EB211 and EB279 exerted growth inhibitory activity by displacing Mg2+ 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.
Background
The emergence of carbapenem-resistant and extensively drug-resistant Acinetobacter baumannii as well as inadequate effective antibiotics calls for an urgent effort to find new antibacterial agents. The therapeutic efficacy of two human scFvs, EB211 and EB279, showing growth inhibitory activity against A. baumannii in vitro, was investigated in immunocompromised mice with A. baumannii pneumonia.
Results
The data revealed that infected mice treated with EB211, EB279, and a cocktail of two scFvs showed better survival, reduced bacterial load in the lungs, and no marked pathological abnormalities in the kidneys, liver, and lungs when compared to the control groups receiving normal saline or an irrelevant scFv.
Conclusions
These results suggest that the scFvs with direct growth inhibitory activity could lead to promising outcomes in immunosuppressed patients with A. baumannii infection.
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