Repurposing auranofin combined with colistin to effectively combat fish-pathogenic Edwardsiella piscicida

Edwardsiella piscicida ( E. piscicida ) is an important opportunistic fish pathogen, and infects various species of cultured fish posing a great challenge to the aquaculture industry globally. To address the frequent emergence of multi-antibiotics resistant E. piscicida and the shortage of effective antimicrobial treatments, this study aimed to develop a novel combination therapy against E. piscicida . The combination of the FDA approved drug auranofin (originally used to treat rheumatoid arthritis) with colistin exerted great bactericidal effects against E. piscicida both in vitro and in vivo . Auranofin disrupted the thioredoxin system, and decreased the minimum inhibitory concentration (MIC colistin ) of E. piscicida . Further, auranofin combined with colistin significantly inhibited the growth of E. piscicida isolates, even resulting in cell death. A mechanistic study revealed that the combination remarkably damaged the bacterial membrane increasing its permeability, and it also promoted a decrease of membrane potential, thereby exacerbating oxidative stress and the bactericidal effects of colistin. Importantly, auranofin combined with colistin strongly attenuated the essential bacterial two-component system, lipopolysaccharide modification, motility and virulence pathways. Moreover, the combination of both chemicals also efficiently reduced E. piscicida burden in the gill, intestine and head kidney of infected zebrafish ( Danio rerio ), and significantly improved their survival rate. Overall, these results suggested that the thioredoxin-mediated redox cascade is a potential target for novel antimicrobials, and that auranofin combined with colistin could be repurposed to effectively combat Edwardsiella infections.