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 (MICcolistin) 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.