Interplay between ferric uptake regulator Fur and horizontally acquired virulence regulator EsrB coordinates virulence gene expression in Edwardsiella piscicida.

Edwardsiella piscicida mediates hemorrhagic septicemia and is a leading pathogen of fish. E. piscicida invades and colonizes macrophages using type III and VI secretion systems (T3/T6SS) that are controlled by a two-component system (TCS) EsrA-EsrB. Iron acquisition is essential for E. piscicida pathogenesis and coordination between iron and TCS signaling in modulating bacterial virulence is not well understood. Here, we show that iron uptake systems are co-regulated by ferric uptake regulator (Fur) in E. piscicida. Fur bound to 98 genes that harbored conserved Fur-box to globally control the expression of ∼755 genes, including those encoding iron uptake systems, T3/T6SS, and Icc, cAMP phosphodiesterase that represses biofilm formation. Additionally, Fur, in complex with iron, bound to the esrB promoter to repress expression and ultimately attenuated virulence. Conversely, EsrB activated the expression of T3/T6SS and iron uptake systems to mitigate a shortage of intracellular iron during iron scarcity. Furthermore, EsrB directly bound to and activated the fur promoter, leading to Fur-ferrous ion-dependent esrB repression in the presence of iron. Finally, Fur-EsrB interplay was essential for bacterial fitness during in vivo infection and survival in seawater environments. Collectively, we highlight the mechanisms that underlie the reciprocal regulatory networks of iron homeostasis and virulence systems in E. piscicida.