Functional insights of a two-component system sensor kinase GacS in a fish pathogen, Pseudomonas plecoglossicida

Pseudomonas plecoglossicida is an important pathogenic bacterial species in aquaculture, causing visceral granulomas in large yellow croaker (Larimichthys crocea) and several other economically important fish species. The GacS/GacA is a two-component regulatory system (TCS) primarily found in Pseudomonads for its role in virulence, quorum sensing, and biofilm formation. To investigate the potential role of GacS/GacA system of P. plecoglossicida, a gacS deficient mutant (ΔgacS) was generated based on a clinical isolate XSDHY-P. Compared to the wild-type (WT) strain, the ΔgacS strain showed almost no growth defect in vitro but was impaired in swimming motility and biofilm formation. Furthermore, the ΔgacS strain exhibited significantly more sensitivity to the bactericidal action of normal fish serum mediated by the complement system. In a cell model originating from the head-kidney of large yellow croaker, the ΔgacS showed lower capacities of adhesion, invasion, and intracellular survival compared to the WT strain. In addition, the virulence of the ΔgacS and WT bacterial strains were evaluated in large yellow croaker. The results showed that the deficiency of gacS drastically decreased bacterial tissue loads and substantially attenuated P. plecoglossicida virulence in fish. Furthermore, a comparative transcriptome was performed between the ΔgacS and WT strains by RNA-sequencing (RNA-seq). Transcriptomic analysis results revealed 25 upregulated and 273 downregulated genes in the ΔgacS strain compared with the WT strain, and the downregulated genes were mainly involved in flagellar assembly, metabolism, bacterial chemotaxis, and biofilm formation, which are consistent with their respective phenotypic traits. These findings will provide functional insight into the role of GacS/GacA in P. plecoglossicida.