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Pseudomonas plecoglossicida is a facultative fish pathogen that possesses three distinct type VI secretion systems (named T6SS-1, T6SS-2, and T6SS-3). Our previous work indicated that only T6SS-2 of P. plecoglossicida mediates interbacterial competition. However, the antibacterial T6SS effectors and their functions are unclear. Here, we reported two T6SS effectors that mediate antibacterial activity. We first identified four putative antibacterial effectors (denoted as Txe1, Txe2, Txe3, and Txe4) and their cognate immunity proteins encoded in P. plecoglossicida strain XSDHY-P by analyzing the regions downstream of three vgrG genes. We showed that the growth of Escherichia coli cells expressing Txe1, Txe2, and Txe4 was inhibited, and these three effectors exhibited nuclease activity in vivo. The interbacterial competition assays with single- or multi-effector deletion mutants as attackers revealed that Txe1 was the predominant T6SS toxin of P. plecoglossicida strain XSDHY-P mediating the interbacterial killing. This work contributes to our understanding of bacterial effectors involved in the interbacterial competition. [ABSTRACT FROM AUTHOR]

Pseudomonas plecoglossicida is a temperature-dependent opportunistic pathogen mediating visceral granulomas in many piscine species including the large yellow croaker (Larimichthys crocea) but the underlying mechanisms are unclear. RpoE is an alternative sigma (σ) factor involved in regulated intramembrane proteolytic (RIP) cascade, enabling bacterial pathogens to coordinate the expression of genetic traits associated with stress adaptation and virulence determinants in response to diverse stimuli in vitro and in vivo of the hosts. In this study, genes associated to RIP cascade in P. plecoglossicida were identified and characterized to show various sequence similarities to their counterparts in Escherichia coli and P. aeruginosa. The expression of P. plecoglossicida RIP locus was induced by higher temperatures. Moreover, RNA sequencing approach revealed that RpoE regulated the expression of ∼297 and ∼261 genes at virulent (18 °C) and non-virulent (28 °C) temperatures, respectively. RpoE regulon genes are involved in various processes associated with bacterial signal transduction, membrane homeostasis, energy metabolism and virulence. In particular, RpoE positively controlled expression of csrA encoding an RNA binding protein essential for central carbon metabolism. In addition, P. plecoglossicida RpoE was validated to regulate type VI secretion system (T6SS) expression, bacteria competition, biofilm formation and reproduction in macrophages. Collectively, RpoE-centered RIP cascade appeared to play important roles in control of the expression of genes involved in adaptation in vivo and in vitro niches by thermal sensing in P. plecoglossicida. These results facilitates to reveal the pathogenic mechanisms of P. plecoglossicida causing fish diseases and provides new perspectives to control bacterial infection. [ABSTRACT FROM AUTHOR]