| Summary: | <i>Vibrio splendidus</i> is a pathogen that infects a wide range of hosts, especially the sea cucumber species <i>Apostichopus japonicus</i>. Previous studies showed that the level of L-glutamic acid (L-Glu) significantly increased under heat stress, and it was found to be one of the best carbon sources used by <i>V. splendidus</i> AJ01. In this study, the effects of exogenous L-Glu on the coelomocyte viability, tissue status, and individual mortality of sea cucumbers were analyzed. The results showed that 10 mM of L-Glu decreased coelomocyte viability and increased individual mortality, with tissue rupture and pyknosis, while 0.1 mM of L-Glu slightly affected the survival of sea cucumbers without obvious damage at the cellular and tissue levels. Transcriptomic analysis showed that exogenous L-Glu upregulated 343 and downregulated 206 genes. Gene Ontology (GO) analysis showed that differentially expressed genes (DEGs) were mainly enriched in signaling and membrane formation, while a Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that DEGs were significantly enriched in the upregulated endocytosis and downregulated lysosomal pathways. The coelomocyte viability further decreased by 20% in the simultaneous presence of exogenous L-Glu and <i>V. splendidus</i> AJ01 compared with that in the presence of <i>V. splendidus</i> AJ01 infection alone. Consequently, a higher sea cucumber mortality was also observed in the presence of exogenous L-Glu challenged by <i>V. splendidus</i> AJ01. Real-time reverse transcriptase PCR showed that L-Glu specifically upregulated the expression of the <i>fliC</i> gene coding the subunit protein of the flagellar filament, promoting the swimming motility activity of <i>V. splendidus</i>. Our results indicate that L-Glu should be kept in a state of equilibrium, and excess L-Glu at the host–pathogen interface prompts the virulence of <i>V. splendidus</i> via the increase of bacterial motility.
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