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Vibrio harveyi is a normal flora in natural marine habitats and a significant opportunistic pathogen in marine animals. This bacterium can cause a series of lesions after infecting marine animals, in which muscle necrosis and ulcers are the most common symptoms. This study explored the adaptation mechanisms of V. harveyi from the seawater environment to host fish muscle environment. The comprehensive transcriptome analysis revealed dramatic changes in the transcriptome of V. harveyi during its adaptation to the host fish muscle environment. Based on Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, flagellar assembly, oxidative phosphorylation, bacterial chemotaxis, and two-component systems play crucial roles in V. harveyi's adaptation to host fish muscle. A comparison of biological phenotypes revealed that V. harveyi displayed a significant increase in flagellar length, swimming, twitching, chemotaxis, adhesion, and biofilm formation after induction by host fish muscle, and its dominant amino acids, especially bacterial chemotaxis induced by host muscle, Ala and Arg. It could be speculated that the enhancement of bacterial chemotaxis induced by amino acids plays a key role in the adaptation of V. harveyi from seawater to the muscle of the host fish. [ABSTRACT FROM AUTHOR]

Aeromonas hydrophila infections are common in aquaculture. Our previous studies found that the A. hydrophila B11 strain can survive in fish macrophages for at least 24 h and the two‐component system EnvZ/OmpR may be involved in intracellular survival. To reveal the role and mechanism of the two‐component system EnvZ/OmpR in intracellular survival of A. hydrophila, the genes of envZ/ompR were silenced by shRNAi. The results showed that the survival rates of the envZ‐RNAi and ompR‐RNAi strains were only 2.05% and 3.75%, respectively, which were decreased by 91% and 83.6% compared with that of the wild‐type strain. The escape ability of envZ‐RNAi and ompR‐RNAi was also decreased by 51.4% and 19.7%, respectively. The comparative transcriptome analysis revealed that the functional genes directly related to bacterial intracellular survival mainly included the genes related to anti‐stress capacity, and the genes related to Zn2+ and Mg2+ transport. Further research confirmed that two‐component system EnvZ/OmpR can regulate the expression of the important molecular chaperones, such as groEL, htpG, dnaK, clpB and grpE. The expression of these molecular chaperones in wild‐type strain was up‐regulated with the increase in H2O2 concentrations, while the expression of these molecular chaperones in silent strains did not change significantly. Cells that phagocytosed wild‐type strain had higher ROS content than cells that phagocytosed silent strains. Two‐component system EnvZ/OmpR could also regulate zinc transporter (znuA, znuB, znuC) and zinc efflux protein (zntA) to maintain zinc homeostasis in cells, thus affecting the ability of bacteria to survive in phagocytes. Moreover, two‐component system EnvZ/OmpR could affect the growth and intracellular survival of A. hydrophila by regulating the expression of MgtA, MgtC and MgtE and participating in bacterial Mg2+ homeostasis in fish macrophages. [ABSTRACT FROM AUTHOR]