Comparative Transcriptome Analysis of Vibrio splendidus JZ6 Reveals the Mechanism of Its Pathogenicity at Low Temperatures

Yesso scallop-pathogenic Vibrio splendidus strain JZ6 was found to have the highest virulence at 10 degrees C, while its pathogenicity was significantly reduced with increased temperature and completely incapacitated at 28 degrees C. In the present study, comparative transcriptome analyses of JZ6 and another nonpathogenic V. splendidus strain, TZ19, were conducted at two crucial culture temperatures (10 degrees C and 28 degrees C) in order to determine the possible mechanism of temperature regulation of virulence. Comparisons among four libraries, constructed from JZ6 and TZ19 cultured at 10 degrees C and 28 degrees C (designated JZ6_10, JZ6_28, TZ19_10, and TZ19_28), revealed that 241 genes were possibly related to the increased virulence of JZ6 at 10 degrees C. There were 10 genes, including 2 encoding Flp pilus assembly proteins (FlhG and VS_2437), 6 encoding proteins of the "Vibrio cholerae pathogenic cycle" (ToxS, CqsA, CqsS, RpoS, HapR, and Vsm), and 2 encoding proteins in the Sec-dependent pathway (SecE and FtsY), that were significantly upregulated in JZ6_10 (P < 0.05) compared to those in JZ6_28, TZ19_10, and TZ19_28, which were supposed to be responsible for adhesion, quorum sensing, virulence, and protein secretion of V. splendidus. When cultured at 10 degrees C, JZ6 cells were larger and tended to aggregate more than those cultured at 28 degrees C. The virulence factor (extracellular metalloprotease) was also found to be highly expressed in the extracellular product (ECP) of JZ6 at 10 degrees C, and this ECP exhibited obvious cytotoxicity to oyster primary hemocytes, A549 cells, and L929 cells. These results indicated that low temperatures (10 degrees C) could enhance adhesion, activate the quorum sensing systems, upregulate virulence factor synthesis and secretion, and, lastly, increase the pathogenicity of JZ6.