CqsA inhibits the virulence of Vibrio harveyi to the pearl gentian grouper (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatus)

Abstract

Vibrio harveyi is an opportunistic Gram-negative pathogen that threatens future development of global aquaculture as well as human health. In this study, to gain insight into virulence regulation of cqsA in V. harveyi, a ΔcqsA mutant strain and complementation of the ΔcqsA mutant (C-cqsA) were constructed and phenotypically characterized. Deletion of cqsA led to enhanced mobility, iron absorption, resistance to H2O2, and biofilm formation ability, and reduced the ability to secrete extracellular proteases. In addition, pearl gentian grouper (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatu) challege experiment was conducted to observe the changes in virulence at the absence of cqsA in V. harveyi. The results showed that the cumulative mortalities of the pearl gentian grouper challenged with the ΔcqsA+ mutant by intraperitoneal injection reached 91.67%, which was significantly higher than that of wild-type (WT+) (41.67%) and C-cqsA (41.67%) strains. Deletion of cqsA also resulted in a significant increase in the ability of V. harveyi to colonize the spleen and liver of the pearl gentian grouper simultaneously. Meanwhile, the early expression levels of immune-related genes of the pearl gentian grouper in the ΔcqsA+ mutant group, including TNF-α, IL-1β, MHC-Iα, IL-16, and IgM were up-regulated post-challenge, indicating that the ΔcqsA+ mutant may hijack the immune signaling pathway and cause early overexpression of some host immune-related genes. The expression levels of the V. harveyi virulence-associated genes flaA, flaC, fliA, fliS, cheR, lafA, aphA, and fur were up-regulated, while vhh and hapR were down-regulated. Collectively, these findings point to broad regulatory functions of cqsA in V. harveyi cells, which likely underlie an important role in pathogenicity.