AcsS inhibits the hemolytic activity and thermostable direct hemolysin (TDH) gene expression in <i>Vibrio parahaemolyticus</i>

<i>Vibrio parahaemolyticus</i> produces a key virulent factor known as thermostable direct hemolysin (TDH). TDH exhibits diverse biological activities, including hemolytic activity. The β-type hemolysis observed on Wagatsuma agar due to TDH is recognized as the Kanagawa phenomenon (KP). The <i>tdh2</i> gene is primarily responsible for TDH production and the associated KP. AcsS was originally identified as an activator of swimming and swarming motility in <i>V. parahaemolyticus</i>. However, its potential roles in other cellular pathways remain unclear. In this study, we investigated the regulatory effects of AcsS on the hemolytic activity and <i>tdh2</i> expression in <i>V. parahaemolyticus</i> using phenotypic tests for KP, quantitative real-time polymerase chain reaction, LacZ fusion, and electrophoretic mobility shift assays. The data showed that <i>V. parahaemolyticus</i> hemolytic activity and <i>tdh2</i> transcription were under the negative control of AcsS. Additionally, in-vitro binding assays revealed that His-AcsS could not bind to the regulatory DNA region of <i>tdh2</i>. However, overexpression of AcsS in an <i>Escherichia coli</i> strain suppressed the expression of <i>tdh2</i>. Collectively, these results suggested that AcsS suppresses the hemolytic activity of <i>V. parahaemolyticus</i> through the downregulation of <i>tdh2</i> transcription. The data enhanced our understanding of the regulatory networks governing <i>tdh2</i> expression and the roles of AcsS in this bacterium.