AcsS inhibits the hemolytic activity and thermostable direct hemolysin (TDH) gene expression in .

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 gene is primarily responsible for TDH production and the associated KP. AcsS was originally identified as an activator of swimming and swarming motility in . 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 expression in using phenotypic tests for KP, quantitative real-time polymerase chain reaction, LacZ fusion, and electrophoretic mobility shift assays. The data showed that hemolytic activity and 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 . However, overexpression of AcsS in an strain suppressed the expression of . Collectively, these results suggested that AcsS suppresses the hemolytic activity of through the downregulation of transcription. The data enhanced our understanding of the regulatory networks governing expression and the roles of AcsS in this bacterium.