Acetyltransferase toxin SsGNAT affects adhesion of Streptococcus suis

Abstract:

[Objective] Streptococcus suis is a major zoonotic pathogen. This study aims to characterize the acetyltransferase toxin of the toxin-antitoxin system in Streptococcus suis and analyze the functions of the toxin, which may contribute to exploring the role of the toxin-antitoxin system in the infection of S. suis. [Methods] We first predicted the putative type Ⅱ toxin-antitoxin system in the genome of S. suis serotype 5 strain HN105. We further determined the activity and analyzed the phylogenetic relationship of the toxin-antitoxin system. Western blotting was employed to determine the expression of the toxin in S. suis. We constructed the toxin-deficient strain, antitoxin-deficient strain, and toxin-antitoxin-deficient strain with HN105 as the parental strain to explore the effects of the system on the adhesion, biofilm formation, anti-phagocytosis, and intracellular survival of S. suis. [Results] The Gcn5-related N-acetyltransferase (GNAT) toxin of the toxin-antitoxin system encoded by DF184_RS00980-DF184_RS00985 in the genome of the strain HN105 was predicted and identified, and the system was named SsMarR-SsGNAT based on the conserved domain. SsGNAT toxin exerted toxicity in the periplasmic space of Escherichia coli, and the antitoxin could neutralize the toxicity. SsGNAT toxin showed low amino acid sequence identity and distant genetic relationship with other acetyltransferase toxins. SsGNAT toxin was cleaved in S. suis, and the reduced expression of antitoxin upregulated the expression of SsGNAT toxin. SsGNAT toxin had no effect on the biofilm formation, anti-phagocytosis or intracellular survival of S. suis, while it affected the adhesion of S. suis to human laryngeal cancer epithelial cells (Hep-2) and human brain microvascular endothelial cells (HBMEC) cells. [Conclusion] We discovered and identified the novel type Ⅱ toxin-antitoxin system SsMarR-SsGNAT in S. suis. SsGNAT toxin exerted toxicity in the periplasmic space of E. coli, and its deletion promoted bacterial adhesion to cells. The findings provide a reference for further unraveling the pathogenic mechanism of S. suis.