Inhibitors targeting the autophosphorylation of serine/threonine kinase of Streptococcus suis show potent antimicrobial activity

Antimicrobial resistance (AMR) poses a serious global threat to public health. One promising strategy to combat AMR is the development of novel antibiotics targeting previously unexplored bacterial processes. Recent studies have highlighted the critical roles of serine/threonine kinases (STKs) in the physiology and pathogenesis of several major bacterial pathogens, positioning them as attractive targets for antimicrobial drug development.
We previously demonstrated that the STK in Streptococcus suis (ssSTK)—a significant zoonotic pathogen—regulates key processes including bacterial cell division, metabolism, and virulence. In this study, we identified Thr167 and Ser175 in the activation loop of ssSTK as autophosphorylation sites. A mutant strain deficient in autophosphorylation displayed similar phenotypes to the stk deletion strain, including impaired growth in minimal medium, abnormal cellular morphology, and reduced virulence compared to the wild-type SC19 strain.
Building on these findings, we developed a screening platform to identify ssSTK inhibitors by evaluating S. suis growth in minimal medium and measuring ATP consumption in ssSTK-catalyzed autophosphorylation reactions. Using a commercial kinase inhibitor library, we identified several compounds with inhibitory activity against ssSTK, including Staurosporine, K252a, AT9283, and APY29. These compounds exhibited antimicrobial activity in vitro.
Importantly, APY29 also demonstrated in vivo efficacy in a Galleria mellonella infection model. Molecular docking predicted that these inhibitors interact directly with the ssSTK active site, further supporting their potential mechanism of action.
Overall, our findings highlight the essential role of ssSTK autophosphorylation in S. suis physiology and pathogenicity and suggest that ssSTK inhibitors—particularly APY29—represent promising lead compounds for the development of novel antimicrobials.