Antimicrobial and antivirulence function of cinnamaldehyde against Streptococcus suis type 2.

Streptococcus suis type 2 (SS2) is an important zoonotic pathogen for swine and humans. The increasing prevalence of antimicrobial resistance in S. suis isolates poses a threat to public health. This study investigated the antimicrobial activity and therapeutic potential of cinnamaldehyde (CA), a natural compound from cinnamon, against SS2. CA showed significant antimicrobial activity with a minimal inhibition concentration of 0.25 µg/mL and prolonged post-antibiotic effect of over 7 h in SS2. Increased bacterial cell membrane permeability and blocked protein synthesis of SS2 were observed after being treated with CA. CA could effectively prevent biofilm formation. CA treatment reduced the crucial virulence factor of suilysin expression and secretion in SS2 cells through a probable interaction with the suilysin protein. CA treatment could prominently alleviate both epithelial HEp-2 and phagocytic RAW264.7 cell cytotoxicity induced by SS2. The pathogenic SS2 strain was attenuated by CA, as demonstrated by the diminished adherence in HEp-2 cells, increased clearance by RAW264.7 and mice whole blood, and improved survival rate in a mouse infection model. CA possessed therapeutic efficacy since the CA treatment exhibited a 50% improvement in mouse survival rate, which surpassed the traditional ampicillin therapy control group. Alleviated clinical symptoms and histopathological phenotypes, with reduced bacterial burden in mouse organs after CA treatment, were examined. Overall, this study identified cinnamaldehyde as a novel antibacterial compound against SS2 with potential therapeutic protective effects, offering an alternative drug for controlling SS2 prevalence and infection.

Importance: Widespread infections caused by Streptococcus suis type 2 (SS2) have garnered significant attention in the realm of public health due to their zoonotic nature. In recent years, antimicrobial resistance phenotypes in SS2 have emerged and intensified within the context of animal husbandry. Herbal compounds and medicinal plants are increasingly recognized as promising therapeutic alternatives for mitigating or addressing the challenges posed by antimicrobial resistance. The aim of this present study was to explore a novel compound of cinnamaldehyde, which obtained significant antimicrobial activity and potential therapeutic protective effect against SS2 infection. The research has made an innovative discovery that the bactericidal effect of cinnamaldehyde is associated with its antivirulence strategies, such as targeting the key virulence factors of SS2 and countering the bacterial infection process.