Quercetin a major biomarker of Psidium guajava L. inhibits SepA protease activity of Shigella flexneri in treatment of infectious diarrhoea.

The leaves of the plant Psidium guajava L. (Myrtaceae) has been traditionally used in treatment of various gastrointestinal disorders including diarrhoea and have also been reported for its potent antidiarrhoeal activity on various chemical induced diarrhoea models. The objective of our present study was to evaluate the potency of the leaf extract of the plant Psidium guajava (PGE) along with its major biomarker quercetin against Shigella flexneri-induced sub chronic model of infectious diarrhoea. PGE at 100, 200 and 400 mg/kg, p.o. and quercetin at 50 mg/kg, p.o. were administered to Shigella flexneri-induced diarrhoeal rats for five days and various behavioural parameters were evaluated on 1st, 3rd and 5th day of treatment. This was followed by assessment of stool water content, density of Shigella flexneri in stools and blood parameters examination. After treatment, colon and small intestine of rats was dissected and subjected to biochemical estimations, cytokine profiling, antioxidant evaluations, ion concentration determination, Na⁺/K⁺-ATPase activity and histopathology. Molecular docking studies on crystal structure of Secreted Extracellular Protein A (SepA) from Shigella flexneri with biomarker quercetin was also performed. PGE at 200 mg/kg followed by quercetin depicted maximum antidiarrhoeal potential, which was confirmed through diarrhoea score and % protection, while PGE at 400 mg/kg showed similar effect to PGE 200 mg/kg thus, the later may have ceiling effect. PGE and quercetin also significantly reduced the density of Shigella flexneri in stools, water content of stools and restored the alterations observed in blood parameters, antioxidant status and pro-inflammatory cytokines (IL-6 and TNF-α) expression. These parameters contributed in normalization of electrolyte balance, reactivation of Na⁺/K⁺-ATPase activity and repairing of epithelial tissue damage, confirmed through histopathology. Docking simulation studies revealed the role of quercetin in inactivating the protease activity of SepA, a protein secreted by Shigella, which disrupts epithelial barrier integrity during infection and also manages its signal production. Thus, the overall results confirmed the role of quercetin as a major biomarker for the observed antidiarrhoeal potential of P. guajava against Shigella flexneri induced infectious diarrhoea.