The Therapeutic Effect Of Oroxylum Indicum Extract Against Staphylococcus Aureus Pneumonia In Mice

Staphylococcus aureus(S. aureus), an important clinical opportunistic bacterium belongs to zoonotic gram positive pathogens, could cause various lethal infections in humans and animals. The long term use of antibiotics coupled with the widespread dissemination and access to drug resistance, especially the emergence of MRSA and VRSA, leads to the treatment of S. aureus infection increasingly difficult, which has caused serious economic loss and harm to the society. Recently, lots of reports have revealed that the virulence of S. aureus is closely related to the virulence factors secreted by this pathogenic bacterium. α-hemolysin, secreted by most pathogenic S. aureus, is one of the members of pore-forming toxin and an essential virulence factor for S. aureus virulence, which could lyse the targeted cells and lead to the occurrence of disease, indicating that α-hemolysin represents a promising target for developing agents for S. aureus infection.In this study, the inhibitors of α-hemolysin have been screened by hemolysis assay based on its function. The results suggested that oroxylum indicum extract could directly inhibit the hemolytic activity of α-hemolysin at the concentrations of 4 μg/m L-32 μg/m L. However, no inhibition of α-hemolysin secretion by S. aureus was observed in the presence of oroxylum indicum extract(4 μg/m L-32 μg/m L). Additionally, the MICs of oroxylum indicum extract against S. aureus ranges 512 μg/m L to 1024 μg/m L, suggesting that oroxylum indicum extract has little antibacterial activity against S. aureus. Furthermore, at the concentrations required for neutralizing the hemolytic activity of α-hemolysin, oroxylum indicum extract has no influence on the growth of S. aureus. These results indicated that oroxylum indicum extract without antibacterial activity could directly neutralize the hemolytic activity of α-hemolysin, which indicates that oroxylum indicum extract represents a promising candidate for the development of anti-virulence compounds for S. aureus infection.Live/Dead assay and LDH assay were employed to further analyze the influence of oroxylum indicum extract on S. aureus virulence using the co-culture system of S. aureus and A549 cells. The results indicated that the addition of oroxylum indicum extract into the co-culture system could significantly inhibit the cell injuries induced by S. aureus, as indicated by lower cell death, improved cell morphology and lower level of LDH release. Finally, the protective effect of oroxylum indicum extract on the pathogenicity of S. aureus in vivo was analyzed using the mouse model of S. aureus pneumonia. The results indicated that treatment with oroxylum indicum extract significantly decreased the mortality of infected mice, alleviated the pathological injury of lung and reduced the bacterial load in lung.Taken together, oroxylum indicum extract, the natural compounds without anti-S. aureus activity, could directly inhibit the hemolytic activity of α-hemolysin at the concentrations of 4 μg/m L-32 μg/m L. Furthermore, the virulence of S. aureus was both reduced by oroxylum indicum extract in vivo and in vitro. Our study suggests that oroxylum indicum extract represents a promising candidate for the development of anti-virulence agents for S. aureus infection, which would also provide a novel strategy and compounds for the battle between us and S. aureus and other pathogenic bacteria.