M21 affecting the expression and production of selected virulence factors. (A) USA300 growth monitored by OD600 in the presence of different concentrations of M21. (B) Expressions of psm, hla, agrA, lukS, clfA, and sarA in the presence of M21 at different concentrations as monitored by qPCR. (C) Production of protein A, α-toxin, and PVL in the presence of M21 assayed by Western blot in USA300. (D) Production of protein A and α-toxin in the presence of M21 assayed by Western blot in Mu3. (E) M21 inhibition on hemolysis of human erythrocyte caused by S. aureus and on leukotoxic effect of S. aureus against mouse macrophage cell line (J774.1). (F) M21 inhibition on the adherence and invasion effect of S. aureus against A549. Compound pretreated bacteria were used in the adherence assay and invasion assay. All the experiments were repeated at least three times. Data were analyzed by Student t test. All data represent mean values ±SD (*P < 0.05; **P < 0.01; ***P < 0.001). P values were determined using GraphPad Prism.