aureus isolates originating from community and nosocomial sources necessitates the development of new and improved antimicrobial agents for the prevention and treatment of these life-threatening infections (Hall et al., 2003). To date, many studies have focused on naturally occurring compounds

(Smith-Palmer et al., 2004). Our previous research has shown that the MICs of licochalcone A against 27 S. aureus strains ranged from 2 to 8 μg mL−1(Qiu et al., 2009). It is uncommon for compounds isolated from medical plants to have such powerful antimicrobial activities on both selleck MSSA and MRSA. Consequently, licochalcone A may potentially be used as a lead compound for the design of more potent antibacterial agents (based on the chalcone template) to be used to fight drug-resistant S. aureus strains. On the other hand, an alternative strategy that is now gaining interest to treat with S. aureus infections is the targeting of bacterial virulence factors (e.g. haemolysins, enterotoxins, adhesins) (Song et al., 2009). A number of virulence factors secreted by S. aureus play a significant role in pathogenesis. Therefore, the clinical performance of antibiotics used for the treatment of S. aureus infections not only depends on the respective bacteriostatic or bactericidal effects but also on the ability R428 chemical structure to prevent the release of virulence factors by dying

or stressed bacteria (Bernardo et al., 2004). Previous studies have indicated that enterotoxins secreted by S. aureus are affected by many antibiotics, especially

at suboptimal concentrations. Protein synthesis inhibitors such as linezolid can reduce the expression of S. aureus virulence factors including enterotoxins A and B at subgrowth-inhibitory concentrations (Bernardo et al., 2004). In contrast, β-lactam antibiotics induce or increase enterotoxin production, suggesting that the symptoms of S. aureus PLEKHB2 infections, especially MRSA infections, may be aggravated when patients are treated with these antibiotics (Stevens et al., 2007). Furthermore, it has been shown that some plant compounds (e.g. oleuropein) and plant essential oils (e.g. oils of bay, cinnamon, and clove) can also influence the production of enterotoxins when used at subinhibitory concentrations (Tranter et al., 1993; Smith-Palmer et al., 2004). The antibiotic-induced regulation of virulence factors may result in either aggravation or attenuation of the disease. Therefore, the up- or downregulation of toxin secretion is significant for diseases caused by S. aureus, and the ability of antibiotics to affect these properties may be an important criterion in selecting an antibiotic for therapy (Blickwede et al., 2005). In this study, licochalcone A was shown by Western blot assay, TNF release assay, murine T-cell proliferation assay, and real-time RT-PCR to repress SEA and SEB secretion by S. aureus in a dose-dependent manner. The expression of most virulence factors by S.