The growth curve of S. aureus ATCC 29213 is shown in Fig. 1. We found that 1/16 × MIC, 1/8 × MIC, and 1/4 × MIC of licochalcone A had no obvious click here effects on the growth of S. aureus. Although S. aureus grew in the presence of 1/2 × MIC of licochalcone A, the growth velocity was much slower, and after 30 min, the OD value was only 51.5% of that of the control culture. However, after 360 min of licochalcone A treatment, there was no significant difference in the OD value among all the cultures. The secretion of two major enterotoxins (SEA and SEB) by S. aureus, when exposed to subinhibitory concentrations of licochalcone A, was analysed in the study; both MSSA ATCC 29213 and MRSA strain 2985 were investigated. As shown
in Fig. 2, the addition of licochalcone A reduced the secretion of SEA and SEB in a dose-dependent manner. Growth in the presence of 1/16 × MIC licochalcone A led to a measurable reduction
in SEA and SEB secretion; at 1/2 × MIC, no immunoreactive protein could be detected in cultures of ATCC 29213 and MRSA 2985. The proteolytic activity of the cultures was determined to confirm whether the reduction of SEA and SEB secretion by S. aureus was due to an increase in protease secretion induced by licochalcone A. There was no significant effect on protease secretion by ATCC 29213 or MRSA 2985 cultured with 1/2 × MIC of licochalcone A (data not shown). It is well known that among the proteins released, enterotoxins are the most important exotoxins secreted by S. aureus that could act as superantigens, stimulating T cells to release proinflammatory cytokines and stimulating T-cell proliferation selleck (Balaban & Rasooly, 2000). Therefore, in this study, a TNF release assay and a murine T-cell proliferation assay were performed to clarify the biological relevance of the reduction in SEA and SEB secretion caused by licochalcone A. As expected, the culture supernatants of S. aureus grown in the presence of graded subinhibitory concentrations of licochalcone A elicited much lower TNF-α production by spleen cells (Fig. 3) and stimulated a significantly lower level of T-cell
proliferation (Fig. 4). In addition, licochalcone A itself did not induce TNF release or stimulate T-cell activation at 1 × MIC or 2 × MIC concentrations. Apparently, licochalcone A reduced the TNF-inducing and T-cell-activating activities in a Glutathione peroxidase dose-dependent manner. Real-time RT-PCR was performed to evaluate the transcriptional level of sea, seb, and agrA after treatment with subinhibitory concentrations of licochalcone A. As shown in Fig. 5, licochalcone A markedly decreased the transcription of sea, seb, and agrA in S. aureus strains ATCC 29213. When cultured with 1/2 × MIC of licochalcone A, the transcriptional levels of sea, seb, and agrA in strain ATCC 29213 were decreased by 6.2-, 7.6-, and 4.2-fold, respectively. The investigated genes were affected by licochalcone A at the transcriptional level in a dose-dependent manner.