1045 Original Paper Cell Physiol Biochem 2011;28:1045-1050 Accepted: July 25, 2011 Cellular Physiology Cellular Physiology Cellular Physiology Cellular Physiology Cellular Physiology and Biochemistr and Biochemistr and Biochemistr and Biochemistr and Biochemistry Copyright © 2011 S. Karger AG, Basel Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com © 2011 S. Karger AG, Basel 1015-8987/11/0285-1045$38.00/0 Accessible online at: www.karger.com/cpb Baicalein Suppresses the SOS Response System of Staphylococcus Aureus Induced by Ciprofloxacin Qing Peng 1 , Shuqin Zhou 2 , Fen Yao 1 , Bing Hou 3 , Yuanchun Huang 4 , Dexing Hua 5 , Yanshan Zheng 1 and Yuanshu Qian 1 1 Pharmacology Department, Shantou University Medical College, Shantou, Guangdong, 2 Zhujiang Hospital, Southern medical University, Guangzhou, Guangdong, 3 Clinical Laboratory of Dermatosis Hospital, Shantou, Guangdong, 4 Clinical Laboratory of First Affiliated Hospital, Shantou University Medical College, Shantou, Guangdong 5 Clinical Laboratory of Yuebei Hospital, Shaoguan, Guangdong Y. S. Qian Pharmacology Department, Shantou University Medical College Shantou, Guangdong, 515041 (R. P. China) Tel. + 86 754 8890 0432, Fax +86 754 8855 7562 E-Mail beckypeng@yahoo.cn Key Words Bacailein • SOS response • Staphylococcus aureus • Ciprofloxacin Abstract Numerous antibiotics can induce an SOS repair system in bacteria that leads to antibiotic-resistant mutation of the bacterium. Therefore, searching for drugs that can prevent the SOS response and thus improve the long-term viability of some antibiotics is important. In this study, we aimed to detect the suppressive effects of baicalein on the SOS system and rifampin-resistant mutation in Staphylococcus aureus. We determined the reactive oxygen species (ROS) formation and intracellular ATP level in S. aureus with baicalein treatment to investigate the mechanisms involved in its effects on the SOS system. The results showed that baicalein was a potent inhibitor of the expression of the SOS genes RecA, LexA and SACOL1400. The rifampin-resistant mutation rate of S. aureus induced by ciprofloxacin was significantly reduced after treatment with baicalein. Treatment with baicalein led to a significant decrease in intracellular reactive oxygen species (ROS) formation and ATP level. Our findings indicate that baicalein may be an SOS-response inhibitor in S. aureus through inhibiting ROS formation and ATP production and may be used to prevent excessive mutation induced by antibiotics. Introduction Staphylococcus aureus infection can be difficult to treat because of multidrug resistance and the organism’s remarkable ability to persist in the host. Persistence and the evolution of resistance may be related to several complex regulatory networks such as the SOS response, which is essential in bacterial mutations. SOS system uses the RecA protein, which is stimulated by single-stranded DNA, to inactivate the LexA repressor and induce the response. In addition to the fluoroquinolones [1-3], other antibiotics, such as rifampin [4], aminoglycosides [5], and β-lactams [5, 6], can induce the SOS response and cause mutations resulting in multidrug resistance. Therefore,