A 58.8–60% increase in ROS was caused by glutathione in the strain in which there was a significant decrease in the MIC (resistant S. aureus 22), whereas in the sensitive strain, glutathione increases the production of ROS only by 12.8–16.6%, without any significant change occurring in MIC. There was a correlation between the stimulus of ROS and the decrease of MIC caused by exogenus glutathione. The glutathione stimulated intracellular ROS, even in strains without the antibiotic, and also increased the oxidative selleck stress at all concentrations of the antibiotics assayed. However, this enhancement was more marked at the higher concentrations of both antibiotics (Figs 3 and 4). The

exogenous glutathione decreased the extracellular ROS, up to a maximum of 86% in the two strains treated with ciprofloxacin, with similar results being obtained with gentamicin. It was previously Selleck AZD6244 shown that synthetic quinolone antibiotics

promoted the formation of the hydroxyl radical that contributed to cell death (Kohanski et al., 2007), and it was proposed that oxidative damage contributes to bactericidal cell death following gyrase poisoning with an oxygen-dependent death pathway appearing to amplify the primary effect on gyrase (Dwyer et al., 2007). Glutathione was chosen because it is a scavenger of ROS, which has been shown to be involved in protecting the cell either directly or indirectly. This might constitute an adaptive response to oxidative damage, which is known to increase in the presence of the antibiotic (Prinz et al., 1997; Carmel-Harel & Storz, 2000; Pomposiello & Demple, 2002). Compounds such as glutathione can rapidly cross the cell L-NAME HCl membrane, due to their hydrophobic nature, low molecular weight and the presence of specific transporters for these antioxidants in the cell membrane, thus allowing them to produce an antioxidant action in the cytosol (Parry & Clark, 2002; Zhang et al., 2003). A previous study conducted on Escherichia coli suggests that glutathione modulates

the effect of antibiotics (Goswami & Jawali, 2007). These authors reported a reduction in MIC for ampicillin and penicillin, from 8 to 4 μg mL−1 and from 64 to 48 μg mL−1, respectively, which is not as marked as that found in our study for ciprofloxacin and gentamicin in S. aureus. According to our results, there exists the possibility of modifying the sensitivity of resistant strains of S. aureus by the addition of glutathione. These antecedents sustain the hypothesis of our work, which suggests that the antioxidants are useful to improve the bactericidal action of ciprofloxacin. Considering that the antioxidant defense in S. aureus is transcriptionally regulated, and that the expression of oxyR genes occurs in response to external conditions via a glutathione-dependent redox enzyme (Zheng et al., 2001; Uziel et al.