3.4 Comparison of the inhibitory effects of indigo, indirubin and indoxyl
Previous studies showed that indigo and indirubin exhibited antimicrobial effects (Chiang et al., 2013). On the other hand, there have been no studies on the inhibitory effect of indoxyl. Because indoxyl is unstable in the presence of oxygen, it is possible that parts of indoxyl may be oxidized to indigo and indirubin during microbial growth. To determine if the indigo and indirubin produced during the oxidation of indoxyl had an inhibitory effect on the growth of S. aureus , the bacteria were cultured in the media containing the same concentrations of indigo and indirubin with indoxyl.
As shown in Fig. 3e and 3f, indigo and indirubin had little effect on glucose consumption and acetate production. On the other hand, the inhibition by indoxyl of glucose consumption and acetic acid formation was significantly higher than those of indigo and indirubin. This could be because indigo and indirubin had poor solubility in aqueous solution so that they did not have a strong antibacterial effect, whereas it was easier for indoxyl as a soluble small molecular substance to enter into cells, affecting their growth.