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.