3.3 Antimicrobial effect of indoxyl
Indoxyl played a key role in the changes of indole alkaloids (Fig. 1), and during the dynamic change of the indole alkaloids, the content of indoxyl exhibited the highest inhibitory effect on the growth ofS. aureus (Fig. 2). To investigate the inhibitory effect of indoxyl, indoxyl solution was first prepared by enzymatic treatment of the indican solution obtained at 100 ℃ in a nitrogen environment. Because indican extract contains a large number of impurities such as protein, organic acid and chlorophyll (Feng et al., 2016), ethanol precipitation was used to remove the impurities from the indican solution in this study, followed by rotary evaporation to remove ethanol. The indoxyl concentration in the resulting solution was 1.71 g/L.
Indoxyl of different concentrations was added to the medium for S. aureus culture. As shown in Fig. 3a, OD600 decreased with the increase in indoxyl concentration. When the indoxyl content was low, the growth of S. aureus in the experimental group was similar to that in the control group, and glucose could be completely consumed within 10 h (Fig. 3b). With the increase in indoxyl concentration, the trend of glucose consumption gradually weakened. In particular, when the inhibitory concentration was 0.6 g/L, the value of OD600 in the solution remains basically unchanged, little glucose was consumed, and the production of acetic acid was suppressed (Fig. 3c). To further confirm the inhibitory effect of indoxyl on S. aureus , cells were cultured for 10 h and then coated with a plate to calculate the number of living cells. The results showed that the cell activity of S. aureus would be affected by indoxyl. The number of living cells per unit volume was inversely proportional to the concentration of indoxyl (Fig. 3d).