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).