Indole concentration was measured by HPLC analysis (Hitachi Primaide, Japan)27 (link). Strain growth was characterized by optical density at 660 nm (OD660) using UV-vis spectrophotometer (Metash UV-9000, China). The indole transformation products at the final cycle of each stage were analyzed using HPLC-MS analysis on a quadrupole ion trap instrument (Agilent HP 1100 LC/MSD, USA) equipped with an atmospheric pressure chemical ionization (APCI) source.
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