Biodegradation and Biotransformation of Indole: Advances and Perspectives

Abstract

Indole is long regarded as a typical N-heterocyclic aromatic pollutant in industrial and agricultural wastewater, and recently it has been identified as a versatile signaling molecule with wide environmental distributions. An exponentially growing number of researches have been reported on indole due to its significant roles in bacterial physiology, pathogenesis, animal behavior and human diseases. From the viewpoint of both environmental bioremediation and biological studies, the researches on metabolism and fates of indole are important to realize environmental treatment and illuminate its biological function. Indole can be produced from tryptophan by tryptophanase in many bacterial species. Meanwhile, various bacterial strains have obtained the ability to transform and degrade indole. The characteristics and pathways for indole degradation have been investigated for a century, and the functional genes for indole aerobic degradation have also been uncovered recently. Interestingly, many oxygenases have proven to be able to oxidize indole to indigo, and this historic and motivating case for biological applications has attracted intensive attention for decades. Herein, the bacteria, enzymes and pathways for indole production, biodegradation and biotransformation are systematically summarized, and the future researches on indole-microbe interactions are also prospected.

Keywords: biodegradation; biotransformation; functional study; indigo; indole; signaling molecule.

Figure 1

Indole synthesis pathway in bacteria.

Figure 2

Indole synthesis pathway in plants.

Figure 3

Summary of indole aerobic degradation pathways. A, a Gram-negative bacterium isolated from tap water (Sakamoto et al., 1953); B, Alcaligenes spec strain In 3 (Claus and Kutzner, 1983); C, C. sp. SHE (Qu et al., 2017); D, P. sp. ST-200 (Doukyu and Aono, 1997); E, a Gram-positive coccus (Fujioka and Wada, 1968) and A. sp. O153 (Sadauskas et al., 2017); F, C. sp. KK10 and A. pittii L1 (Fukuoka et al., ; Yang et al., 2017).

Figure 4

Summary of indole aerobic degradation functional enzymes. (A) overall indole degradation pathway and corresponding enzymes; (B) iif gene cluster organization in strain A. sp. O153 and C. sp. SHE.

Figure 5

Summary of indole anaerobic pathways under different conditions.

Figure 6

Indigoids biosynthesis pathway by naphthalene dioxygenase.

Figure 7

Indigoids biosynthesis pathway by cytochrome P450 monooxygenases.

Figure 8

Indigoids biosynthesis pathway by phenol hydroxylase.

Figure 9

Indigoids biosynthesis pathway by flavin-containing monooxygenase.

Figure 10

A novel glucosyl protecting group based indigo production and dying strategy.

Figure 11

Research perspectives on indole-microbe interactions.