Isolation and Antibacterial Activity of Indole Alkaloids from Pseudomonas aeruginosa UWI-1

Abstract

In this study, we report the first isolation of three antibiotic indole alkaloid compounds from a Pseudomonad bacterium, Pseudomonas aeruginosa UWI-1. The bacterium was batch fermented in a modified Luria Broth medium and compounds were solvent extracted and isolated by bioassay-guided fractionation. The three compounds were identified as (1) tris(1H-indol-3-yl) methylium, (2) bis(indol-3-yl) phenylmethane, and (3) indolo (2, 1b) quinazoline-6, 12 dione. A combination of 1D and 2D NMR, high-resolution mass spectrometry data and comparison from related data from the literature was used to determine the chemical structures of the compounds. Compounds 1-3 were evaluated in vitro for their antimicrobial activities against a wide range of microorganisms using the broth microdilution technique. Compounds 1 and 2 displayed antibacterial activity against only Gram-positive pathogens, although 1 had significantly lower minimum inhibitory concentration (MIC) values than 2. Compound 3 displayed potent broad-spectrum antimicrobial activity against a range of Gram positive and negative bacteria. Several genes identified from the genome of P. aeruginosa UWI-1 were postulated to contribute to the biosynthesis of these compounds and we attempted to outline a possible route for bacterial synthesis. This study demonstrated the extended metabolic capability of Pseudomonas aeruginosa in synthesizing new chemotypes of bioactive compounds.

Keywords: 12 dione; 1b) quinazoline-6; Pseudomonas aeruginosa; antibacterial compounds; bis(indol-3-yl) phenylmethane; indole alkaloids; indolo (2; natural products; tris(1H-indol-3-yl) methylium.

Figure 1

Antibacterial compounds isolated from Pseudomonas aeruginosa UWI-1. (1) tris(1H-indol-3-yl) methylium; (2) bis(indol-3-yl)phenylmethane; and (3) indolo (2, 1b) quinazoline-6, 12 dione (Tryptanthrin).

Figure 2

Proposed biosynthesis mechanisms for Compounds 1 and 2 by P. aeruginosa UWI-1. Indole is proposed to originate from tryptophan degradation via an unclassified carbon–carbon lyase. The oxidative polymerization of homogentisate (HGA) to HGA melanin serves as the catalyst for the formation of compounds 1 and 2. Compound 1 is putatively formed via condensation of two molecules of indole and one molecule of indole-3-carboxaldehyde. Indole-3-carboxaldehyde is an intermediate product in the IAA synthesis pathway found in Pseudomonads. Compound 2 is putatively formed by the condensation of two molecules of indole and one molecule of benzaldehyde. Benzaldehyde is proposed to be derived as a product of polycyclic aromatic hydrocarbon metabolism by naphthalene 1,2-dioxygenase. The location of each gene involved in the biosynthesis is given in parentheses and is based on the genome of P. aeruginosa UWI-1 available from the European Nucleotide Archive (ENA) project number PRJEB32405 (https://www.ebi.ac.uk/ena/data/view/PRJEB32405).

Figure 3

Proposed biosynthesis mechanism for tryptanthrin by P. aeruginosa UWI-1. Indole is proposed to originate from tryptophan degradation via an unclassified carbon–carbon lyase. Indole is oxygenated either into 3-oxyindole by indole monooxygenases (IM) and then further oxygenated by inosine 5-monophosphate dehydrogenase (IMPD) into isatin. In another route, indole may be converted directly to isatin via naphthalene 1,2-dioxygenase (NDO). Anthranilic acid is obtained through conversion of chorismic acid by the action of anthranilate synthase (AS). Both isatin and anthranilic acid fuse to form tryptanthrin. The location of each gene involved in the biosynthesis is given in parentheses and is based on the genome of P. aeruginosa UWI- 1 available from the European Nucleotide Archive (ENA) project number PRJEB32405 (https://www.ebi.ac.uk/ena/data/view/PRJEB32405).