Indole and 7-hydroxyindole diminish Pseudomonas aeruginosa virulence

Indole Derivatives Obtained from Egyptian Enterobacter sp. Soil Isolates Exhibit Antivirulence Activities against Uropathogenic Proteus mirabilis

Proteus mirabilis is a frequent cause of catheter associated urinary tract infections (CAUTIs). Several virulence factors contribute to its pathogenesis, but swarming motility, biofilm formation, and urease activity are considered the hallmarks. The increased prevalence in antibiotic resistance among uropathogens is alarming and requires searching for new treatment alternatives. With this in mind, our study aims to investigate antivirulence activity of indole derivatives against multidrug resistant P. mirabilis isolates. Ethyl acetate (EtOAc) extracts from Enterobacter sp. (rhizobacterium), isolated from Egyptian soil samples were tested for their ability to antagonize the virulence capacity and biofilm activity of P. mirabilis uropathogens. Extracts of two Enterobacter sp. isolates (coded Zch127 and Cbg70) showed the highest antivirulence activities against P. mirabilis. The two promising rhizobacteria Zch127 and Cbg70 were isolated from soil surrounding: Cucurbita pepo (Zucchini) and Brassica oleracea var. capitata L. (Cabbage), respectively. Sub-minimum inhibitory concentrations (Sub-MICs) of the two extracts showed potent antibiofilm activity with significant biofilm reduction of ten P. mirabilis clinical isolates (p-value < 0.05) in a dose-dependent manner. Interestingly, the Zch127 extract showed anti-urease, anti-swarming and anti-swimming activity against the tested strains. Indole derivatives identified represented key components of indole pyruvate, indole acetamide pathways; involved in the synthesis of indole acetic acid. Additional compounds for indole acetonitrile pathway were detected in the Zch127 extract which showed higher antivirulence activity. Accordingly, the findings of the current study model the feasibility of using these extracts as promising antivirulence agent against the P. mirabilis uropathogens and as potential therapy for treatment of urinary tract infections (UTIs).

The Primary Physiological Roles of Autoinducer 2 in Escherichia coli Are Chemotaxis and Biofilm Formation

Autoinducer 2 (AI-2) is a ubiquitous metabolite but, instead of acting as a "universal signal," relatively few phenotypes have been associated with it, and many scientists believe AI-2 is often a metabolic byproduct rather than a signal. Here, the aim is to present evidence that AI-2 influences both biofilm formation and motility (swarming and chemotaxis), using Escherichia coli as the model system, to establish AI-2 as a true signal with an important physiological role in this bacterium. In addition, AI-2 signaling is compared to the other primary signal of E. coli, indole, and it is shown that they have opposite effects on biofilm formation and virulence.

Discerning the role of polymicrobial biofilms in the ascent, prevalence, and extent of heteroresistance in clinical practice

Antimicrobial therapy is facing a worrisome and underappreciated challenge, the phenomenon of heteroresistance (HR). HR has been gradually documented in clinically relevant pathogens (e.g. Pseudomonas aeruginosa, Staphylococcus aureus, Burkholderia spp., Acinetobacter baumannii, Klebsiella pneumoniae, Candida spp.) towards several drugs and is believed to complicate the clinical picture of chronic infections. This type of infections are typically mediated by polymicrobial biofilms, wherein microorganisms inherently display a wide range of physiological states, distinct metabolic pathways, diverging refractory levels of stress responses, and a complex network of chemical signals exchange. This review aims to provide an overview on the relevance, prevalence, and implications of HR in clinical settings. Firstly, related terminologies (e.g. resistance, tolerance, persistence), sometimes misunderstood and overlapped, were clarified. Factors generating misleading HR definitions were also uncovered. Secondly, the recent HR incidences reported in clinically relevant pathogens towards different antimicrobials were annotated. The potential mechanisms underlying such occurrences were further elucidated. Finally, the link between HR and biofilms was discussed. The focus was to recognize the presence of heterogeneous levels of resistance within most biofilms, as well as the relevance of polymicrobial biofilms in chronic infectious diseases and their role in resistance spreading. These topics were subject of a critical appraisal, gaining insights into the ascending clinical implications of HR in antimicrobial resistance spreading, which could ultimately help designing effective therapeutic options.

EmhR is an indole-sensing transcriptional regulator responsible for the indole-induced antibiotic tolerance in Pseudomonas fluorescens

Indole is well known as an interspecies signalling molecule to modulate bacterial physiology; however, it is not clear how the indole signal is perceived and responded to by plant growth promoting rhizobacteria (PGPR) in the rhizosphere. Here, we demonstrated that indole enhanced the antibiotic tolerance of Pseudomonas fluorescens 2P24, a PGPR well known for its biocontrol capacity. Proteomic analysis revealed that indole influenced the expression of multiple genes including the emhABC operon encoding a major multidrug efflux pump. The expression of emhABC was regulated by a TetR-family transcription factor EmhR, which was demonstrated to be an indole-responsive regulator. Molecular dynamics simulation showed that indole allosterically affected the distance between the two DNA-recognizing helices within the EmhR dimer, leading to diminished EmhR-DNA interaction. It was further revealed the EmhR ortholog in Pseudomonas syringae was also responsible for indole-induced antibiotic tolerance, suggesting this EmhR-dependent, indole-induced antibiotic tolerance is likely to be conserved among Pseudomonas species. Taken together, our results elucidated the molecular mechanism of indole-induced antibiotic tolerance in Pseudomonas species and had important implications on how rhizobacteria sense and respond to indole in the rhizosphere.

Identification of Genetic Variants via Bacterial Respiration Gas Analysis

Indole is a signal molecule derived from the conversion of tryptophan, and it is present in bacterial respiratory gas. Besides influencing bacterial growth, indole exhibits effects on human health, including a positive effect on inflammation and protection against pathogens. However, a high fecal indole concentration (FIC) can suggest an unbalanced gut flora or the presence of certain pathogens. To analyze the indole produced by bacteria, its collection and detection is required. Traditional methods usually require centrifugation of liquid bacterial culture medium and subsequent extraction of indole from the medium or partial purification of indole from fecal samples (e.g., by distillation or extraction). In this study, we demonstrate the possibility of identifying gas contents directly from bacteria, and we distinguish the difference in species and their genetics without the need to centrifuge or extract. Using an absorbent sheet placed above a liquid culture, we were able to collect gas content directly from bacteria. Gas chromatography-mass spectrometry (GC-MS) was used for the analysis. The GC-MS results showed a clear peak attributed to indole for wild-type Escherichia coli cells (MG1655 and MC4100 strains), whereas the indole peak was absent in the chromatograms of cells where proteins, part of the indole production pathway from tryptophan (TnaA and TnaB), were not expressed (by using tnaAB-deleted cells). The indole observed was measured to be present in a low nmol-range. This method can distinguish whether the bacterial genome contains the tnaAB gene or not and can be used to collect gas compounds from bacterial cultures quickly and easily. This method is useful for other goals and future research, such as for measurements in restrooms, for food-handling facilities, and for various applications in medical settings.

Antibiofilm and Antivirulence Properties of Indoles Against Serratia marcescens

Indole and its derivatives have been shown to interfere with the quorum sensing (QS) systems of a wide range of bacterial pathogens. While indole has been previously shown to inhibit QS in Serratia marcescens, the effects of various indole derivatives on QS, biofilm formation, and virulence of S. marcescens remain unexplored. Hence, in the present study, we investigated the effects of 51 indole derivatives on S. marcescens biofilm formation, QS, and virulence factor production. The results obtained revealed that several indole derivatives (3-indoleacetonitrile, 5-fluoroindole, 6-fluoroindole, 7-fluoroindole, 7-methylindole, 7-nitroindole, 5-iodoindole, 5-fluoro-2-methylindole, 2-methylindole-3-carboxaldehyde, and 5-methylindole) dose-dependently interfered with quorum sensing (QS) and suppressed prodigiosin production, biofilm formation, swimming motility, and swarming motility. Further assays showed 6-fluoroindole and 7-methylindole suppressed fimbria-mediated yeast agglutination, extracellular polymeric substance production, and secretions of virulence factors (e.g., proteases and lipases). QS assays on Chromobacterium violaceum CV026 confirmed that indole derivatives interfered with QS. The current results demonstrate the antibiofilm and antivirulence properties of indole derivatives and their potentials in applications targeting S. marcescens virulence.

Diversity of Bacteria and Bacterial Products as Antibiofilm and Antiquorum Sensing Drugs Against Pathogenic Bacteria

The increase in antibiotic resistance of pathogenic bacteria has led to the development of new therapeutic approaches to inhibit biofilm formation as well as interfere quorum sensing (QS) signaling systems. The QS system is a phenomenon in which pathogenic bacteria produce signaling molecules that are involved in cell to cell communication, production of virulence factors, biofilm maturation, and several other functions. In the natural environment, several non-pathogenic bacteria are present as mixed population along with pathogenic bacteria and they control the behavior of microbial community by producing secondary metabolites. Similarly, non-pathogenic bacteria also take advantages of the QS signaling molecule as a sole carbon source for their growth through catabolism with enzymes. Several enzymes are produced by bacteria which disrupt the biofilm architecture by degrading the composition of extracellular polymeric substances (EPS) such as exopolysaccharide, extracellular- DNA and protein. Thus, the interference of QS system by bacterial metabolic products and enzymatic catalysis, modification of the QS signaling molecules as well as enzymatic disruption of biofilm architecture have been considered as the alternative therapeutic approaches. This review article elaborates on the diversity of different bacterial species with respect to their metabolic products as well as enzymes and their molecular modes of action. The bacterial enzymes and metabolic products will open new and promising perspectives for the development of strategies against the pathogenic bacterial infections.

Rapid Killing and Biofilm Inhibition of Multidrug-Resistant Acinetobacter baumannii Strains and Other Microbes by Iodoindoles

Multi-drug resistant Acinetobacter baumannii is well-known for its rapid acclimatization in hospital environments. The ability of the bacterium to endure desiccation and starvation on dry surfaces for up to a month results in outbreaks of health care-associated infections. Previously, indole and its derivatives were shown to inhibit other persistent bacteria. We found that among 16 halogenated indoles, 5-iodoindole swiftly inhibited A. baumannii growth, constrained biofilm formation and motility, and killed the bacterium as effectively as commercial antibiotics such as ciprofloxacin, colistin, and gentamicin. 5-Iodoindole treatment was found to induce reactive oxygen species, resulting in loss of plasma membrane integrity and cell shrinkage. In addition, 5-iodoindole rapidly killed three Escherichia coli strains, Staphylococcus aureus, and the fungus Candida albicans, but did not inhibit the growth of Pseudomonas aeruginosa. This study indicates the mechanism responsible for the activities of 5-iodoindole warrants additional study to further characterize its bactericidal effects on antibiotic-resistant A. baumannii and other microbes.

3D-Printed Microfluidic Devices for Enhanced Online Sampling and Direct Optical Measurements

3D printing has emerged as a robust technique to fabricate reliable and reproducible microfluidic devices. However, a limitation of 3D-printed devices has been the low transparency even when printed in a "clear" material. There are currently no reports regarding direct optical measurements through a 3D-printed device. Here, we present for the first time that the printing orientation can affect the transparency of a 3D-printed object. With the optimal orientation, we printed a microfluidic detector that was sufficiently transparent (transmittance ≈ 80%) for optical quantitation. This finding is significant because it shows the feasibility to directly 3D-print optical components for analytical applications. In addition, we created a novel microfluidic dialysis device via 3D printing, which enabled higher flow rates (for sampling with high temporal resolution) and increased extraction efficiency than commercially available ones. By coupling the microfluidic detector and dialysis probe, we successfully measured the release kinetics of indole from biofilms in a continuous, automated, and near real-time fashion. Indole is an intercellular signaling molecule in biofilms, which may regulate antibiotic resistance. The release kinetics of this molecule had not been quantitated likely because of the lack of a suitable analytical tool. Our results fill this knowledge gap.

Comprehensive Host Cell-Based Screening Assays for Identification of Anti-Virulence Drugs Targeting Pseudomonas aeruginosa and Salmonella Typhimurium

The prevalence of bacterial pathogens being resistant to antibiotic treatment is increasing worldwide, leading to a severe global health challenge. Simultaneously, the development and approval of new antibiotics stagnated in the past decades, leading to an urgent need for novel approaches to avoid the spread of untreatable bacterial infections in the future. We developed a highly comprehensive screening platform based on quantification of pathogen driven host-cell death to detect new anti-virulence drugs targeting Pseudomonas aeruginosa (Pa) and Salmonella enterica serovar Typhimurium (ST), both known for their emerging antibiotic resistance. By screening over 10,000 small molecules we could identify several substances showing promising effects on Pa and ST pathogenicity in our in vitro infection model. Importantly, we could detect compounds potently inhibiting bacteria induced killing of host cells and one novel comipound with impact on the function of the type 3 secretion system (T3SS) of ST. Thus, we provide proof of concept data of rapid and feasible medium- to high-throughput drug screening assays targeting virulence mechanisms of two major Gram-negative pathogens.

Indole signaling decreases biofilm formation and related virulence of Listeria monocytogenes

Bacterial communication system known as quorum sensing (QS) is a pivotal system for bacterial survival, adaptation and pathogenesis. Members in the multicellular community may synthesize or acquire a signaling molecule in order to elicit downstream cellular processes. Roles of indole and derivatives, a new class of quorum-sensing signal molecules, in various bacterial physiologies and virulence have been reported recently. Indole is normally found in mammal gastrointestinal tract as a metabolite of tryptophan metabolism by microbiota. Therefore, interspecies connection via indole signaling among commensal bacteria and enteric pathogens could be anticipated. Effects of indole exposure on the virulence of Listeria monocytogenes were investigated by phenotypic and molecular approaches. Results demonstrated that synthetic indole and indole-rich conditioned medium significantly diminished biofilm formation and related virulence of L. monocytogenes including motility, cell aggregation and exopolysaccharide production. Transcript levels of virulence-associated (pssE, dltA, flaA, fliI, motB, agrA and hly) and regulatory genes (codY, sigB, prfA and gmaR) were substantially downregulated in indole-treated cells. Only mogR gene encoding for a repressor of motility genes was upregulated after indole exposure. Our findings raise the possibility that L. monocytogenes may acquire indole signaling from gut microbiota for resource-effective adaptation upon transition to new environment.

Combatting Persister Cells With Substituted Indoles

Given that a subpopulation of most bacterial cells becomes dormant due to stress, and that the resting cells of pathogens can revive and reconstitute infections, it is imperative to find methods to treat dormant cells to eradicate infections. The dormant bacteria that are not spores or cysts are known as persister cells. Remarkably, in contrast to the original report that incorrectly indicated indole increases persistence, a large number of indole-related compounds have been found in the last few years that kill persister cells. Hence, in this review, along with a summary of recent results related to persister cell formation and resuscitation, we focus on the ability of indole and substituted indoles to combat the persister cells of both pathogens and non-pathogens.

Postbiotic-Enabled Targeting of the Host-Microbiota-Pathogen Interface: Hints of Antibiotic Decline?

Mismanagement of bacterial infection therapies has undermined the reliability and efficacy of antibiotic treatments, producing a profound crisis of the antibiotic drug market. It is by now clear that tackling deadly infections demands novel strategies not only based on the mere toxicity of anti-infective compounds. Host-directed therapies have been the first example as novel treatments with alternate success. Nevertheless, recent advances in the human microbiome research have provided evidence that compounds produced by the microbial metabolism, namely postbiotics, can have significant impact on human health. Such compounds target the host-microbe-pathogen interface rescuing biotic and immune unbalances as well as inflammation, thus providing novel therapeutic opportunities. This work discusses critically, through literature review and personal contributions, these novel nonantibiotic treatment strategies for infectious disease management and resistance prevention, which could represent a paradigm change rocking the foundation of current antibiotic therapy tenets.

Local and Universal Action: The Paradoxes of Indole Signalling in Bacteria

Indole is a signalling molecule produced by many bacterial species and involved in intraspecies, interspecies, and interkingdom signalling. Despite the increasing volume of research published in this area, many aspects of indole signalling remain enigmatic. There is disagreement over the mechanism of indole import and export and no clearly defined target through which its effects are exerted. Progress is hindered further by the confused and sometimes contradictory body of indole research literature. We explore the reasons behind this lack of consistency and speculate whether the discovery of a new, pulse mode of indole signalling, together with a move away from the idea of a conventional protein target, might help to overcome these problems and enable the field to move forward.

Synergistic action of substituted indole derivatives and clinically used antibiotics against drug-resistant bacteria

Aim: The current report describes the discovery of indole derivatives that synergize with standard antibiotics. Materials & methods: The antibacterial activities were determined using an optimized time-kill method, while viability of mammalian cells was assessed using the (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Results: The synergy is observed with methicillin- and vancomycin-resistant Staphylococcus aureus bacterial strains, against which the standard antibiotics show no activities of their own. Our indole derivatives in combination with antibiotics lack toxicity toward mammalian cells, do not promote the evolution of resistance of S. aureus in comparison to clinically established antibiotics, and likely work by permeabilizing bacterial cell membranes. Conclusion: The above-mentioned findings demonstrate the potential clinical applications of our indole derivatives.

Chemical Mechanisms of Colonization Resistance by the Gut Microbial Metabolome

The gut microbiome, the collection of 100 trillion microorganisms that resides in the intestinal lumen, plays major roles in modulating host physiology. One well-established function of the gut microbiota is that of colonization resistance or the ability of the microbial collective to protect the host against enteric pathogens. Although evidence suggests that these microbes may outcompete some pathogens, there remains a lack of mechanistic understanding that underlies this competitive exclusion. In recent years, there has been great interest in small-molecule metabolites that are produced by the gut microbiota and in understanding how these molecules regulate host-pathogen interactions. In this review, we briefly summarize these findings by focusing on several classes of metabolites that mediate this important process. Understanding these host-microbe interactions in the gut may lead to identification of potential candidates for the development of prophylactics and therapeutics for many infectious diseases that are impacted by the gut microbiome.

The Signaling Molecule Indole Inhibits Induction of the AR2 Acid Resistance System in Escherichia coli

Induction of the AR2 acid response system of Escherichia coli occurs at a moderately low pH (pH 5.5) and leads to high levels of resistance to pH levels below 2.5 in the presence of glutamate. Induction is mediated in part by the EvgAS two component system. Here, we show that the bacterial signaling molecule indole inhibits the induction of key promoters in the AR2 system and blocks the development of glutamate-dependent acid resistance. The addition of tryptophan, the precursor for indole biosynthesis, had the same effects, and this block was relieved in a tnaA mutant, which is unable to synthesize indole. Expression of a constitutively active EvgS protein was able to relieve the inhibition caused by indole, consistent with EvgS being inhibited directly or indirectly by indole. Indole had no effect on autophosphorylation of the isolated cytoplasmic domain of EvgS. This is consistent with a model where indole directly or indirectly affects the ability of EvgS to detect its inducing signal or to transduce this information across the cytoplasmic membrane. The inhibitory activity of indole on the AR2 system is not related to its ability to act as an ionophore, and, conversely, the ionophore CCCP had no effect on acid-induced AR2 promoter activity, showing that the proton motive force is unlikely to be a signal for induction of the AR2 system.

Biphasic chemotaxis of Escherichia coli to the microbiota metabolite indole

Bacterial chemotaxis to prominent microbiota metabolites such as indole is important in the formation of microbial communities in the gastrointestinal (GI) tract. However, the basis of chemotaxis to indole is poorly understood. Here, we exposed Escherichia coli to a range of indole concentrations and measured the dynamic responses of individual flagellar motors to determine the chemotaxis response. Below 1 mM indole, a repellent-only response was observed. At 1 mM indole and higher, a time-dependent inversion from a repellent to an attractant response was observed. The repellent and attractant responses were mediated by the Tsr and Tar chemoreceptors, respectively. Also, the flagellar motor itself mediated a repellent response independent of the receptors. Chemotaxis assays revealed that receptor-mediated adaptation to indole caused a bipartite response-wild-type cells were attracted to regions of high indole concentration if they had previously adapted to indole but were otherwise repelled. We propose that indole spatially segregates cells based on their state of adaptation to repel invaders while recruiting beneficial resident bacteria to growing microbial communities within the GI tract.

Resistome in Lake Bolonha, Brazilian Amazon: Identification of Genes Related to Resistance to Broad-Spectrum Antibiotics

Resistance to antibiotics is one of the most relevant public health concerns in the world. Aquatic environments play an important role because they are reservoirs for antibiotic resistance genes and antibiotic-resistant strains, contributing to the spread of resistance. The present study investigated the resistome in Lake Bolonha (three sampling sites) in the Amazon region using a metagenomics approach and culture-dependent methods. Whole-metagenome-based results showed that the most abundant phyla were Protobacteria, Actinobacteria, Firmicutes, Bacteroidetes and Cyanobacteria. The composition of the resistome demonstrated that the genes that confer resistance to β-lactams were prevalent at all sampling sites, followed by genes conferring resistance to aminoglycosides and tetracycline. Acquired genes encoding extended-spectrum β-lactamases (e.g., bla_CTX–M) and resistance to carbapenems (e.g., bla_IMP and bla_VIM) were detected through metagenome analysis. Bacteria were isolated from culture medium supplemented with cefotaxime or imipenem, and isolates were identified and analyzed for their antibiotic susceptibility profiles and resistance genes. In total, 98 bacterial isolates belonging to the genera Pseudomonas (37), Acinetobacter (32), Klebsiella (13), Enterobacter (9), Pantoe (3), Stenotrophomonas (3), and Methylobacterium (1) were obtained. Among isolates, the most abundant genes were bla_CTX–M (28.3%), bla_SHV (22.6%) and bla_TEM (18.8%) in isolates from cefotaxime-supplemented medium and bla_VIM (28.8%) and bla_IMP (22.2%) in isolates recovered from imipenem-supplemented medium. The genes intl1 and intl2 were detected in 19.3% and 7.1% of isolates. Antibiograms showed that 94.9% (from cefotaxime-supplemented medium) and 85.7% (from imipenem-supplemented medium) of the isolates were multidrug resistant. Besides cefotaxime and imipenem, isolates were mostly resistant to aztreonam (91.8%), amoxicillin (98.8%), ampicillin (82.6%), and nalidixic acid (77.5%). Hence, the present study demonstrates that Lake Bolonha is a reservoir of bacteria resistant to antibiotics and resistance genes, some of which are of critical importance to human health.

Antibacterial and photocatalytic activities of 5-nitroindole capped bimetal nanoparticles against multidrug resistant bacteria

The emergence of antibiotic resistance to commercially- available antibiotics is becoming a major health crisis worldwide. Non-antibiotic strategies are needed to combat biofilm-associated infectious diseases caused by multidrug resistant (MDR) bacterial pathogens. In this study, MBR1 was isolated from a membrane bioreactor used in wastewater treatment plants, and the resistance profile was explored. 5-Nitroindole (5 N)-capped CuO/ZnO bimetal nanoparticles (5 NNP) were synthesized using a one pot method to improve the antibacterial and antibiofilm activities of 5 N against Gram-negative (Escherichia coli ATCC700376 and Pseudomonas aeruginosa PA01) and positive (Staphylococcus aureus ATCC6538) human pathogens. 5 NNP containing 1 mM of 5 N exhibited strong antibacterial and antibiofilm properties to most MDR bacteria. In addition, the photocatalytic activity of CuO/ZnO reduced bacterial cell growth by 1.8 log CFU/mL maximum when exposed to visible light. Scanning electron microscopy showed that 5 NNP reduced the cell density and biofilm attachment of MBR1 by >90% under static conditions. In addition to the antimicrobial and antibiofilm activities, 5 NNP inhibited the persister cell formation of MDR bacterial strains P. aeruginosa, MBR1, E. coli and S. aureus. Therefore, it is speculated that 5 NNP potentially inhibits biofilm and persister cells; hence, 5 NNP could be an alternative agent to combat MDR infectious diseases using a non-antibiotic therapeutic approach.

Indole and Derivatives Modulate Biofilm Formation and Antibiotic Tolerance of Klebsiella pneumoniae

Intercellular communication is a crucial process for the multicellular community in both prokaryotes and eukaryotes. Indole has been recognized as a new member of the signal molecules which enables the regulated control of various bacterial phenotypes. To elucidate the inter-species relationship among enteric microorganisms via indole signaling, Klebsiella pneumoniae (KP) culture was treated with indole solution and examined for the pathogenicity by using various phenotypic tests. Both synthetic and naturally-produced indole preparations had no deteriorating effect on growth and autoaggregative capacity of KP. The results showed that biofilm formation of carbapenem-susceptible K. pneumoniae (KP-S) was clearly induced by indole exposure (≈ 2-10 folds), whereas no significant difference was observed in the resistant counterpart. In addition, the tolerance to β-lactam antibiotics of KP was altered upon exposure to indole and/or derivatives assessed by Kirby-Bauer disk diffusion test. Taken together, our finding indicates the functional role of indole in changing or modulating pathogenic behaviors of other bacteria.

Metabolic output defines Escherichia coli as a health-promoting microbe against intestinal Pseudomonas aeruginosa

Gut microbiota acts as a barrier against intestinal pathogens, but species-specific protection of the host from infection remains relatively unexplored. Although lactobacilli and bifidobacteria produce beneficial lactic and short-chain fatty acids in the mammalian gut, the significance of intestinal Escherichia coli producing these acids is debatable. Taking a Koch’s postulates approach in reverse, we define Escherichia coli as health-promoting for naturally colonizing the gut of healthy mice and protecting them against intestinal colonization and concomitant mortality by Pseudomonas aeruginosa. Reintroduction of faecal bacteria and E. coli in antibiotic-treated mice establishes a high titre of E. coli in the host intestine and increases defence against P. aeruginosa colonization and mortality. Strikingly, high sugar concentration favours E. coli fermentation to lactic and acetic acid and inhibits P. aeruginosa growth and virulence in aerobic cultures and in a model of aerobic metabolism in flies, while dietary vegetable fats - not carbohydrates or proteins - favour E. coli fermentation and protect the host in the anaerobic mouse gut. Thus E. coli metabolic output is an important indicator of resistance to infection. Our work may also suggest that the lack of antimicrobial bacterial metabolites in mammalian lungs and wounds allows P. aeruginosa to be a formidable microbe at these sites.

Black sea cucumber (Holothuria atra Jaeger, 1833) rescues Pseudomonas aeruginosa-infected Caenorhabditis elegans via reduction of pathogen virulence factors and enhancement of host immunity

A strategy to circumvent the problem of multidrug resistant pathogens is the discovery of anti-infectives targeting bacterial virulence or host immunity. Black sea cucumber (Holothuria atra) is a tropical sea cucumber species traditionally consumed as a remedy for many ailments. There is a paucity of knowledge on the anti-infective capacity of H. atra and the underlying mechanisms involved. The objective of this study is to utilize the Caenorhabditis elegans-P. aeruginosa infection model to elucidate the anti-infective properties of H. atra. A bioactive H. atra extract and subsequently its fraction were shown to have the capability of promoting the survival of C. elegans during a customarily lethal P. aeruginosa infection. The same entities also attenuate the production of elastase, protease, pyocyanin and biofilm in P. aeruginosa. The treatment of infected transgenic lys-7::GFP worms with this H. atra fraction restores the repressed expression of the defense enzyme lys-7, indicating an improved host immunity. QTOF-LCMS analysis revealed the presence of aspidospermatidine, an indole alkaloid, and inosine in this fraction. Collectively, our findings show that H. atra possesses anti-infective properties against P. aeruginosa infection, by inhibiting pathogen virulence and, eventually, reinstating host lys-7 expression.

Selective Cu-Catalyzed Intramolecular Annulation of 3-Aryl/Heteryl-2-(diazoacetyl)-1H-pyrroles: Synthesis of Benzo/Furo/Thieno[e]-Fused 1H-Indol-7-oles and Their Transformations

The Co(III)-catalyzed reaction of 1,3-dicarbonyl compounds with 2-(diazoacetyl)-2H-azirines, prepared by a simplified procedure from 2H-azirin-2-carbonyl chlorides, led in high yields to the formation of 2-(diazoacetyl)pyrroles, while leaving the diazoacetyl function intact. The intramolecular aromatic substitution reaction of 2-(diazoacetyl)pyrroles, catalyzed by Cu(OTf)₂, provided selectively previously unknown benzo[e]- and hetero[e]-fused indol-7-oles in good yields. Formylation of benzo[e]indol-4-ol led selectively to the 5-formyl derivative, which is a good precursor for an unusual salen ligand and its Ni-complex. Triflates prepared from benzo[e]indol-4-oles gave various 4-substituted benzo[e]indoles carrying aryl, 2-thienyl, 2-pyridyl, and alkynyl groups, in excellent yields using cross-coupling reactions. 4-(2-Pyridyl)benzo[e]indoles, upon treatment with BF₃·Et₂O/Et₃N, afforded a new type of fluorescent boron complexes with large Stokes shifts.

Pantoea agglomerans YS19 poly(A) polymerase I gene possesses the indole-sensing sequence in the promoter region

Pantoea agglomerans YS19 is a predominant diazotrophic endophyte with multiple growth-promoting effects on its host plant that was isolated from rice. Indole is confirmed to induce many changes of physiological and biochemical characteristics in bacteria. Although YS19 cannot produce indole, it can sense indole in the environment and be regulated by indole. Here, using gfp as a reporter gene, we constructed a series of recombinant plasmids containing the promoter region of the poly(A) polymerase I gene (pcnB) fused with gfp, and compared the green fluorescence intensity at different concentrations of exogenous indole by a flow cytometer. In this research, we confirmed that exogenous indole significantly inhibited the expression of pcnB by its promoter; the regulation sequence sensitive to indole in the promoter region of the pcnB gene (In-pcnB) was between -129 and -88 bp. In-pcnB is widely distributed and strictly conserved in the same genus. These results suggest novel roles of In-pcnB in P. agglomerans YS19, showing its special relation to the indole regulatory pathway.

Indole Degradation in a Model System and in Poultry Manure by Acinetobacter spp

Indole degradation in a model system and in poultry manure was studied using an enrichment culture of two Acinetobacter species; Acinetobacter toweneri NTA1-2A and Acinetobacter guillouiae TAT1-6A. Degradation of indole was quantified using reverse phase high performance liquid chromatography (HPLC). The two strains were capable of degrading initial concentrations of indole ranging from 58.58–300 mg/L. The degradation efficiency was 66.36% (NTA1-2A), 94.87% (TAT1-6A), and 96.00% (mix) in 6 days when the initial concentration <300 mg/L. The strains were tested for enzymatic activity using 120 mg/L indole. The enzyme extracts of NTA1-2A and TAT1-6A from culture medium degraded indole completely, and no appreciable change of indole concentration was witnessed in the control group. The NTA1-2A, TAT1-6A, and the mix of strains were also used for in vivo poultry manure fermentation and removed 78.67%, 83.28%, and 83.70% of indole, respectively in 8 d. The strains showed a statistically significant difference (p < 0.05) in indole removal efficiency compared with the control, but no significant difference between the two strains and the mix in indole removal capacity. We concluded that A. toweneri NTA1-2A and A. guillouiae TAT1-6A are promising strains to remove indole and its derivatives to control the notorious odor in poultry and other livestock industries.

Clostridium difficile Modulates the Gut Microbiota by Inducing the Production of Indole, an Interkingdom Signaling and Antimicrobial Molecule

Clostridium (Clostridioides) difficile infection (CDI) is associated with dysbiosis. C. difficile has a characteristic propensity to persist and recur 1 to 4 weeks after treatment, but the mechanism is unknown. We hypothesized that C. difficile may persist by manipulating the intestinal microenvironment, thereby hampering gut microbiota reconstitution following antibiotic-mediated dysbiosis. By screening stools from CDI patients for unique markers, a metabolite identified to be indole by mass spectrometry and Fourier transform infrared spectroscopy was identified. The average fecal indole concentration detected in CDI patients (n = 216; mean, 1,684.0 ± 84.4 µM) was significantly higher than in stools of patients with non-C. difficile diarrhea (n = 204; mean, 762.8 ± 53.8 µM). Certain intestinal bacteria, but not C. difficile, produce indole, a potent antimicrobial antioxidant. Remarkably, C. difficile induced other indole-producing gut microbes to produce increasing amounts of indole. Furthermore, a C. difficile accessory gene regulator 1 quorum sensing system mutant cannot induce indole, but complementation of the mutant strain with the wild-type gene restored its ability to induce indole production. Indole tolerance assays indicated that the amount of indole required to inhibit growth of most gut-protective bacteria was within the range detected in the CDI stools. We think that a high indole level limits the growth of beneficial indole-sensitive bacteria in the colon and alters colonization resistance and this might allow C. difficile to proliferate and persist. Together, these results reveal a unique mechanism of C. difficile persistence and provide insight into complex interactions and chemical warfare among the gut microbiota. IMPORTANCE Clostridium difficile infection is the leading cause of hospital-acquired and antibiotic-associated diarrhea worldwide. C. difficile flourishes in the colon after the diversity of the beneficial and protective gut microbiota have been altered by antibiotic therapy. C. difficile tends to persist, as does dysbiosis, encouraging recurrence a few days to weeks after treatment, and this further complicates treatment options. Here, we show that C. difficile might persist by manipulating the indigenous microbiota to produce indole, a bioactive molecule that inhibits the growth and reconstitution of the protective gut microbiota during infection. This discovery may explain a unique strategy C. difficile uses to control other bacteria in the colon and provide insight into the complex interactions and chemical warfare among the gut microbiota.

A benzimidazole-based ruthenium(IV) complex inhibits Pseudomonas aeruginosa biofilm formation by interacting with siderophores and the cell envelope, and inducing oxidative stress

Pseudomonas aeruginosa biofilm-associated infections are a serious medical problem, and new compounds and therapies acting through novel mechanisms are much needed. Herein, the authors report a ruthenium(IV) complex that reduces P. aeruginosa PAO1 biofilm formation by 84%, and alters biofilm morphology and the living-to-dead cell ratio at 1 mM concentration. Including the compound in the culture medium altered the pigments secreted by PAO1, and fluorescence spectra revealed a decrease in pyoverdine. Scanning electron microscopy showed that the ruthenium complex did not penetrate the bacterial cell wall, but accumulated on external cell structures. Fluorescence quenching experiments indicated strong binding of the ruthenium complex to both plasmid DNA and bovine serum albumin. Formamidopyrimidine DNA N-glycosylase (Fpg) protein digestion of plasmid DNA isolated after ruthenium(IV) complex treatment revealed the generation of oxidative stress, which was further proved by the observed upregulation of catalase and superoxide dismutase gene expression.

Suppression of Fluconazole Resistant Candida albicans Biofilm Formation and Filamentation by Methylindole Derivatives

Candida albicans is an opportunistic fungal pathogen and most prevalent species among clinical outbreaks. It causes a range of infections, including from mild mucosal infections to serious life-threatening candidemia and disseminated candidiasis. Multiple virulence factors account for the pathogenic nature of C. albicans, and its morphological transition from budding yeast to hyphal form and subsequent biofilm formation is regarded as the most important reason for the severity of Candida infections. To address the demanding need for novel antifungals, we investigated the anti-biofilm activities of various methylindoles against C. albicans using a crystal violet assay, and the metabolic activity was assessed by using a 2,3-bis (2-methoxy-4-nitro-5-sulfo-phenyl)-2H-tetrazolium-5-carboxanilide reduction assay. Changes in biofilm morphologies and thicknesses were determined by confocal laser scanning microscopy and scanning electron microscopy, respectively. Of the 21 methylindoles tested, 1-methylindole-2-carboxylic acid (1MI2CA) at 0.1 mM (17.5 μg ml^-1) and 5-methylindole-2-carboxylic acid (5MI2CA) at 0.1 mM effectively inhibited biofilm formation by C. albicans DAY185 and ATCC10231 strains. Moreover, 1MI2CA and 5MI2CA both effectively inhibited hyphal formation, and thus, improved C. albicans infected nematode survival without inducing acute toxic effects. Furthermore, our in silico molecular modeling findings were in-line with in vitro observations. This study provides information useful for the development of novel strategies targeting candidiasis and biofilm-related infections.

Biodegradation and Biotransformation of Indole: Advances and Perspectives

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.

Efficacy of 7-benzyloxyindole and other halogenated indoles to inhibit Candida albicans biofilm and hyphal formation

Certain pathogenic bacteria and yeast form biofilms on biotic and abiotic surfaces including medical devices and implants. Hence, the development of antibiofilm coating materials becomes relevant. The virulence of those colonizing pathogens can be reduced by inhibiting biofilm formation rather than killing pathogens using excessive amounts of antimicrobials, which is touted as one of the main reasons for the development of drug resistance. Candida albicans is an opportunistic fungal pathogen, and the transition of yeast cells to hyphal cells is believed to be a crucial virulence factor. Previous studies have shown that indole and its derivatives possess antivirulence properties against various bacterial pathogens. In this study, we used various indole derivatives to investigate biofilm-inhibiting activity against C. albicans. Our study revealed that 7-benzyloxyindole, 4-fluoroindole and 5-iodoindole effectively inhibited biofilm formation compared to the antifungal agent fluconazole. Particularly, 7-benzyloxyindole at 0.02 mM (4.5 μg ml-1 ) significantly reduced C. albicans biofilm formation, but had no effect on planktonic cells, and this finding was confirmed by a 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) assay and three-dimensional confocal laser scanning microscopy. Scanning electron microscopy analyses revealed that 7-benzyloxyindole effectively inhibited hyphal formation, which explains biofilm inhibition. Transcriptomic analysis showed that 7-benzyloxyindole downregulated the expressions of several hypha/biofilm-related genes (ALS3, ECE1, HWP1 and RBT1). A C. albicans-infected Caenorhabditis elegans model system was used to confirm the antivirulence efficacy of 7-benzyloxyindole.

Synthetic small molecules as anti-biofilm agents in the struggle against antibiotic resistance

Biofilm formation significantly contributes to microbial survival in hostile environments and it is currently considered a key virulence factor for pathogens responsible for serious chronic infections. In the last decade many efforts have been made to identify new agents able to modulate bacterial biofilm life cycle, and many compounds have shown interesting activities in inhibiting biofilm formation or in dispersing pre-formed biofilms. However, only a few of these compounds were tested using in vivo models for their clinical significance. Contrary to conventional antibiotics, most of the anti-biofilm compounds act as anti-virulence agents as they do not affect bacterial growth. In this review we selected the most relevant literature of the last decade, focusing on the development of synthetic small molecules able to prevent bacterial biofilm formation or to eradicate pre-existing biofilms of clinically relevant Gram-positive and Gram-negative pathogens. In addition, we provide a comprehensive list of the possible targets to counteract biofilm formation and development, as well as a detailed discussion the advantages and disadvantages of the different current biofilm-targeting strategies.

Characterization and regulation of AcrABR, a RND-type multidrug efflux system, in Agrobacterium tumefaciens C58

Agrobacterium tumefaciens AcrR is the transcriptional repressor of the acrABR operon. The AcrAB efflux pump confers resistance to various toxic compounds, including antibiotics [ciprofloxacin (CIP), nalidixic acid (NAL), novobiocin (NOV) and tetracycline (TET)], a detergent [sodium dodecyl sulfate (SDS)] and a biocide [triclosan (TRI)]. The sequence to which AcrR specifically binds in the acrA promoter region was determined by EMSA and DNase I footprinting. The AcrR-DNA interaction was abolished by adding NAL, SDS and TRI. Quantitative real time-PCR analysis showed that induction of the acrA transcript occurred when wild-type cells were exposed to NAL, SDS and TRI. Indole is a signaling molecule that increases the antibiotic resistance of bacteria, at least in part, through activation of efflux pumps. Expression of the A. tumefaciens acrA transcript was also inducible by indole in a dose-dependent manner. Indole induced protection against CIP, NAL and SDS but enhanced susceptibility to NOV and TRI. Additionally, the TET resistance of A. tumefaciens was not apparently modulated by indole. A. tumefaciens AcrAB played a dominant role and was required for tolerance to high levels of the toxic compounds. Understanding the regulation of multidrug efflux pumps and bacterial adaptive responses to intracellular and extracellular signaling molecules for antibiotic resistance is essential. This information will be useful for the rational design of effective treatments for bacterial infection to overcome possible multidrug-resistant pathogens.

Antibacterial Properties of Four Novel Hit Compounds from a Methicillin-Resistant Staphylococcus aureus-Caenorhabditis elegans High-Throughput Screen

There is an urgent need for the discovery of effective new antimicrobial agents to combat the rise of bacterial drug resistance. High-throughput screening (HTS) in whole-animal infection models is a powerful tool for identifying compounds that show antibacterial activity and low host toxicity. In this report, we characterize the activities of four novel antistaphylococcal compounds identified from an HTS campaign conducted using Caenorhabditis elegans nematodes infected with methicillin-resistant Staphylococcus aureus (MRSA). The hit compounds included an N-hydroxy indole-1, a substituted melamine derivative-2, N-substituted indolic alkyl isothiocyanate-3, and p-difluoromethylsulfide analog-4 of the well-known protonophore carbonyl cyanide m-chlorophenyl hydrazone. Minimal inhibitory concentrations (MICs) of the four compounds ranged from 2 to 8 μg/ml against MRSA-MW2 and Enterococcus faecium and all were bacteriostatic. The compounds were mostly inactive against Gram-negative pathogens, with only 1 and 4 showing slight activity (MIC = 32 μg/ml) against Acinetobacter baumanii. Compounds 2 and 3 (but not 1 or 4) were found to perturb MRSA membranes. In phagocytosis assays, compounds 1, 2, and 4 inhibited the growth of internalized MRSA in macrophages, whereas compound 3 showed a remarkable ability to clear intracellular MRSA at its MIC (p < 0.001). None of the compounds showed hemolytic activity at concentrations below 64 μg/ml (p = 0.0021). Compounds 1, 2, and 4 (but not 3) showed synergistic activity against MRSA with ciprofloxacin, while compound 3 synergized with erythromycin, gentamicin, streptomycin, and vancomycin. In conclusion, we describe four new antistaphylococcal compounds that warrant further study as novel antibacterial agents against Gram-positive organisms.

Colony analysis and deep learning uncover 5-hydroxyindole as an inhibitor of gliding motility and iridescence in Cellulophaga lytica

Iridescence is an original type of colouration that is relatively widespread in nature but has been either incompletely described or entirely neglected in prokaryotes. Recently, we reported a brilliant 'pointillistic' iridescence in agar-grown colony biofilms of Cellulophaga lytica and some other marine Flavobacteria that exhibit gliding motility. Bacterial iridescence is created by a unique self-organization of sub-communities of cells, but the mechanisms underlying such living photonic crystals are unknown. In this study, we used Petri dish assays to screen a large panel of potential activators or inhibitors of C. lytica's iridescence. Derivatives potentially interfering with quorum-sensing and other communication or biofilm formation processes were tested, as well as metabolic poisons or algal exoproducts. We identified an indole derivative, 5-hydroxyindole (5HI, 250 µM) which inhibited both gliding and iridescence at the colonial level. 5HI did not affect growth or cell respiration. At the microscopic level, phase-contrast imaging confirmed that 5HI inhibits the gliding motility of cells. Moreover, the lack of iridescence correlated with a perturbation of self-organization of the cell sub-communities in both the WT and a gliding-negative mutant. This effect was proved using recent advances in machine learning (deep neuronal networks). In addition to its effect on colony biofilms, 5HI was found to stimulate biofilm formation in microplates. Our data are compatible with possible roles of 5HI or marine analogues in the eco-biology of iridescent bacteria.

Quorum-Sensing Systems as Targets for Antivirulence Therapy

The development of novel therapies to control diseases caused by antibiotic-resistant pathogens is one of the major challenges we are currently facing. Many important plant, animal, and human pathogens regulate virulence by quorum sensing, bacterial cell-to-cell communication with small signal molecules. Consequently, a significant research effort is being undertaken to identify and use quorum-sensing-interfering agents in order to control diseases caused by these pathogens. In this review, an overview of our current knowledge of quorum-sensing systems of Gram-negative model pathogens is presented as well as the link with virulence of these pathogens, and recent advances and challenges in the development of quorum-sensing-interfering therapies are discussed.

Unveiling the biotransformation mechanism of indole in a Cupriavidus sp. strain

Indole, an important signaling molecule as well as a typical N-heterocyclic aromatic pollutant, is widespread in nature. However, the biotransformation mechanisms of indole are still poorly studied. Here, we sought to unlock the genetic determinants of indole biotransformation in strain Cupriavidus sp. SHE based on genomics, proteomics and functional studies. A total of 177 proteins were notably altered (118 up- and 59 downregulated) in cells grown in indole mineral salt medium when compared with that in sodium citrate medium. RT-qPCR and gene knockout assays demonstrated that an indole oxygenase gene cluster was responsible for the indole upstream metabolism. A functional indole oxygenase, termed IndA, was identified in the cluster, and its catalytic efficiency was higher than those of previously reported indole oxidation enzymes. Furthermore, the indole downstream metabolism was found to proceed via the atypical CoA-thioester pathway rather than conventional gentisate and salicylate pathways. This unusual pathway was catalyzed by a conserved 2-aminobenzoyl-CoA gene cluster, among which the 2-aminobenzoyl-CoA ligase initiated anthranilate transformation. This study unveils the genetic determinants of indole biotransformation and will provide new insights into our understanding of indole biodegradation in natural environments and its functional studies.

Chemical and Biological Aspects of Nutritional Immunity-Perspectives for New Anti-Infectives that Target Iron Uptake Systems

Upon bacterial infection, one of the defense mechanisms of the host is the withdrawal of essential metal ions, in particular iron, which leads to "nutritional immunity". However, bacteria have evolved strategies to overcome iron starvation, for example, by stealing iron from the host or other bacteria through specific iron chelators with high binding affinity. Fortunately, these complex interactions between the host and pathogen that lead to metal homeostasis provide several opportunities for interception and, thus, allow the development of novel antibacterial compounds. This Review focuses on iron, discusses recent highlights, and gives some future perspectives which are relevant in the fight against antibiotic resistance.

Biotechnology of siderophores in high-impact scientific fields

Different aspects of bacterial and fungal siderophore biotechnological applications will be discussed. Areas of application presented include, but are not limited to agriculture, medicine, pharmacology, bioremediation, biodegradation and food industry. In agriculture-related applications, siderophores could be employed to enhance plant growth due to their uptake by rhizobia. Siderophores hindered the presence of plant pathogens in biocontrol strategies. Bioremediation studies on siderophores discuss mostly the mobilization of heavy metals and radionuclides; the emulsifying effects of siderophore-producing microorganisms in oil-contaminated environments are also presented. The different applications found in literature based in medicine and pharmacological approaches range from iron overload to drug delivery systems and, more recently, vaccines. Additional research should be done in siderophore production and their metabolic relevance to have a deeper understanding for future biotechnological advances.

Indole Biodegradation in Acinetobacter sp. Strain O153: Genetic and Biochemical Characterization

Indole is a molecule of considerable biochemical significance, acting as both an interspecies signal molecule and a building block of biological elements. Bacterial indole degradation has been demonstrated for a number of cases; however, very little is known about genes and proteins involved in this process. This study reports the cloning and initial functional characterization of genes (iif and ant cluster) responsible for indole biodegradation in Acinetobacter sp. strain O153. The catabolic cascade was reconstituted in vitro with recombinant proteins, and each protein was assigned an enzymatic function. Degradation starts with oxidation, mediated by the IifC and IifD flavin-dependent two-component oxygenase system. Formation of indigo is prevented by IifB, and the final product, anthranilic acid, is formed by IifA, an enzyme which is both structurally and functionally comparable to cofactor-independent oxygenases. Moreover, the iif cluster was identified in the genomes of a wide range of bacteria, suggesting the potential of widespread Iif-mediated indole degradation. This work provides novel insights into the genetic background of microbial indole biodegradation.IMPORTANCE The key finding of this research is identification of the genes responsible for microbial biodegradation of indole, a toxic N-heterocyclic compound. A large amount of indole is present in urban wastewater and sewage sludge, creating a demand for an efficient and eco-friendly means to eliminate this pollutant. A common strategy of oxidizing indole to indigo has the major drawback of producing insoluble material. Genes and proteins of Acinetobacter sp. strain O153 (DSM 103907) reported here pave the way for effective and indigo-free indole removal. In addition, this work suggests possible novel means of indole-mediated bacterial interactions and provides the basis for future research on indole metabolism.