Intercepting bacterial indole signaling with flustramine derivatives

Synthesis and Staphylococcus aureus biofilm inhibitory activity of indolenine-substituted pyrazole and pyrimido[1,2-b]indazole derivatives

Staphylococcus aureus is a highly adaptable opportunistic pathogen that can form biofilms and generate persister cells, leading to life-threatening infections that are difficult to treat with antibiotics alone. Therefore, there is a need for an effective S. aureus biofilm inhibitor to combat this public health threat. In this study, a small library of indolenine-substituted pyrazoles and pyrimido[1,2-b]indazole derivatives were synthesised, of which the hit compound exhibited promising antibiofilm activities against methicillin-susceptible S. aureus (MSSA ATCC 29213) and methicillin-resistant S. aureus (MRSA ATCC 33591) at concentrations significantly lower than the planktonic growth inhibition. The hit compound could prevent biofilm formation and eradicate mature biofilms of MSSA and MRSA, with a minimum biofilm inhibitory concentration (MBIC50) value as low as 1.56 µg/mL and a minimum biofilm eradication concentration (MBEC50) value as low as 6.25 µg/mL. The minimum inhibitory concentration (MIC) values of the hit compound against MSSA and MRSA were 50 µg/mL and 25 µg/mL, respectively, while the minimum bactericidal concentration (MBC) values against MSSA and MRSA were > 100 µg/mL. Preliminary structure-activity relationship analysis reveals that the fused benzene ring and COOH group of the hit compound are crucial for the antibiofilm activity. Additionally, the compound was not cytotoxic to human alveolar A549 cells, thus highlighting its potential as a suitable candidate for further development as a S. aureus biofilm inhibitor.

Engineered RebH Halogenase Variants Demonstrating a Specificity Switch from Tryptophan towards Novel Indole Compounds

Activating industrially important aromatic hydrocarbons by installing halogen atoms is extremely important in organic synthesis and often improves the pharmacological properties of drug molecules. To this end, tryptophan halogenase enzymes are potentially valuable tools for regioselective halogenation of arenes, including various industrially important indole derivatives and similar scaffolds. Although endogenous enzymes show reasonable substrate scope towards indole compounds, their efficacy can often be improved by engineering. Using a structure-guided semi-rational mutagenesis approach, we have developed two RebH variants with expanded biocatalytic repertoires that can efficiently halogenate several novel indole substrates and produce important pharmaceutical intermediates. Interestingly, the engineered enzymes are completely inactive towards their natural substrate tryptophan in spite of their high tolerance to various functional groups in the indole ring. Computational modelling and molecular dynamics simulations provide mechanistic insights into the role of gatekeeper residues in the substrate binding site and the dramatic switch in substrate specificity when these are mutated.

Strategies and Approaches for Discovery of Small Molecule Disruptors of Biofilm Physiology

Biofilms, the predominant growth mode of microorganisms, pose a significant risk to human health. The protective biofilm matrix, typically composed of exopolysaccharides, proteins, nucleic acids, and lipids, combined with biofilm-grown bacteria’s heterogenous physiology, leads to enhanced fitness and tolerance to traditional methods for treatment. There is a need to identify biofilm inhibitors using diverse approaches and targeting different stages of biofilm formation. This review discusses discovery strategies that successfully identified a wide range of inhibitors and the processes used to characterize their inhibition mechanism and further improvement. Additionally, we examine the structure–activity relationship (SAR) for some of these inhibitors to optimize inhibitor activity.

Newer guar gum ester/chicken feather keratin interact films for tissue engineering

This work intends to synthesis newer guar gum indole acetate ester and design film scaffolds based on protein-polysaccharide interactions for tissue engineering applications. Guar gum indole acetate(GGIA) was synthesized for the first time from guar gum in presence of aprotic solvent activated hofmeister ions. The newer biopolymer was fully characterized in FT-IR,¹³C NMR, XRD and TGA analysis. High DS (Degree of Substitution, DS = 0.61) GGIA was cross-linked with hydrolyzed keratin, extracted from chicken feather wastes. Films were synthesized from different biopolymer ratios and the surface chemistry appeared interesting. Physicochemical properties for GGIA-keratin association were notable. Fully bio-based films were non-cytotoxic and exhibited excellent biocompatibility for human dermal fibroblast cell cultivations. The film scaffold showed 63% porosity and the recorded tensile strength at break was 6.4 MPa. Furthermore, the standardised film exerted superior antimicrobial activity against both the Gram-positive and Gram-negative bacteria. MICs were recorded at 130 μg/mL and 212 μg/mL for E. coli and S. aureus respectively. In summary, GGIA-keratin film scaffolds represented promising platforms for skin tissue engineering applications.

Natural products as inspiration for the development of bacterial antibiofilm agents

Natural products have historically been a rich source of diverse chemical matter with numerous biological activities, and have played an important role in drug discovery in many areas including infectious disease. Synthetic and medicinal chemistry have been, and continue to be, important tools to realize the potential of natural products as therapeutics and as chemical probes. The formation of biofilms by bacteria in an infection setting is a significant factor in the recalcitrance of many bacterial infections, conferring increased tolerance to many antibiotics and to the host immune response, and as yet there are no approved therapeutics for combatting biofilm-based bacterial infections. Small molecules that interfere with the ability of bacteria to form and maintain biofilms can overcome antibiotic tolerance conferred by the biofilm phenotype, and have the potential to form combination therapies with conventional antibiotics. Many natural products with anti-biofilm activity have been identified from plants, microbes, and marine life, including: elligic acid glycosides, hamamelitannin, carolacton, skyllamycins, promysalin, phenazines, bromoageliferin, flustramine C, meridianin D, and brominated furanones. Total synthesis and medicinal chemistry programs have facilitated structure confirmation, identification of critical structural motifs, better understanding of mechanistic pathways, and the development of more potent, more accessible, or more pharmacologically favorable derivatives of anti-biofilm natural products.

Bromotryptamine and Imidazole Alkaloids with Anti-inflammatory Activity from the Bryozoan Flustra foliacea

Chemical investigation of the marine bryozoan Flustra foliacea collected in Iceland resulted in isolation of 13 new bromotryptamine alkaloids, flustramines Q-W (1-7) and flustraminols C-H (8-13), and two new imidazole alkaloids, flustrimidazoles A and B (14 and 15), together with 12 previously described compounds (16-27). Their structures were established by detailed spectroscopic analysis using 1D and 2D NMR and HRESIMS. Structure 2 was verified by calculations of the 13C and 1H NMR chemical shifts using density functional theory. The relative and absolute configurations of the new compounds were elucidated on the basis of coupling constant analysis, NOESY, [α]D, and ECD spectroscopic data, in addition to chemical derivatization. The compounds were tested for in vitro anti-inflammatory activity using a dendritic cell model. Eight compounds (1, 3, 5, 13, 16, 18, 26, and 27) decreased dendritic cell secretion of the pro-inflammatory cytokine IL-12p40, and two compounds (4 and 14) increased secretion of the anti-inflammatory cytokine IL-10. Deformylflustrabromine B (27) showed the most potent anti-inflammatory effect (IC50 2.9 μM). These results demonstrate that F. foliacea from Iceland expresses a broad range of brominated alkaloids, many without structural precedents. The potent anti-inflammatory activity in vitro of metabolite 27 warrants further investigations into its potential as a lead for inflammation-related diseases.

Tryptophan derived natural marine alkaloids and synthetic derivatives as promising antimicrobial agents

Antimicrobial resistance has become a major threat to public health worldwide, as pathogenic microorganisms are finding ways to evade all known antimicrobials. Therefore, the demand for new and effective antimicrobial agents is also increasing. Natural products have always played an important role in drug discovery, either by themselves or as inspiration for synthetic compounds. The marine environment is a rich source of bioactive metabolites, and among them, tryptophan-derived alkaloids stand out for their abundance and by displaying a variety of biological activities, with antimicrobial properties being among the most significant. This review aims to reveal the potential of marine alkaloids derived from tryptophan as antimicrobial agents. Relevant examples of these compounds and their synthetic analogues reported in the last decades are presented and discussed in detail, with their mechanism of action and synthetic approaches whenever relevant. Several tryptophan-derived marine alkaloids have shown potent and promising antimicrobial activities, whether against bacteria, fungi, or virus. Synthetic approaches to many of the compounds have been developed and recent methodologies are proving to be efficient. Even though most of the studies regarding the antimicrobial activity are still preliminary, this class of compounds has proven to be worth of further investigation and may provide useful lead compounds for the development of antimicrobial agents. Overall, marine alkaloids derived from tryptophan are revealed as a valuable class of antimicrobials and molecular modifications in order to reduce the toxicity of these compounds and additional studies regarding their mechanism of action are interesting topics to explore in the future.

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.

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.

2D Graphdiyne Oxide Serves as a Superior New Generation of Antibacterial Agents

Graphdiyne (GDY) as an emerging 2D carbon-network nanomaterial possesses many fascinating properties that lead to numerous exciting applications, but the use of GDY and its derivatives in the antibacterial field has not yet been discovered. In this study, we first report on the use and evaluation of GDY and graphdiyne oxide (GDYO) as antibacterial agents and propose the antibacterial mechanisms of GDY-based nanomaterials. GDYO has been synthesized via the surface oxidation of GDY, and the antibacterial activity of GDYO has been compared with that of GDY through a series of antibacterial tests. Surprisingly, surface oxidation endowed inert GDY with superior antibacterial capability against two representative bacterial models: Escherichia coli and Staphylococcus aureus. Antibacterial mechanism experiments disclose that the antibacterial function of GDYO is a result of reactive oxygen species-dependent oxidation stress when a dispersed GDYO suspension has a direct contact with bacteria especially under visible light irradiation.

Marine Alkaloid 2,2-Bis(6-bromo-3-indolyl) Ethylamine and Its Synthetic Derivatives Inhibit Microbial Biofilms Formation and Disaggregate Developed Biofilms

The antimicrobial activity of the marine bisindole alkaloid 2,2-bis(6-bromo-3-indolyl) ethylamine (1) and related synthetic analogues (compounds 2–8) against target microorganisms was investigated by Minimum Inhibitory Concentration (MIC) determination. Compound 1 showed the greatest antimicrobial activity with the lowest MIC (8 mg/L) against Escherichia coli, Staphylococcus aureus, and Klebsiella pneumoniae, while the derivatives exhibited higher MICs values (from 16 to 128 mg/L). Compounds 1, 3, 4, and 8, the most active ones, were then tested against E. coli, S. aureus, K. pneumoniae, and Candida albicans during biofilms formation as well as on 24 h developed biofilms. The natural alkaloid 1 inhibited the biofilm formation of all the tested microorganisms up to 82.2% and disaggregated biofilms of E. coli, S. aureus, K. pneumoniae, and C. albicans after 30 min of contact, as assessed by viable plate count and crystal violet (CV) staining (optical density at 570 nm). Synthetic derivatives 3, 4, and 8 displayed anti-biofilm activity toward individual bacterial populations. This study highlights the potential of marine bisindole alkaloid 1 as anti-biofilm agent and shows, through a preliminary structure activity relationship (SAR), the importance of halogens and ethylamine side chain for the antimicrobial and antibiofilm activities of this bisindole series.

Strategies for combating bacterial biofilms: A focus on anti-biofilm agents and their mechanisms of action

Biofilm refers to the complex, sessile communities of microbes found either attached to a surface or buried firmly in an extracellular matrix as aggregates. The biofilm matrix surrounding bacteria makes them tolerant to harsh conditions and resistant to antibacterial treatments. Moreover, the biofilms are responsible for causing a broad range of chronic diseases and due to the emergence of antibiotic resistance in bacteria it has really become difficult to treat them with efficacy. Furthermore, the antibiotics available till date are ineffective for treating these biofilm related infections due to their higher values of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), which may result in in-vivo toxicity. Hence, it is critically important to design or screen anti-biofilm molecules that can effectively minimize and eradicate biofilm related infections. In the present article, we have highlighted the mechanism of biofilm formation with reference to different models and various methods used for biofilm detection. A major focus has been put on various anti-biofilm molecules discovered or tested till date which may include herbal active compounds, chelating agents, peptide antibiotics, lantibiotics and synthetic chemical compounds along with their structures, mechanism of action and their respective MICs, MBCs, minimum biofilm inhibitory concentrations (MBICs) as well as the half maximal inhibitory concentration (IC₅₀) values available in the literature so far. Different mode of action of anti biofilm molecules addressed here are inhibition via interference in the quorum sensing pathways, adhesion mechanism, disruption of extracellular DNA, protein, lipopolysaccharides, exopolysaccharides and secondary messengers involved in various signaling pathways. From this study, we conclude that the molecules considered here might be used to treat biofilm-associated infections after significant structural modifications, thereby investigating its effective delivery in the host. It should also be ensured that minimum effective concentration of these molecules must be capable of eradicating biofilm infections with maximum potency without posing any adverse side effects on the host.

Meridianin D Analogues Display Antibiofilm Activity against MRSA and Increase Colistin Efficacy in Gram-Negative Bacteria

In the last 30 years, development of new classes of antibiotics has slowed, increasing the necessity for new options to treat multidrug resistant bacterial infections. Development of antibiotic adjuvants that increase the effectiveness of currently available antibiotics is a promising alternative approach to classical antibiotic development. Reports of the ability of the natural product meridianin D to modulate bacterial behavior have been rare. Herein, we describe the ability of meridianin D to inhibit biofilm formation of methicillin-resistant Staphylococcus aureus (MRSA) and to increase the potency of colistin against colistin-resistant and sensitive Gram-negative bacteria. Analogues were identified that are capable of inhibiting and dispersing MRSA biofilms and lowering the colistin MIC to below the CLSI breakpoint against Acinetobacter baumannii, Klebsiella pneumoniae, and Escherichia coli.

Understanding and Improving the Activity of Flavin-Dependent Halogenases via Random and Targeted Mutagenesis

Flavin-dependent halogenases (FDHs) catalyze the halogenation of organic substrates by coordinating reactions of reduced flavin, molecular oxygen, and chloride. Targeted and random mutagenesis of these enzymes have been used to both understand and alter their reactivity. These studies have led to insights into residues essential for catalysis and FDH variants with improved stability, expanded substrate scope, and altered site selectivity. Mutations throughout FDH structures have contributed to all of these advances. More recent studies have sought to rationalize the impact of these mutations on FDH function and to identify new FDHs to deepen our understanding of this enzyme class and to expand their utility for biocatalytic applications.

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.

Halogenated indoles eradicate bacterial persister cells and biofilms

The emergence of antibiotic resistance has necessitated new therapeutic approaches to combat recalcitrant bacterial infections. Persister cells, often found in biofilms, are metabolically dormant, and thus, are highly tolerant to all traditional antibiotics and represent a major drug resistance mechanism. In the present study, 36 diverse indole derivatives were investigated with the aim of identifying novel compounds that inhibit persisters and biofilm formation by Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. 5-Iodoindole and other halogenated indoles, 4-fluoroindole, 7-chloroindole, and 7-bromoindole, eradicated persister formation by E. coli and S. aureus, and 5-iodoindole most potently inhibited biofilm formation by the two bacteria. Unlike other antibiotics, 5-iodoindole did not induce persister cell formation, and 5-iodoindole inhibited the production of the immune-evasive carotenoid staphyloxanthin in S. aureus; hence, 5-iodoindole diminished the production of virulence factors in this strain. These results demonstrate halogenated indoles are potentially useful for controlling bacterial antibiotic resistance.

Marine Indole Alkaloids

Marine indole alkaloids comprise a large and steadily growing group of secondary metabolites. Their diverse biological activities make many compounds of this class attractive starting points for pharmaceutical development. Several marine-derived indoles were found to possess cytotoxic, antineoplastic, antibacterial and antimicrobial activities, in addition to the action on human enzymes and receptors. The newly isolated indole alkaloids of marine origin since the last comprehensive review in 2003 are reported, and biological aspects will be discussed.

Directed evolution of RebH for site-selective halogenation of large biologically active molecules

We recently characterized the substrate scope of wild-type RebH and proceeded to evolve variants of this enzyme with improved stability for biocatalysis. The substrate scopes of both RebH and the stabilized variants, however, are limited primarily to compounds similar in size to tryptophan. A substrate walking approach was used to further evolve RebH variants with expanded substrate scope. Two particularly notable variants were identified: 3-SS, which provides high conversion of tricyclic tryptoline derivatives; and 4-V, which accepts a broad range of large indoles and carbazoles. This constitutes the first reported use of directed evolution to enable the functionalization of substrates not accepted by wild-type RebH and demonstrates the utility of RebH variants for the site-selective halogenation of biologically active compounds.

Osteocompatibility of biofilm inhibitors

The demand for infection prevention therapies has led to the discovery of several biofilm inhibitors. These inhibiting signals are released by bacteria, fungi, or marine organisms to signal biofilm dispersal or disruption in Gram-positive, Gram-negative, and fungal microorganisms. The purpose of this study was to test the biocompatibility of five different naturally-produced biofilm chemical dispersal and inhibition signals with osteoblast-like cells: D-amino acids (D-AA), lysostaphin (LS), farnesol, cis-2-decenoic acid (C2DA), and desformyl flustrabromine (dFBr). In this preliminary study, compatibility of these anti-biofilm agents with differentiating osteoblasts was examined over a 21 days period at levels above and below concentrations active against bacterial biofilm. Anti-biofilm compounds listed above were serially diluted in osteogenic media and added to cultures of MC3T3 cells. Cell viability and cytotoxicity, after exposure to each anti-biofilm agent, were measured using a DNA assay. Differentiation characteristics of osteoblasts were determined qualitatively by observing staining of mineral deposits and quantitatively with an alkaline phosphatase assay. D-AA, LS, and C2DA were all biocompatible within the reported biofilm inhibitory concentration ranges and supported osteoblast differentiation. Farnesol and dFBr induced cytotoxic responses within the reported biofilm inhibitory concentration range and low doses of dFBr were found to inhibit osteoblast differentiation. At high concentrations, such as those that may be present after local delivery, many of these biofilm inhibitors can have effects on cellular viability and osteoblast function. Concentrations at which negative effects on osteoblasts occur should serve as upper limits for delivery to orthopaedic trauma sites and guide development of these potential therapeutics for orthopaedics.

Controlling bacterial behavior with indole-containing natural products and derivatives

Indole has recently been implicated as an important small molecule signal utilized by many bacteria to coordinate various forms of behavior. Indole plays a role in numerous bacterial processes, including: biofilm formation and maintenance, virulence factor production, antibiotic resistance and persister cell formation. Intercepting indole-signaling pathways with appropriately designed small molecules provides a n opportunity to control unwanted bacterial behaviors, and is an attractive anti-virulence therapeutic strategy. In this review, we give an overview of the process controlled by indole signaling, and summarize current efforts to design indole-containing small molecules to intercept these pathways, and detail the synthetic efforts towards accessing indole derived bioactive small molecules.

Building an antifouling zwitterionic coating on urinary catheters using an enzymatically triggered bottom-up approach

Catheter associated urinary tract infections are common during hospitalization due to the formation of bacterial biofilms on the indwelling device. In this study, we report an innovative biotechnology-based approach for the covalent functionalization of silicone catheters with antifouling zwitterionic moieties to prevent biofilm formation. Our approach combines the potential bioactivity of a natural phenolics layer biocatalytically conjugated to sulfobetaine-acrylic residues in an enzymatically initiated surface radical polymerization with laccase. To ensure sufficient coating stability in urine, the silicone catheter is plasma-activated. In contrast to industrial chemical methods, the methacrylate-containing zwitterionic monomers are polymerized at pH 5 and 50 °C using as an initiator the phenoxy radicals solely generated by laccase on the phenolics-coated catheter surface. The coated catheters are characterized by X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared (FTIR) analysis, atomic force microscopy (AFM), and colorimetrically. Contact angle and protein adsorption measurements, coupled with in vitro tests with the Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus in static and dynamic conditions, mimicking the operational conditions to be faced by the catheters, demonstrate reduced biofilm formation by about 80% when compared to that of unmodified urinary catheters. The zwitterionic coating did not affect the viability of the human fibroblasts (BJ-5ta) over seven days, corresponding to the extended useful life of urinary catheters.

Highly active modulators of indole signaling alter pathogenic behaviors in Gram-negative and Gram-positive bacteria

Indole is a universal signal that regulates various bacterial behaviors, such as biofilm formation and antibiotic resistance. To generate mechanistic probes of indole signaling and control indole-mediated pathogenic phenotypes in both Gram-positive and Gram-negative bacteria, we have investigated the use of desformylflustrabromine (dFBr) derivatives to generate highly active indole mimetics. We have developed non-microbicidal dFBr derivatives that are 27-2000 times more active than indole in modulating biofilm formation, motility, acid resistance, and antibiotic resistance. The activity of these analogues parallels indole, because they are dependent on temperature, the enzyme tryptophanase TnaA, and the transcriptional regulator SdiA. This investigation demonstrates that molecules based on the dFBr scaffold can alter pathogenic behaviors by mimicking indole-signaling pathways.

Indole/triazole conjugates are selective inhibitors and inducers of bacterial biofilms

Herein is described a method of accessing indole/triazole and benzothiophene/triazole analogues that selectively promote or inhibit biofilm formation by Gram-positive and Gram-negative bacteria. Structure/function studies revealed that the addition of a bromine atom at the 2-position of the indole/triazole scaffold altered activity against both Gram-negative and Gram-positive bacteria and could transform a biofilm inhibitor into a biofilm inducer. Isosteric replacement of the indole core by a benzothiophene significantly impaired anti-biofilm activity. A competition assay exposing Escherichia coli to the most potent biofilm inducer and an inhibitor of E. coli biofilm formation was performed. The inducer exhibited the ability to mute the effect of the anti-biofilm compound for this targeted bacterial population.

Indole and 7-benzyloxyindole attenuate the virulence of Staphylococcus aureus

Human pathogens can readily develop drug resistance due to the long-term use of antibiotics that mostly inhibit bacterial growth. Unlike antibiotics, antivirulence compounds diminish bacterial virulence without affecting cell viability and thus, may not lead to drug resistance. Staphylococcus aureus is a major agent of nosocomial infections and produces diverse virulence factors, such as the yellow carotenoid staphyloxanthin, which promotes resistance to reactive oxygen species (ROS) and the host immune system. To identify novel antivirulence compounds, bacterial signal indole present in animal gut and diverse indole derivatives were investigated with respect to reducing staphyloxanthin production and the hemolytic activity of S. aureus. Treatment with indole or its derivative 7-benzyloxyindole (7BOI) caused S. aureus to become colorless and inhibited its hemolytic ability without affecting bacterial growth. As a result, S. aureus was more easily killed by hydrogen peroxide (H₂O₂) and by human whole blood in the presence of indole or 7BOI. In addition, 7BOI attenuated S. aureus virulence in an in vivo model of nematode Caenorhabditis elegans, which is readily infected and killed by S. aureus. Transcriptional analyses showed that both indole and 7BOI repressed the expressions of several virulence genes such as α-hemolysin gene hla, enterotoxin seb, and the protease genes splA and sspA and modulated the expressions of the important regulatory genes agrA and sarA. These findings show that indole derivatives are potential candidates for use in antivirulence strategies against persistent S. aureus infection.

Small molecule control of bacterial biofilms

Bacterial biofilms are defined as a surface attached community of bacteria embedded in a matrix of extracellular polymeric substances that they have produced. When in the biofilm state, bacteria are more resistant to antibiotics and the host immune response than are their planktonic counterparts. Biofilms are increasingly recognized as being significant in human disease, accounting for 80% of bacterial infections in the body and diseases associated with bacterial biofilms include: lung infections of cystic fibrosis patients, colitis, urethritis, conjunctivitis, otitis, endocarditis and periodontitis. Additionally, biofilm infections of indwelling medical devices are of particular concern, as once the device is colonized infection is virtually impossible to eradicate. Given the prominence of biofilms in infectious diseases, there has been an increased effort toward the development of small molecules that will modulate bacterial biofilm development and maintenance. In this review, we highlight the development of small molecules that inhibit and/or disperse bacterial biofilms through non-microbicidal mechanisms. The review discuses the numerous approaches that have been applied to the discovery of lead small molecules that mediate biofilm development. These approaches are grouped into: (1) the identification and development of small molecules that target one of the bacterial signaling pathways involved in biofilm regulation, (2) chemical library screening for compounds with anti-biofilm activity, and (3) the identification of natural products that possess anti-biofilm activity, and the chemical manipulation of these natural products to obtain analogues with increased activity.

The acyl homoserine lactone receptor, SdiA, of Escherichia coli and Salmonella enterica serovar Typhimurium does not respond to indole

In this study, we tested the hypothesis that the SdiA proteins of Escherichia coli and Salmonella enterica serovar Typhimurium respond to indole. While indole was found to have effects on gene expression and biofilm formation, these effects were not sdiA dependent. However, high concentrations of indole did inhibit N-acyl-l-homoserine lactone (AHL) sensing by SdiA. We conclude that SdiA does not respond to indole but indole can inhibit SdiA activity in E. coli and Salmonella.