Bacterial quorum sensing: signals, circuits, and implications for biofilms and disease

Anti-biofilm compounds derived from marine sponges

Bacterial biofilms are surface-attached communities of microorganisms that are protected by an extracellular matrix of biomolecules. In the biofilm state, bacteria are significantly more resistant to external assault, including attack by antibiotics. In their native environment, bacterial biofilms underpin costly biofouling that wreaks havoc on shipping, utilities, and offshore industry. Within a host environment, they are insensitive to antiseptics and basic host immune responses. It is estimated that up to 80% of all microbial infections are biofilm-based. Biofilm infections of indwelling medical devices are of particular concern, since once the device is colonized, infection is almost impossible to eliminate. Given the prominence of biofilms in infectious diseases, there is a notable effort towards developing small, synthetically available molecules that will modulate bacterial biofilm development and maintenance. Here, we highlight the development of small molecules that inhibit and/or disperse bacterial biofilms specifically through non-microbicidal mechanisms. Importantly, we discuss several sets of compounds derived from marine sponges that we are developing in our labs to address the persistent biofilm problem. We will discuss: discovery/synthesis of natural products and their analogues—including our marine sponge-derived compounds and initial adjuvant activity and toxicological screening of our novel anti-biofilm compounds.

Anti-quorum sensing activity of essential oils from Colombian plants

Essential oils from Colombian plants were characterised by GC-MS, and assayed for anti-quorum sensing activity in bacteria sensor strains. Two major chemotypes were found for Lippia alba, the limonene-carvone and the citral (geranial-neral). For other species, the main components included α-pinene (Ocotea sp.), β-pinene (Swinglea glutinosa), cineol (Elettaria cardamomun), α-zingiberene (Zingiber officinale) and pulegone (Minthostachys mollis). Several essential oils presented promising inhibitory properties for the short chain AHL quorum sensing (QS) system, in Escherichia coli containing the biosensor plasmid pJBA132, in particular Lippia alba. Moderate activity as anti-QS using the same plasmid, were also found for selected constituents of essential oils studied here, such as citral, carvone and α-pinene, although solely at the highest tested concentration (250 µg mL(-1)). Only citral presented some activity for the long chain AHL QS system, in Pseudomonas putida containing the plasmid pRK-C12. In short, essential oils from Colombian flora have promising properties as QS modulators.

Bacterial interactions in dental biofilm

Biofilms are masses of microorganisms that bind to and multiply on a solid surface, typically with a fluid bathing the microbes. The microorganisms that are not attached but are free floating in an aqueous environment are termed planktonic cells. Traditionally, microbiology research has addressed results from planktonic bacterial cells. However, many recent studies have indicated that biofilms are the preferred form of growth of most microbes and particularly those of a pathogenic nature. Biofilms on animal hosts have significantly increased resistance to various antimicrobials compared to planktonic cells. These microbial communities form microcolonies that interact with each other using very sophisticated communication methods (i.e., quorum-sensing). The development of unique microbiological tools to detect and assess the various biofilms around us is a tremendously important focus of research in many laboratories. In the present review, we discuss the major biofilm mechanisms and the interactions among oral bacteria.

Effects on membrane lateral pressure suggest permeation mechanisms for bacterial quorum signaling molecules

Quorum sensing is an intricate example of "social" behavior in microbial communities mediated by small secreted molecules (autoinducers). The mechanisms of membrane permeation remain elusive for many of them. Here we present the assessment of membrane permeability for three natural autoinducers and four synthetic analogues based on their polarity, surface activity, affinity for lipid monolayers, and ability to induce lateral pressure changes in the inner E. coli membrane sensed by the bacterial tension-activated channel MscS. AI-1 (N-(3-oxodecanoyl)-l-homoserine lactone) is surface-active, and it robustly inserts into lipid monolayers, indicating strong propensity toward membranes. When presented to membrane patches from the cytoplasmic side, AI-1 transiently shifts MscS's activation curve toward higher tensions due to intercalation into the cytoplasmic leaflet followed by redistribution to the opposite side. Indole showed no detectable surface activity at the air-water interface but produced a moderate increase of lateral pressure in monolayers and was potent at shifting activation curves of MscS, demonstrating transients on sequential additions. AI-2 (4,5-dihydroxy-2,3-pentanedione, DPD) showed little activity at the interfaces, correspondingly with no effect on MscS activation. After chemical modification with isobutyl, hexyl, or heptyl chains, AI-2 displayed strong surface activity. Hexyl and especially heptyl AI-2 induced robust transient shifts of MscS activation curves. The data strongly suggest that both AI-1 and indole are directly permeable through the membrane. AI-2, more hydrophilic, shows low affinity toward lipids and thus requires a transport system, whereas alkyl analogues of AI-2 should permeate the membrane directly.

Flustramine inspired synthesis and biological evaluation of pyrroloindoline triazole amides as novel inhibitors of bacterial biofilms

Anti-biofilm agents have been developed based upon the flustramine family of alkaloids isolated from Flustra foliacea. A Garg interrupted Fischer indolization reaction was employed to access a core pyrroloindoline scaffold that was subsequently employed to create a pyrroloindoline triazole amide library. Screening for the ability to modulate biofilm formation against strains of Gram-positive and Gram-negative bacteria identified several compounds with low micromolar, non-toxic IC(50) values.

Programming microbial population dynamics by engineered cell-cell communication

A major aim of synthetic biology is to program novel cellular behavior using engineered gene circuits. Early endeavors focused on building simple circuits that fulfill simple functions, such as logic gates, bistable toggle switches, and oscillators. These gene circuits have primarily focused on single-cell behaviors since they operate intracellularly. Thus, they are often susceptible to cell-cell variations due to stochastic gene expression. Cell-cell communication offers an efficient strategy to coordinate cellular behavior at the population level. To this end, we review recent advances in engineering cell-cell communication to achieve reliable population dynamics, spanning from communication within single species to multispecies, from one-way sender-receiver communication to two-way communication in synthetic microbial ecosystems. These engineered systems serve as well-defined model systems to better understand design principles of their naturally occurring counterparts and to facilitate novel biotechnology applications.

Anti-fouling chemistry of chiral monolayers: enhancing biofilm resistance on racemic surface

This work reports the resistance to protein adsorption and bacterial biofilm formation by chiral monolayers of polyol-terminated alkanethiols surrounding micrometer-sized patterns of methyl-terminated alkanethiols on gold films. We discover that patterned surfaces surrounded by chiral polyol monolayers can distinguish different stages of biofilm formation. After inoculation on the surfaces, bacteria first reversibly attached on the chiral polyol monolayers. Over time, the bacteria detached from the polyol surfaces, and attached on the hydrophobic micropatterns to form biofilms. Interestingly, while both enantiomers of gulitol- and mannonamide-terminated monolayer resisted adsorption of proteins (bovine serum albumin, lysozyme, and fibrinogen) and confined biofilms formed on the micropatterns, the monolayers formed by the racemic mixture of either pair of enantiomers exhibited stronger antifouling chemistry against both protein adsorption and biofilm formation than monolayers formed by one enantiomer alone. These results reveal the different chemistries that separate the different stages of biofilm formation, and the stereochemical influence on resisting biofoulings at a molecular-level.

Characterization of quorum sensing signals in coral-associated bacteria

Marine environment habitats, such as the coral mucus layer, are abundant in nutrients and rich with diverse populations of microorganisms. Since interactions among microorganisms found in coral mucus can be either mutualistic or competitive, understanding quorum sensing-based acyl homoserine lactone (AHL) language may shed light on the interaction between coral-associated microbial communities in the native host. More than 100 bacterial isolates obtained from different coral species were screened for their ability to produce AHL. When screening the isolated coral bacteria for AHL induction activity using the reporter strains Escherichia coli K802NR-pSB1075 and Agrobacterium tumefaciens KYC55, we found that approximately 30% of the isolates tested positive. Thin layer chromatography separation of supernatant extracts revealed different AHL profiles, with detection of at least one active compound in the supernatant of those bacterial extracts being able to induce AHL activity in the two different bioreporter strains. The active extract of bacterial isolate 3AT 1-10-4 was subjected to further analysis by preparative thin layer chromatography and liquid chromatography tandem mass spectrometry. One of the compounds was found to correspond with N-(3-hydroxydecanoyl)-L-homoserine lactone. 16S rRNA gene sequencing of the isolates with positive AHL activity affiliated them with the Vibrio genus. Understanding the ecological role of AHL in the coral environment and its regulatory circuits in the coral holobiont-associated microbial community will further expand our knowledge of such interactions.

Single cell time-resolved quorum responses reveal dependence on cell density and configuration

Bacterial communication via quorum sensing has been extensively investigated in recent years. Bacteria communicate in a complex manner through the production, release, and reception of diffusible low molecular weight chemical signaling molecules. Much work has focused on understanding the basic mechanisms of quorum sensing. As more and more bacteria grow resistant to conventional antibiotics, the development of drugs that do not kill bacteria but instead interrupt their communication is of increasing interest. This study presents a method for analyzing bacterial communication by investigating single cell responses. Most conventional analysis methods for bacterial communication are based on the averaged response from many bacteria, masking how individual cells respond to their immediate environment. We applied a fiber-optic microarray to record cellular communication from single cells. Single cell quorum sensing systems have previously been employed, but the highly ordered array reported here is an improvement because it allows us to simultaneously investigate cellular communication in many different environments with known cellular densities and configurations. We employed this method to detect how genes under quorum regulation are induced or repressed over time on the single cell level and to determine whether cellular density and configuration are indicative of the single cell temporal patterns of gene expression.

Quorum sensing between Pseudomonas aeruginosa biofilms accelerates cell growth

This manuscript describes the fabrication of arrays of spatially confined chambers embossed in a layer of poly(ethylene glycol) diacrylate (PEGDA) and their application to studying quorum sensing between communities of Pseudomonas aeruginosa. We hypothesized that biofilms may produce stable chemical signaling gradients in close proximity to surfaces, which influence the growth and development of nearby microcolonies into biofilms. To test this hypothesis, we embossed a layer of PEGDA with 1.5-mm wide chambers in which P. aeruginosa biofilms grew, secreted homoserine lactones (HSLs, small molecule regulators of quorum sensing), and formed spatial and temporal gradients of these compounds. In static growth conditions (i.e., no flow), nascent biofilms secreted N-(3-oxododecanoyl) HSL that formed a gradient in the hydrogel and was detected by P. aeruginosa cells that were ≤8 mm away. Diffusing HSLs increased the growth rate of cells in communities that were <3 mm away from the biofilm, where the concentration of HSL was >1 μM, and had little effect on communities farther away. The HSL gradient had no observable influence on biofilm structure. Surprisingly, 0.1-10 μM of N-(3-oxododecanoyl) HSL had no effect on cell growth in liquid culture. The results suggest that the secretion of HSLs from a biofilm enhances the growth of neighboring cells in contact with surfaces into communities and may influence their composition, organization, and diversity.

Detection of quorum sensing signal molecules in the family Vibrionaceae

AIMS

The aim of this study was to detect the production of three kinds of quorum sensing (QS) signal molecules, i.e. the N-acyl-homoserine lactone (AHL), the autoinducer-2 (AI-2) and the cholerae autoinducer-1-like (CAI-1-like) molecules in 25 Vibrionaceae strains.

METHODS AND RESULTS

The QS signal molecules in 25 Vibrionaceae strains were detected with different biosensors. Except Salinivibrio costicola VIB288 and Vibrio natriegens VIB299, all the other 23 Vibrionaceae strains could produce one or more kinds of detectable QS signal molecules. Twenty-one of the 25 strains were found to produce AHL signal molecules by using Vibrio harveyi JMH612 and Agrobacterium tumefaciens KYC55 (pJZ372; pJZ384; pJZ410) as biosensors. The AHL fingerprints of eight strains were detected by thin-layer chromatography with Ag. tumefaciens KYC55, and two of them, i.e. V. mediterranei VIB296 and Aliivibrio logei VIB414 had a high diversity of AHLs. Twenty of the 25 strains were found to have the AI-2 activity, and the luxS gene sequences in 18 strains were proved to be conserved by PCR amplification and sequencing. Only six (five Vibrio strains and A. logei VIB414) of the 25 strains possessed the CAI-1-like activity. A. logei VIB414, V. campbellii VIB285, V. furnissii VIB293, V. pomeroyi LMG20537 and two V. harveyi strains VIB571 and VIB645 were found to produce all the three kinds of QS signal molecules.

CONCLUSIONS

The results indicated that the QS signal molecules, especially AHL and AI-2 molecules, were widespread in the family Vibrionaceae.

SIGNIFICANCE AND IMPACT OF THE STUDY

In response to a variety of environmental conditions and selection forces, the family Vibrionaceae produced QS signal molecules with great diversity and complexity. The knowledge we obtained from this study will be useful for further research on the roles of different QS signal molecules in this family.

Quorum sensing in biofilms--how to destroy the bacterial citadels or their cohesion/power?

Biofilms or microbial communities formed by adherent and cohesive cells on cellular or inert substrata (like medical devices), are involved in ≈ 60% of all infections and characterized by moderate intensity symptoms, chronic evolution and resistance to antibiotics. Biofilms' pathogenicity, even of those formed by opportunistic microorganisms, is amplified by two major biofilm characteristics: 1) the increased resistance to antimicrobials; 2) the protection of cells against the host's defence mechanisms. The studies at the molecular level shown that the biofilms formation is controlled by cell-to-cell signalling mechanisms and the gene regulation during biofilm growth is due to the accumulation of signal molecules. In this regard, quorum sensing mechanism (QS) is defined as a cell-density dependent bacterial intercellular communication, involved in gene expression (e.g. virulence genes for exoenzymes, exopolysaccharides) and the consequent changed behaviour of biofilm's cells, including the resistance to stress conditions; this resistance is different of well known antibioresistance, being named phenotypical resistance or tolerance. Considering the differences in physiology and susceptibility to antibiotics of biofilm embedded bacteria, as well as their increased power against the host defence responses, there are necessary new strategies for prevention and therapy of biofilm associated infections. The dental plaque is a typical example of biofilm, involved in the ethiology of cariogenesis and periodontal diseases associated with local chronic inflammation and cytokines production. The genetical and phenotypical versatility of the biofilm's cells represent a challenge for discovering new methods of treatment and prevention of biofilm associated infections. A novel class of antibiofilm and antipathogenic therapeutics which are interfering with a new target - the QS pathway, not based on growth inhibition and called QS inhibitors, natural, with different origins or artificial, are now developing as an alternative to antibiotherapy.

Synthesis and biological activity of 2-aminoimidazole triazoles accessed by Suzuki-Miyaura cross-coupling

A pilot library of 2-aminoimidazole triazoles (2-AITs) was synthesized and assayed against Acinetobacter baumannii and methicillin-resistant Staphylococus aureus (MRSA). Results from these studies show that these new derivatives have improved biofilm dispersal activities as well as antibacterial properties against A. baumannii. With MRSA biofilms they are found to possess biofilm inhibition capabilities at low micromolar concentrations.

Engineering biofilm formation and dispersal

Anywhere water is in the liquid state, bacteria will exist as biofilms, which are complex communities of cells that are cemented together. Although frequently associated with disease and biofouling, biofilms are also important for engineering applications, such as bioremediation, biocatalysis and microbial fuel cells. Here, we review approaches to alter genetic circuits and cell signaling towards controlling biofilm formation, and emphasize utilizing these tools for engineering applications. Based on a better understanding of the genetic basis of biofilm formation, we find that biofilms might be controlled by manipulating extracellular signals, and that they might be dispersed using conserved intracellular signals and regulators. Biofilms could also be formed at specific locations where they might be engineered to make chemicals or treat human disease.

Identification of a key amino acid of LuxS involved in AI-2 production in Campylobacter jejuni

Autoinducer-2 (AI-2) mediated quorum sensing has been associated with the expression of virulence factors in a number of pathogenic organisms and has been demonstrated to play a role in motility and cytolethal distending toxin (cdt) production in Campylobacter jejuni. We have initiated the work to determine the molecular basis of AI-2 synthesis and the biological functions of quorum sensing in C. jejuni. In this work, two naturally occurring variants of C. jejuni 81116 were identified, one producing high-levels of AI-2 while the other is defective in AI-2 synthesis. Sequence analysis revealed a G92D mutation in the luxS gene of the defective variant. Complementation of the AI-2⁻ variant with a plasmid encoded copy of the wild-type luxS gene or reversion of the G92D mutation by site-directed mutagenesis fully restored AI-2 production by the variant. These results indicate that the G92D mutation alone is responsible for the loss of AI-2 activity in C. jejuni. Kinetic analyses showed that the G92D LuxS has a ∼100-fold reduced catalytic activity relative to the wild-type enzyme. Findings from this study identify a previously undescribed amino acid that is essential for AI-2 production by LuxS and provide a unique isogenic pair of naturally occurring variants for us to dissect the functions of AI-2 mediated quorum sensing in Campylobacter.

Emerging knowledge of regulatory roles of D-amino acids in bacteria

The d-enantiomers of amino acids have been thought to have relatively minor functions in biological processes. While l-amino acids clearly predominate in nature, d-amino acids are sometimes found in proteins that are not synthesized by ribosomes, and d-Ala and d-Glu are routinely found in the peptidoglycan cell wall of bacteria. Here, we review recent findings showing that d-amino acids have previously unappreciated regulatory roles in the bacterial kingdom. Many diverse bacterial phyla synthesize and release d-amino acids, including d-Met and d-Leu, which were not previously known to be made. These noncanonical d-amino acids regulate cell wall remodeling in stationary phase and cause biofilm dispersal in aging bacterial communities. Elucidating the mechanisms by which d-amino acids govern cell wall remodeling and biofilm disassembly will undoubtedly reveal new paradigms for understanding how extracytoplasmic processes are regulated as well as lead to development of novel therapeutics.

Biological nanofactories target and activate epithelial cell surfaces for modulating bacterial quorum sensing and interspecies signaling

In order to control the behavior of bacteria present at the surface of human epithelial cells, we have created a biological "nanofactory" construct that "coats" the epithelial cells and "activates" the surface to produce the bacterial quorum sensing signaling molecule, autoinducer-2 (AI-2). Specifically, we demonstrate directed modulation of signaling among Escherichia coli cells grown over the surface of human epithelial (Caco-2) cells through site-directed attachment of biological nanofactories. These "factories" comprise a fusion protein expressed and purified from E. coli containing two AI-2 bacterial synthases (Pfs and LuxS), a protein G IgG binding domain, and affinity ligands for purification. The final factory is fabricated ex vivo by incubating with an anti-CD26 antibody that binds the fusion protein and specifically targets the CD26 dipeptidyl peptidase found on the outer surface of Caco-2 cells. This is the first report of the intentional "in vitro" synthesis of bacterial autoinducers at the surface of epithelial cells for the redirection of quorum sensing behaviors of bacteria. We envision tools such as this will be useful for interrogating, interpreting, and disrupting signaling events associated with the microbiome localized in human intestine and other environments.

Quorum sensing and a global regulator TsrA control expression of type VI secretion and virulence in Vibrio cholerae

Vibrio cholerae is a human pathogen that causes the life-threatening diarrheal disease cholera. A type VI secretion system (T6SS) was recently shown to be required for full virulence in the O37 serogroup strain V52, which causes only sporadic human disease, but T6SS is not expressed in seventh pandemic O1 El Tor strains under standard laboratory conditions. In this study, we show that in the O1 El Tor strain C6706, T6SS is repressed by both quorum sensing and the uncharacterized protein VC0070 (TsrA). Disruption of TsrA and the quorum sensing regulator LuxO induces expression and secretion of the T6SS substrate Hcp, and this is dependent on the downstream regulator HapR, which directly binds to the promoter region of the T6SS genes hcp1 and hcp2 to induce expression. The activated T6SS in C6706 is functional and can translocate the effector protein VgrG-1 into macrophage cells, and T6SS activation leads to fecal diarrhea and intestinal inflammation in infant rabbits. Using an infant mouse infection model, we show that deletion of tsrA results in a 9.3-fold increase in intestinal colonization compared with wild type. TsrA functions as a global regulator to activate expression of hemagglutinin protease and repress cholera toxin and toxin coregulated pilus. Our findings provide significant insight into the molecular mechanism of T6SS and ToxT regulon gene regulation by quorum sensing and TsrA.

Minimal models of growth and decline of microbial populations

Dynamics of growth and decline of microbial populations were analysed and respective models were developed in this investigation. Analysis of the dynamics was based on general considerations concerning the main properties of microorganisms and their interactions with the environment which was supposed to be affected by the activity of the population. Those considerations were expressed mathematically by differential equations or systems of the equations containing minimal sets of parameters characterizing those properties. It has been found that: (1) the factors leading to the decline of the population have to be considered separately, namely, accumulation of metabolites (toxins) in the medium and the exhaustion of resources; the latter have to be separated again into renewable ('building materials') and non-renewable (sources of energy); (2) decline of the population is caused by the exhaustion of sources of energy but no decline is predicted by the model because of the exhaustion of renewable resources; (3) the model determined by the accumulation of metabolites (toxins) in the medium does not suggest the existence of a separate 'stationary phase'; (4) in the model determined by the exhaustion of energy resources the 'stationary' and 'decline' phases are quite discernible; and (5) there is no symmetry in microbial population dynamics, the decline being slower than the rise. Mathematical models are expected to be useful in getting insight into the process of control of the dynamics of microbial populations. The models are in agreement with the experimental data.

Role of planktonic and sessile extracellular metabolic byproducts on Pseudomonas aeruginosa and Escherichia coli intra and interspecies relationships

Bacterial species are found primarily as residents of complex surface-associated communities, known as biofilms. Although these structures prevail in nature, bacteria still exist in planktonic lifestyle and differ from those in morphology, physiology, and metabolism. This study aimed to investigate the influence of physiological states of Pseudomonas aeruginosa and Escherichia coli in cell-to-cell interactions. Filtered supernatants obtained under planktonic and biofilm cultures of each single species were supplemented with tryptic soy broth (TSB) and used as the growth media (conditioned media) to planktonic and sessile growth of both single- and two-species cultures. Planktonic bacterial growth was examined through OD(640) measurement. One-day-old biofilms were evaluated in terms of biofilm biomass (CV), respiratory activity (XTT), and CFU number. Conditioned media obtained either in biofilm or in planktonic mode of life triggered a synergistic effect on planktonic growth, mainly for E. coli single cultures growing in P. aeruginosa supernatants. Biofilms grown in the presence of P. aeruginosa biofilms-derived metabolites presented less mass and activity. These events highlight that, when developed in biofilm, P. aeruginosa release signals or metabolites able to prejudice single and binary biofilm growth of others species and of their own species. However, products released by their planktonic counterparts did not impair biofilm growth or activity. E. coli, living as planktonic or sessile cultures, released signals and metabolites or removed un-beneficial compounds which promoted the growth and activity of all the species. Our findings revealed that inter and intraspecies behaviors depend on the involved bacteria and their adopted mode of life.

Marine natural product drug discovery: Leads for treatment of inflammation, cancer, infections, and neurological disorders

Natural products, secondary metabolites, isolated from plants, animals and microbes are important sources for bioactive molecules that in many cases have been developed into treatments for diseases. This review will focus on describing the potential for finding new treatments from marine natural products for inflammation, cancer, infections, and neurological disorders. Historically terrestrial natural products have been studied to a greater extent and such classic drugs as aspirin, vincristine and many of the antibiotics are derived from terrestrial natural products. The need for new therapeutics in the four areas mentioned is dire. Within the last 30 years marine natural products, with their unique structures and high level of halogenation, have shown many promising activities against the inflammatory response, cancer, infections and neurological disorders. The review will outline examples of such compounds and activities.

Bacterial translocation to mesenteric lymph nodes increases in chronic portal hypertensive rats

PURPOSE

Bacterial translocation is a frequent complication in portal hypertension related to cirrhosis in the human clinical area. The aim of this study was to verify the existence of intestinal bacterial translocation to mesenteric lymph nodes in male Wistar rats with triple partial portal vein ligation during short- (48 h) and long-term (1 month) postoperative evolution.

RESULTS

At 48 h, ileal total aerobes bacteria (p < 0.001) and Lactobacillus decrease in sham-operated (SO) and portal hypertensive (PH) rats. At 1 month, ileal Enterococci and Streptococcus sp. show a statistically significant decrease in SO- and PH-rats. Lactobacillus decreases in the colon in SO- (p < 0.01) and in PH-rats (p < 0.001). At 1 month, colonic Enterococci decreases compared to control (p < 0.001) and SO-rats (p < 0.01). These intestinal microfloral changes are associated with bacterial translocation to mesenteric lymph nodes at 48 h (50%; p = 0.004) and 1 month (100%; p < 0.001) of postoperative evolution in PH-rats.

CONCLUSIONS

The enlargement of the stenosed portal tract related to triple partial portal vein ligation in the rat, since it increases the resistance to the portal blood flow, may be a key factor involved in one of the pathological consequences of portal hypertension, as is bacterial translocation to mesenteric lymph nodes.

Reduced bacterial deposition and attachment by quorum-sensing inhibitor 4-nitro-pyridine-N-oxide: the role of physicochemical effects

Surface-attached chemical groups that resist protein adhesion are commonly characterized as being hydrophilic, H-bond acceptors, non-H-bond donors, and electrically neutral. Quorum-sensing (QS) inhibitor 4-nitropyridine-N-oxide (4-NPO) that previously was found to decrease Pseudomonas aeruginosa biofilm formation possesses all of these characteristics, making this molecule an ideal antiadhesive compound. It was hypothesized that once 4-NPO adsorbs to either the solid surface or bacteria, resultant changes in the physical-chemical surface properties of the solid surface and bacteria will reduce the extent of bacterial adhesion. These physical-chemical effects take place prior to the commencement of already well-established QS biofilm-inhibition mechanisms. Bacterial adhesion experiments to silica conducted in quartz crystal microbalance with dissipation (QCM-D) and parallel plate flow cells demonstrated that 4-NPO reduces bacterial adhesion to silica-coated surfaces by the adsorption of 4-NPO to the silica surface as well to the outer membrane of both gram-negative P. aeruginosa PAO1 and gram-positive Staphylococcus aureus. 4-NPO effectively neutralizes both the bacterial and silica surface charge, and it is proposed that this neutralization of local surface charge heterogeneities by 4-NPO adsorption is the mechanism responsible for decelerating rates of bacterial deposition.

Indirect pathogenicity of Haemophilus influenzae and Moraxella catarrhalis in polymicrobial otitis media occurs via interspecies quorum signaling

Otitis media (OM) is among the leading diseases of childhood and is caused by opportunists that reside within the nasopharynx, such as Haemophilus influenzae and Moraxella catarrhalis. As with most airway infections, it is now clear that OM infections involve multiple organisms. This study addresses the hypothesis that polymicrobial infection alters the course, severity, and/or treatability of OM disease. The results clearly show that coinfection with H. influenzae and M. catarrhalis promotes the increased resistance of biofilms to antibiotics and host clearance. Using H. influenzae mutants with known biofilm defects, these phenotypes were shown to relate to biofilm maturation and autoinducer-2 (AI-2) quorum signaling. In support of the latter mechanism, chemically synthesized AI-2 (dihydroxypentanedione [DPD]) promoted increased M. catarrhalis biofilm formation and resistance to antibiotics. In the chinchilla infection model of OM, polymicrobial infection promoted M. catarrhalis persistence beyond the levels seen in animals infected with M. catarrhalis alone. Notably, no such enhancement of M. catarrhalis persistence was observed in animals infected with M. catarrhalis and a quorum signaling-deficient H. influenzae luxS mutant strain. We thus conclude that H. influenzae promotes M. catarrhalis persistence within polymicrobial biofilms via interspecies quorum signaling. AI-2 may therefore represent an ideal target for disruption of chronic polymicrobial infections. Moreover, these results strongly imply that successful vaccination against the unencapsulated H. influenzae strains that cause airway infections may also significantly impact chronic M. catarrhalis disease by removing a reservoir of the AI-2 signal that promotes M. catarrhalis persistence within biofilm.

Controlling biofilm formation, prophage excision and cell death by rewiring global regulator H-NS of Escherichia coli

The global regulator H-NS of Escherichia coli controls genes related to stress response, biofilm formation and virulence by recognizing curved DNA and by silencing acquired genes. Here, we rewired H-NS to control biofilm formation using protein engineering; H-NS variant K57N was obtained that reduces biofilm formation 10-fold compared with wild-type H-NS (wild-type H-NS increases biofilm formation whereas H-NS K57N reduces it). Whole-transcriptome analysis revealed that H-NS K57N represses biofilm formation through its interaction with the nucleoid-associated proteins Cnu and StpA and in the absence of these proteins, H-NS K57N was unable to reduce biofilm formation. Significantly, H-NS K57N enhanced the excision of defective prophage Rac while wild-type H-NS represses excision, and H-NS controlled only Rac excision among the nine resident E. coli K-12 prophages. Rac prophage excision not only led to the change in biofilm formation but also resulted in cell lysis through the expression of toxin HokD. Hence, the H-NS regulatory system may be evolved through a single-amino-acid change in its N-terminal oligomerization domain to control biofilm formation, prophage excision and apoptosis.

Vitamin D, innate immunity and upper respiratory tract infection

INTRODUCTION

At the turn of the twentieth century, ultraviolet light was successfully used to treat tuberculosis of the skin. Upper respiratory tract infections had been inversely associated with sun exposure. During the last decade, basic scientific research demonstrated that vitamin D has an important anti-infective role.

METHOD

Review of the relevant literature on the influence of vitamin D on innate immunity and respiratory tract infection.

RESULTS

Vitamin D is involved in the production of defensins and cathelicidin - antimicrobial peptides that provide a natural defence against potential microbiological pathogens. Vitamin D supplementation increases cathelicidin production. Low vitamin D levels are associated with an increased incidence of upper respiratory tract infections.

CONCLUSIONS

Vitamin D appears to play an important role in the regulation of innate immunity in the upper respiratory tract. Optimal vitamin D levels and appropriate dosing schedules have yet to be determined.

Host tissue interaction, fate, and risks of degradable and nondegradable gel fillers

BACKGROUND

A constantly increasing number of gel fillers for aesthetic and reconstructive purposes have been introduced during the last 20 years. Most of the new ones are modified versions of the original collagen and hyaluronic acid gels. They have been reconstructed, often by adding cross-bindings to the polymer in order to obtain a more dense molecular structure, which will prolong degradation and filling effect of the gel. Other gel fillers contain particles of organic (poly-lactic acid) or inorganic (calcium hydroxylapatite) material, which have been used in human tissue for other purposes (degradable suture material and bone cement, respectively). The permanent fillers (silicone oil and polyacrylamide gel) have been used for many years, silicone mainly in the US and polyacrylamide gel in most countries outside the US and Canada.

OBJECTIVE

Complications occur, and they appear to be more frequent with particulated fillers, polyacrylamide gel and silicone oil. However, these complications differ in nature and depend on the filler type used.

METHODS AND MATERIALS

This overview presents the different gel filler types, how they interact with host tissue, and what can go wrong. The results and conclusion are based on experimental and clinical observations coupled with a search of the literature.

RESULTS AND CONCLUSION

Complications following homogenous hydrogels are caused by infection with bacteria, which have been inserted into the gel during injection. If not treated with relevant antibiotics (but instead steroids or large doses of NSAIDs) the bacteria form a biofilm, which gives rise to a low-grade chronic infection that is resistant to antibiotics. Complications following particulated gels and silicone oil are not known, but bacteria in a biofilm and/or endotoxins released by these is a possibility which deserves further investigations, primarily by using the fluorescence in situ hybridization (FISH) technique.

Indole as an intercellular signal in microbial communities

Bacteria can utilize signal molecules to coordinate their behavior to survive in dynamic multispecies communities. Indole is widespread in the natural environment, as a variety of both Gram-positive and Gram-negative bacteria (to date, 85 species) produce large quantities of indole. Although it has been known for over 100 years that many bacteria produce indole, the real biological roles of this molecule are only now beginning to be unveiled. As an intercellular signal molecule, indole controls diverse aspects of bacterial physiology, such as spore formation, plasmid stability, drug resistance, biofilm formation, and virulence in indole-producing bacteria. In contrast, many non-indole-producing bacteria, plants and animals produce diverse oxygenases which may interfere with indole signaling. It appears indole plays an important role in bacterial physiology, ecological balance, and possibly human health. Here we discuss our current knowledge and perspectives on indole signaling.

Studying bacterial infections through culture-independent approaches

The ability to characterize accurately the cause of infection is fundamental to effective treatment. The impact of any antimicrobial agents used to treat infection will, however, always be constrained by both the appropriateness of their use and our ability to determine their effectiveness. Traditional culture-based diagnostic microbiology is, in many cases, unable to provide this information. Molecular microbiological approaches that assess the content of clinical samples in a culture-independent manner promise to change dramatically the types of data that are obtained routinely from clinical samples. We argue that, in addition to the technical advance that these methodologies offer, a conceptual advance in the way that we reflect on the information generated is also required. Through the development of both of these advances, our understanding of infection, as well as the ways in which infections can be treated, may be improved. In the analysis of the microbiological content of certain clinical samples, such as blood, cerebrospinal fluid, brain and bone biopsy, culture-independent approaches have been well documented. Herein, we discuss how extensions to such studies can shape our understanding of infection at the many sites of the human body where a mixed flora, or in more ecological terms, a community of microbes, is present. To do this, we consider the underlying principles that underpin diagnostic systems, describe the ways in which these systems can be applied to community characterization, and discuss the significance of the data generated. We propose that at all locations within the human body where infection is routinely initiated within the context of a community of microbes, the same principles will apply. To consider this further, we take insights from areas such as the gut, oral cavity and skin. The main focus here is understanding respiratory tract infection, and specifically the infections of the cystic fibrosis lung. The impact that the use of culture-independent, molecular analyses will have on the way we approach the treatment of infections is also considered.

A naturally occurring brominated furanone covalently modifies and inactivates LuxS

Halogenated furanones, a group of natural products initially isolated from marine red algae, are known to inhibit bacterial biofilm formation, swarming, and quorum sensing. However, their molecular targets and the precise mode of action remain elusive. Herein, we show that a naturally occurring brominated furanone covalently modifies and inactivates LuxS (S-ribosylhomocysteine lyase, EC 4.4.1.21), the enzyme which produces autoinducer-2 (AI-2).

Enterococcus faecalis biofilms eradication by root canal irrigants

The aim of this study was to evaluate the minimal biofilm eradication concentration (MBEC) of sodium hypochlorite (NaOCl), chlorhexidine (CHX), EDTA, and citric and phosphoric acids after 1, 5, and 10 minutes of exposure to biofilms of Enterococcus faecalis. The biofilms grew in the MBEC high-throughput device for 24 hours at 37 degrees C and were exposed to 10 serial two-fold dilutions of each irrigating solution. The viable cell counts were log(10) transformed, and a concentration of an irrigant was considered to eradicate the biofilms when it produced a reduction of > or = 5 logarithmic units. NaOCl was the most effective agent, capable of eradicating the biofilms after 1 minute at a concentration of 0.00625%. CHX eradicated biofilm after 5 minutes at 2%. EDTA and citric and phosphoric acid solutions were not effective against the biofilms at any concentration or time tested.

A quorum-sensing antagonist targets both membrane-bound and cytoplasmic receptors and controls bacterial pathogenicity

Quorum sensing is a process of bacterial communication involving production and detection of secreted molecules called autoinducers. Gram-negative bacteria use acyl-homoserine lactone (AHL) autoinducers, which are detected by one of two receptor types. First, cytoplasmic LuxR-type receptors bind accumulated intracellular AHLs. AHL-LuxR complexes bind DNA and alter gene expression. Second, membrane-bound LuxN-type receptors bind accumulated extracellular AHLs. AHL-LuxN complexes relay information internally by phosphorylation cascades that direct gene expression changes. Here, we show that a small molecule, previously identified as an antagonist of LuxN-type receptors, is also a potent antagonist of the LuxR family, despite differences in receptor structure, localization, AHL specificity, and signaling mechanism. Derivatives were synthesized and optimized for potency, and in each case, we characterized the mode of action of antagonism. The most potent antagonist protects Caenorhabditis elegans from quorum-sensing-mediated killing by Chromobacterium violaceum, validating the notion that targeting quorum sensing has potential for antimicrobial drug development.

LuxS promotes biofilm maturation and persistence of nontypeable haemophilus influenzae in vivo via modulation of lipooligosaccharides on the bacterial surface

Nontypeable Haemophilus influenzae (NTHI) is an extremely common airway commensal which can cause opportunistic infections that are usually localized to airway mucosal surfaces. During many of these infections, NTHI forms biofilm communities that promote persistence in vivo. For many bacterial species, density-dependent quorum-signaling networks can affect biofilm formation and/or maturation. Mutation of luxS, a determinant of the autoinducer 2 (AI-2) quorum signal pathway, increases NTHI virulence in the chinchilla model for otitis media infections. For example, bacterial counts in middle-ear fluids and the severity of the host inflammatory response were increased in luxS mutants compared with parental strains. As these phenotypes are consistent with those that we have observed for biofilm-defective NTHI mutants, we hypothesized that luxS may affect NTHI biofilms. A luxS mutant was generated using the well-characterized NTHI 86-028NP strain and tested to determine the effects of the mutation on biofilm phenotypes in vitro and bacterial persistence and disease severity during experimental otitis media. Quantitation of the biofilm structure by confocal microscopy and COMSTAT analysis revealed significantly reduced biomass for NTHI 86-028NP luxS biofilms, which was restored by a soluble mediator in NTHI 86-028NP supernatants. Analysis of lipooligosaccharide moieties using an enzyme-linked immunosorbent assay and immunoblotting showed decreased levels of biofilm-associated glycoforms in the NTHI 86-028NP luxS strain. Infection studies showed that NTHI 86-028NP luxS had a significant persistence defect in vivo during chronic otitis media infection. Based on these data, we concluded that a luxS-dependent soluble mediator modulates the composition of the NTHI lipooligosaccharides, resulting in effects on biofilm maturation and bacterial persistence in vivo.

Flow-based microfluidic device for quantifying bacterial chemotaxis in stable, competing gradients

Chemotaxis is the migration of cells in gradients of chemoeffector molecules. Although multiple, competing gradients must often coexist in nature, conventional approaches for investigating bacterial chemotaxis are suboptimal for quantifying migration in response to gradients of multiple signals. In this work, we developed a microfluidic device for generating precise and stable gradients of signaling molecules. We used the device to investigate the effects of individual and combined chemoeffector gradients on Escherichia coli chemotaxis. Laminar flow-based diffusive mixing was used to generate gradients, and the chemotactic responses of cells expressing green fluorescent protein were determined using fluorescence microscopy. Quantification of the migration profiles indicated that E. coli was attracted to the quorum-sensing molecule autoinducer-2 (AI-2) but was repelled from the stationary-phase signal indole. Cells also migrated toward higher concentrations of isatin (indole-2,3-dione), an oxidized derivative of indole. Attraction to AI-2 overcame repulsion by indole in equal, competing gradients. Our data suggest that concentration-dependent interactions between attractant and repellent signals may be important determinants of bacterial colonization of the gut.