A review of chemical signaling pathways in the quorum sensing circuit of Pseudomonas aeruginosa

Pseudomonas aeruginosa, an increasingly common competitive and biofilm organism in healthcare infection with sophisticated, interlinked and hierarchic quorum systems (Las, Rhl, PQS, and IQS), creates the greatest threats to the medical industry and has rendered prevailing chemotherapy medications ineffective. The rise of multidrug resistance has evolved into a concerning and potentially fatal occurrence for human life. P. aeruginosa biofilm development is assisted by exopolysaccharides, extracellular DNA, proteins, macromolecules, cellular signaling and interaction. Quorum sensing is a communication process between cells that involves autonomous inducers and regulators. Quorum-induced infectious agent biofilms and the synthesis of virulence factors have increased disease transmission, medication resistance, infection episodes, hospitalizations and mortality. Hence, quorum sensing may be a potential therapeutical target for bacterial illness, and developing quorum inhibitors as an anti-virulent tool could be a promising treatment strategy for existing antibiotics. Quorum quenching is a prevalent technique for treating infections caused by microbes because it diminishes microbial pathogenesis and increases microbe biofilm sensitivity to antibiotics, making it a potential candidate for drug development. This paper examines P. aeruginosa quorum sensing, the hierarchy of quorum sensing mechanism, quorum sensing inhibition and quorum sensing inhibitory agents as a drug development strategy to supplement traditional antibiotic strategies.

Inhibition of biofilm formation, quorum sensing and virulence factor production in Pseudomonas aeruginosa PAO1 by selected LasR inhibitors

The quorum sensing network of Pseudomonas aeruginosa mediates the regulation of genes controlling biofilm formation and virulence factors. The rise of drug resistance to Pseudomonas aeruginosa infections has made quorum sensing-regulated biofilm formation in clinical settings a major issue. In the present study, LasR inhibitors identified in our previous study were evaluated for their antibiofilm and antiquorum sensing activities against P. aeruginosa PAO1. The compounds selected were (3-[2-(3,4-dimethoxyphenyl)-2-(1H-indol-3-yl)ethyl]-1-(2-fluorophenyl)urea) (C1), (3-(4-fluorophenyl)-2-[(3-methylquinoxalin-2-yl)methylsulfanyl]quinazolin-4-one) (C2) and (2-({4-[4-(2-methoxyphenyl)piperazin-1-yl]pyrimidin-2-yl}sulfanyl)-N-(2,4,6-trimethylphenyl)acetamide) (C3). The minimum inhibitory concentrations of C1 and C2 were 1000 μM, whereas that of C3 was 500 μM. At sub-MICs, the compounds showed potent antibiofilm activity without affecting the growth of P. aeruginosa PAO1. Electron microscopy confirmed the disruption of biofilm by the selected compounds. The antiquorum sensing activity of the compounds was revealed by the inhibition of violacein in Chromobacterium violaceum and the inhibition of swimming and swarming motilities in P. aeruginosa PAO1. Furthermore, the compounds also attenuated the production of quorum sensing-mediated virulence factors. The qRT-PCR revealed the downregulation of quorum sensing regulatory genes, namely lasI, lasR, rhlI, rhlR, lasB, pqsA and pqsR. The selected compounds also exhibited lower cytotoxicity against peripheral blood lymphocytes. Thus, this study could pave a way to explore these compounds for the development of therapeutic agent against Pseudomonas aeruginosa biofilm-related infections.

Potential of Impatiens balsamina Leaf Extract against Quorum Sensing in Pseudomonas aeruginosa PA01

Bacteria control gene expression by quorum sensing (QS) mechanism owing to producing small signal molecules associated with population density. Both gram-positive and gram-negative bacteria use QS to manage various physiological characteristics, including bioluminescence, virulence gene expression, biofilm formation, and antibiotic resistance. Impatience balsamina is a flowering, perennial and annual herb indigenous to southern Asia in India. All parts of Impatience balsamina have a therapeutic effect on different diseases. This study evaluated the anti-quorum sensing activity of leaf extract of Impatience balsamina by examining its action on Violacein production by Chromobacterium violaceum, a biosensor strain, and Biofilm, Pyocyanin, Protease, and Chitinase production by the reference strain Pseudomonas aeruginosa PA 01. Minimum inhibitory concentration (MIC) for Pseudomonas aeruginosa PA 01was 3.125mg/ml. A concentration of 1.563mg/ml (sub-MIC) showed inhibition of 100% on Las A protease, 78.42% on chitinase, 30.75% on biofilm, and 93.33% on pyocyanin production by Pseudomonas aeruginosa PA 01. This article displayed the quorum quenching activity of Impatience balsamina by hindering the quorum-sensing controlled characteristics of bacteria without killing it, which reduces the proneness of drug resistance in bacteria, a globally accepted emerging problem in the medical field.

Effect of Biogenic Silver Nanoparticles on the Quorum-Sensing System of Pseudomonas aeruginosa PAO1 and PA14

The increase in multidrug-resistant microorganisms represents a global threat requiring the development novel strategies to fight bacterial infection. This study aimed to assess the effect of silver nanoparticles (bio-AgNPs) on bacterial growth, biofilm formation, production of virulence factors, and expression of genes related to the quorum-sensing (QS) system of P. aeruginosa PAO1 and PA14. Biofilm formation and virulence assays were performed with bio-AgNPs. RT-qPCR was carried out to determine the effect of bio-AgNPs on the QS regulatory genes lasI, lasR, rhlI, rhlR, pqsA, and mvfR. Bio-AgNPs had an MIC value of 62.50 μM, for both strains. Phenotypic and genotypic assays were carried out using sub-MIC values. Experimental results showed that treatment with sub-MICs of bio-AgNPs reduced (p < 0.05) the motility and rhamnolipids and elastase production in P. aeruginosa PAO1. In PA14, bio-AgNPs stimulated swarming and twitching motilities as well as biofilm formation and elastase and pyocyanin production. Bio-AgNP treatment increased (p < 0.05) the expression of QS genes in PAO1 and PA14. Despite the different phenotypic behaviors in both strains, both showed an increase in the expression of QS genes. Demonstrating that the bio-AgNPs acted in the induction of regulation. The possible mechanism underlying the action of bio-AgNPs involves the induction of the rhl and/or pqs system of PAO1 and of the las and/or pqs system of PA14. These results suggest that exposure to low concentrations of bio-AgNPs may promote the expression of QS regulatory genes in P. aeruginosa, consequently inducing the production of virulence factors such as elastase, pyocyanin, and biofilms.

Effect of (+) usnic Acid on Pigment Production in Bacteria

Antibiotic resistance is a universal problem in bacterial infections. Hence it gives high priority for new therapeutic methods to alternate conventional antibiotic treatment. Pigment production is one of the virulence characteristics of bacteria regulated by a quorum-sensing mechanism. Antiquorum sensing activities will not directly affect the viability of bacteria; however, they will reduce the pathogenicity of bacteria. Thereby it gives an adverse probability of the development of drug resistance. Aim of our study is to evaluate the activity of (+) usnic acid on pigment production in Pseudomonas aeruginosa MTCC 2453, Chromobacterium violaceum MTCC 2656 and Serratia marcescens MTCC 8708. (+) usnic acid eluted by column chromatography. Dimethyl sulphoxide (DMSO) was used as the solvent for (+) usnic acid. Antibacterial activity determined by Agar well diffusion and broth microdilution methods. Effect on pigment production was assessed by spectroscopy. (+) usnic acid showed accumulative inhibition as its concentration increases on pigment production by Pseudomonas aeruginosa MTCC 2453, Chromobacterium violaceum MTCC 2656 and Serratia marcescens MTCC 8708. The lowest concentrations of (+) usnic acid manifested 50% inhibition of pigment production was 122.67, 87.73 and 205.26 µg/ml respectively on above mentioned order in bacteria. The concentration of (+) usnic acid that showed pigment production inhibition did not hinder the growth of the bacteria, but it can reduce the virulence of the bacteria. This property can be used to resolve the drug resistance in bacteria. Further studies are required to check the action of (+) usnic acid on other virulence factors of the bacteria to prove the quorum quenching activity.

Effect of sub-inhibitory concentrations of cefepime on biofilm formation by Pseudomonas aeruginosa

This study investigated the effect of cefepime at sub-minimum inhibitory concentrations (sub-MICs) on in vitro biofilm formation (BF) by clinical isolates of Pseudomonas aeruginosa. The effect of cefepime at sub-MIC levels (½-1/256 MIC) on in vitro BF by six clinical isolates of P. aeruginosa was phenotypically assessed following 24 and 48 h of challenge using the tissue culture plate (TCP) assay. Quantitative real-time polymerase chain reaction (qRT-PCR) was employed to observe the change in expression of three biofilm-related genes, namely, a protease-encoding gene (lasA), fimbrial protein-encoding gene (cupA1), and alginate-encoding gene (algC), in a weak biofilm-producing strain of P. aeruginosa following 24 and 48 h of challenge with sub-MICs of cefepime. The BF morphology in response to cefepime was imaged using scanning electron microscopy (SEM). The TCP assay showed strain-, time-, and concentration-dependent changes in in vitro BF in P. aeruginosa following challenge with sub-MICs of cefepime, with a profound increase in strains with inherently no or weak biofilm-producing ability. RT-PCR revealed time-dependent upregulation in the expression of the investigated genes following challenge with ½ and ¼ MIC levels, as confirmed by SEM. Cefepime at sub-MICs could upregulate the expression of BF-related genes and enhance BF by P. aeruginosa clinical isolates.

An ACE inhibitor reduces bactericidal activity of human neutrophils in vitro and impairs mouse neutrophil activity in vivo

Angiotensin-converting enzyme inhibitors (ACEIs) are used by millions of patients to treat hypertension, diabetic kidney disease, and heart failure. However, these patients are often at increased risk of infection. To evaluate the impact of ACEIs on immune responses to infection, we compared the effect of an ACEI versus an angiotensin receptor blocker (ARB) on neutrophil antibacterial activity. ACEI exposure reduced the ability of murine neutrophils to kill methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa, and Klebsiella pneumoniae in vitro. In vivo, ACEI-treated mice infected with MRSA had increased bacteremia and tissue bacteria counts compared to mice treated with an ARB or with no drug. Similarly, ACEIs, but not ARBs, increased the incidence of MRSA-induced infective endocarditis in mice with aortic valve injury. Neutrophils from ACE knockout (KO) mice or mice treated with an ACEI produced less leukotriene B4 (LTB4) upon stimulation with MRSA or lipopolysaccharide, whereas neutrophils overexpressing ACE produced more LTB4 compared to wild-type neutrophils. As a result of reduced LTB4 production, ACE KO neutrophils showed decreased survival signaling and increased apoptosis. In contrast, neutrophils overexpressing ACE had an enhanced survival phenotype. Last, in a cohort of human volunteers receiving the ACEI ramipril for 1 week, ACEI administration reduced neutrophil superoxide and reactive oxygen species production and neutrophils isolated from volunteers during ramipril treatment had reduced bactericidal activity. Together, these data demonstrate that ACEI treatment, but not ARB treatment, can reduce the bacterial killing ability of neutrophils.

In vivo evaluation of a recombinant N-acylhomoserine lactonase formulated in a hydrogel using a murine model infected with MDR Pseudomonas aeruginosa clinical isolate, CCASUP2

Failure in the treatment of P. aeruginosa, due to its broad spectrum of resistance, has been associated with increased patient mortality. One alternative approach for infection control is quorum quenching which was found to decrease virulence of such pathogen. In this study, the efficiency of a recombinant Ahl-1 lactonase formulated as a hydrogel was investigated to control the infection of multidrug resistant (MDR) P. aeruginosa infected burn using a murine model. The recombinant N-acylhomoserine lactonase (Ahl-1) was formulated as a hydrogel. To test its ability to control the infection of MDR P. aeruginosa, a thermal injury model was used. Survival rate, and systemic spread of the infection were evaluated. Histopathological examination of the animal dorsal skin was also done for monitoring the healing and cellular changes at the site of infection. Survival rate in the treated group was 100% relative to 40% in the control group. A decrease of up to 3 logs of bacterial count in the blood samples of the treated animals relative to the control group and a decrease of up to 4 logs and 2.3 logs of bacteria in lung and liver samples, respectively were observed. Histopathological examination revealed more enhanced healing process in the treated group. Accordingly, by promoting healing of infected MDR P. aeruginosa burn and by reducing systemic spread of the infection as well as decreasing mortality rate, Ahl-1 hydrogel application is a promising strategy that can be used to combat and control P. aeruginosa burn infections.

Growing emergence of drug-resistant Pseudomonas aeruginosa and attenuation of its virulence using quorum sensing inhibitors: A critical review

A perilous increase in the number of bacterial infections has led to developing throngs of antibiotics for increasing the quality and expectancy of life. Pseudomonas aeruginosa is becoming resistant to all known conventional antimicrobial agents thereby posing a deadly threat to the human population. Nowadays, targeting virulence traits of infectious agents is an alternative approach to antimicrobials that is gaining much popularity to fight antimicrobial resistance. Quorum sensing (QS) involves interspecies communication via a chemical signaling pathway. Under this mechanism, cells work in a concerted manner, communicate with each other with the help of signaling molecules called auto-inducers (AI). The virulence of these strains is driven by genes, whose expression is regulated by AI, which in turn acts as transcriptional activators. Moreover, the problem of antibiotic-resistance in case of infections caused by P. aeruginosa becomes more alarming among immune-compromised patients, where the infectious agents easily take over the cellular machinery of the host while hidden in the QS mediated biofilms. Inhibition of the QS circuit of P. aeruginosa by targeting various signaling pathways such as LasR, RhlR, Pqs, and QScR transcriptional proteins will help in blocking downstream signal transducers which could result in reducing the bacterial virulence. The anti-virulence agent does not pose an immediate selective pressure on growing bacterium and thus reduces the pathogenicity without harming the target species. Here, we review exclusively, the growing emergence of multi-drug resistant (MDR) P. aeruginosa and the critical literature survey of QS inhibitors with their potential application of blocking P. aeruginosa infections.

Identification, antibiotic resistance, and virulence profiling of Aeromonas and Pseudomonas species from wastewater and surface water

Aquatic environments are hotspots for the spread of antibiotic-resistant bacteria and genes due to pollution caused mainly by anthropogenic activities. The aim of this study was to evaluate the impact of wastewater effluents, informal settlements, hospital, and veterinary clinic discharges on the occurrence, antibiotic resistance profile and virulence signatures of Aeromonas spp. and Pseudomonas spp. isolated from surface water and wastewater. High counts of Aeromonas spp. (2.5 (± 0.8) - 3.3 (± 0.4) log₁₀ CFU mL^-1) and Pseudomonas spp. (0.6 (± 1.0) - 1.8 (± 1.0) log₁₀ CFU mL^-1) were obtained. Polymerase chain reaction (PCR) and MALDI-TOF characterization identified four species of Aeromonas and five of Pseudomonas. The isolates displayed resistance to 3 or more antibiotics (71% of Aeromonas and 94% of Pseudomonas). Aeromonas spp. showed significant association with the antibiotic meropenem (χ² = 3.993, P < 0.05). The virulence gene aer in Aeromonas was found to be positively associated with the antibiotic resistance gene blaOXA (χ² = 6.657, P < 0.05) and the antibiotic ceftazidime (χ² = 7.537, P < 0.05). Aeromonas recovered from both wastewater and surface water displayed high resistance to ampicillin and had higher multiple antibiotic resistance (MAR) indices close to the hospital. Pseudomonas isolates on the other hand exhibited low resistance to carbapenems but very high resistance to the third-generation cephalosporins and cefixime. The results showed that some of the Pseudomonas spp. and Aeromonas spp. isolates were extended-spectrum β-lactamase producing bacteria. In conclusion, the strong association between virulence genes and antibiotic resistance in the isolates shows the potential health risk to communities through direct and indirect exposure to the water.

Biological approaches practised using genetically engineered microbes for a sustainable environment: A review

Conventional methods used to remediate toxic substances from the environment have failed drastically, and thereby, advancement in newer remediation techniques can be one of the ways to improve the quality of bioremediation. The increased environmental pollution led to the exploration of microorganisms and construction of genetically engineered microbes (GEMs) for pollution abatement through bioremediation. The present review deals with the successful bioremediation techniques and approaches practised using genetically modified or engineered microbes. In the present scenario, physical and chemical strategies have been practised for the remediation of domestic and industrial wastes but these techniques are expensive and toxic to the environment. Involving engineered microbes can provide a much safer and cost effective strategy in comparison with the other techniques. With the aid of biotechnology and genetic engineering, GEMs are designed by transforming microbes with a more potent protein to overexpress the desired character. GEMs such as bacteria, fungi and algae have been used to degrade oil spills, camphor, hexane, naphthalene, toluene, octane, xylene, halobenzoates, trichloroethylene etc. These engineered microbes are more potent than the natural strains and have higher degradative capacities with quick adaptation for various pollutants as substrates or cometabolize. The road ahead for the implementation of genetic engineering to produce such organisms for the welfare of the environment and finally, public health is indeed long and worthwhile.

The role of high-density lipoprotein cholesterol, apolipoprotein A and paraoxonase-1 in the pathophysiology of neuroprogressive disorders

Lowered high-density lipoprotein (HDL) cholesterol has been reported in major depressive disorder, bipolar disorder, first episode of psychosis, and schizophrenia. HDL, its major apolipoprotein component, ApoA1, and the antioxidant enzyme paraoxonase (PON)1 (which is normally bound to ApoA1) all have anti-atherogenic, antioxidant, anti-inflammatory, and immunomodulatory roles, which are discussed in this paper. The paper details the pathways mediating the anti-inflammatory effects of HDL, ApoA1 and PON1 and describes the mechanisms leading to compromised HDL and PON1 levels and function in an environment of chronic inflammation. The molecular mechanisms by which changes in HDL, ApoA1 and PON1 might contribute to the pathophysiology of the neuroprogressive disorders are explained. Moreover, the anti-inflammatory actions of ApoM-mediated sphingosine 1-phosphate (S1P) signalling are reviewed as well as the deleterious effects of chronic inflammation and oxidative stress on ApoM/S1P signalling. Finally, therapeutic interventions specifically aimed at improving the levels and function of HDL and PON1 while reducing levels of inflammation and oxidative stress are considered. These include the so-called Mediterranean diet, extra virgin olive oil, polyphenols, flavonoids, isoflavones, pomegranate juice, melatonin and the Mediterranean diet combined with the ketogenic diet.

Traditional Chinese Medicine Tanreqing Inhibits Quorum Sensing Systems in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that can infect a wide variety of hosts including humans, plants, and animals. The production of virulence factors is the determinant of the infection paradigm and is under orchestrated regulation via cell-to-cell communication process called quorum sensing (QS). To disable QS circuits and prevent bacterial infections, a large battery of anti-QS agents, particularly from traditional Chinese medicine have been developed. Here, we used P. aeruginosa as a model microorganism to investigate the effect of traditional Chinese medicine Tanreqing (TRQ) formula on bacterial pathogenicity. Phenotypic analysis showed that TRQ treatment could completely inhibit the production of phenazine pyocyanin and moderately inhibit the production of virulence factors such as rhamnolipids, elastase, and alkaline protease. Further transcriptomic analyses revealed that TRQ treatment could significantly attenuate the expression of QS-regulated genes in P. aeruginosa and TRQ-treated P. aeruginosa regulon shared a large overlap with QS regulon. Component contribution to QS inhibition shed light on the indispensable role of all five components in TRQ formula. Further genetic analysis indicated that upstream regulators of QS systems, including two-component systems GacS/GacA and PprA/PprB, were both inhibited by TRQ treatment. Finally, our TRQ formula could efficiently protect Caenorhabditis elegans from killing by P. aeruginosa. Altogether, we have proved TRQ formula as an effective and specific agent to attenuate bacterial virulence and combat bacterial infections.

Inhibition of quorum sensing-controlled virulence factors with natural substances and novel protease, obtained from Halobacillus karajensis

INTRODUCTION

In recent years, a challenge in clinical treatment has developed due to bacterial resistance to antibiotics. One of the new mechanisms against infections is virulence factor inhibition. Many virulence factors are controlled by quorum sensing pathways such as biofilm formation and pyocyanin production. The goal of the present study was to investigate the effect of an obligate halophilic bacterial strain on Pseudomonas aeruginosa and Staphylococcus aureus, due to its halo-tolerant substances and enzymes.

METHODS

The effect of Halobacillus karajensis on bacterial growth and production of virulence factors was studied in this work. The obligate halophile cells and supernatant fractions were extracted by the methanol/chloroform method and characterized by Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Gas Chromatography-Mass Spectrometry (GC-MS), and zymography. The effects of these fractions were studied on biofilm formation in P. aeruginosa and S. aureus as well as on pyocyanin production in P. aeruginosa. The effective protein in the fraction was analyzed by the SDS-PAGE method, and all protein fragments were studied for pyocyanin inhibition.

RESULTS

The crude supernatant extract, MMS fraction, from H. karajensis was effective for the biofilm reduction in S. aureus (74%) and P. aeruginosa (27%). Two proteases in this fraction, which were recognized by zymography on skim milk, were the probable causes for extracellular polymeric substances (EPS) hydrolysis in the biofilm matrix. Also, halide crystals and branched fatty acids, 12methyl-tetradecanoic acid, in the other fractions decreased the biofilm by 18% in S. aureus. The results showed that a new 25 kD protein, which was obtained from MMS fraction, inhibited pyocyanin production by 60% in P. aeruginosa. The zymogram and bioinformatics studies showed that this protein was a serine alkaline metalloprotease and had an interaction with AHL molecules.

CONCLUSION

The inhibitory effects of the non-toxic natural substances and proteases on biofilm formation and pyocyanin production, specifically the 25 kD protease, are novel in this study and make them a good candidate for infected wound healing and inhibiting the virulence factors.

Association of human serum paraoxonase-1 with some respiratory drugs

In this study, we investigated the effects of certain respiratory drugs, which are mainly used on human serum paraoxonase-1 (hPON1; EC 3.1.8.1). hPON1 was purified from human serum, with 354.91 fold and 45% yield by using two simple step procedures including, first, ammonium sulfate precipitation, then, Sepharose-4B-l-tyrosine-1-naphthylamine hydrophobic interaction chromatography. SDS-polyacrylamide gel electrophoresis showed a single protein band belonging to hPON1 with 43 kDa. All the pharmaceutical compounds inhibited the PON1 enzyme highly at the micromolar level. The obtained IC₅₀ values for nine different pharmaceutics ranged from 0.219 μM (salbutamol sulfate) to 67.205 μM (montelukast sodium). So, all drugs could be considered as potent hPON1 inhibitors. K_i values and inhibition types were determined by Lineweaver-Burk graphs. While varenicline tartrate and moxifloxacin hydrochloride inhibited the enzyme in a noncompetitive manner, others inhibited it in a mixed manner.

Host suppression of quorum sensing during catheter-associated urinary tract infections

Chronic bacterial infections on medical devices, including catheter-associated urinary tract infections (CAUTI), are associated with bacterial biofilm communities that are refractory to antibiotic therapy and resistant to host immunity. Previously, we have shown that Pseudomonas aeruginosa can cause CAUTI by forming a device-associated biofilm that is independent of known biofilm exopolysaccharides. Here, we show by RNA-seq that host urine alters the transcriptome of P. aeruginosa by suppressing quorum sensing regulated genes. P. aeruginosa produces acyl homoserine lactones (AHLs) in the presence of urea, but cannot perceive AHLs. Repression of quorum sensing by urine implies that quorum sensing should be dispensable during infection of the urinary tract. Indeed, mutants defective in quorum sensing are able to colonize similarly to wild-type in a murine model of CAUTI. Quorum sensing-regulated processes in clinical isolates are also inhibited by urea. These data show that urea in urine is a natural anti-quorum sensing mechanism in mammals.

Overexpressed recombinant quorum quenching lactonase reduces the virulence, motility and biofilm formation of multidrug-resistant Pseudomonas aeruginosa clinical isolates

The increasing occurrence of resistance among Pseudomonas aeruginosa clinical isolates necessitates finding alternatives to antibiotics for controlling the infection of such pathogenic bacteria. In this study, lactonase gene ahl-1 from Bacillus weihenstephanensis isolate-P65 was successfully cloned and expressed in Escherichia coli BL21 (DE3) under the control of T7 promoter for utilizing its quorum quenching activity against three multidrug-resistant (MDR) P. aeruginosa clinical isolates. The biological activity of the overexpressed lactonase enzyme (Ahl-1), tested using a synthetic signal and Chromobacterium violaceum CV026 as a biosensor, displayed good catalytic activity using hexanoyl homoserine lactone (HHL) as a substrate and Chromobacterium violaceum (CV026) as a biosensor (77.2 and 133 nm min^-1 for the crude and the purified Ahl-lactonase enzymes, respectively). Upon challenging its ability to inhibit the virulence of three MDR P. aeruginosa clinical isolates, recombinant Ahl-1 successfully prevented the accumulation of acylhomoserine lactone signals resulting in a significant reduction in the investigated virulence determinants; protease (from 40 up to 75.5%), pyocyanin (48-75.9%), and rhamnolipids (52.7-63.4%) (P value < 0.05). Ahl-1 also displayed significant inhibitory activities on the swarming motility and biofilm formation of the three tested MDR P. aeruginosa clinical isolates (P value < 0.05). Consequently, Ahl-1 lactonase enzyme in this study is considered a promising therapeutic agent to inhibit P. aeruginosa pathogenicity with no fear of emergence of resistance.

Rhamnolipids from Pseudomonas aeruginosa disperse the biofilms of sulfate-reducing bacteria

Biofilm formation is an important problem for many industries. Desulfovibrio vulgaris is the representative sulfate-reducing bacterium (SRB) which causes metal corrosion in oil wells and drilling equipment, and the corrosion is related to its biofilm formation. Biofilms are extremely difficult to remove since the cells are cemented in a polymer matrix. In an effort to eliminate SRB biofilms, we examined the ability of supernatants from Pseudomonas aeruginosa PA14 to disperse SRB biofilms. We found that the P. aeruginosa supernatants dispersed more than 98% of the biofilm. To determine the biochemical basis of this SRB biofilm dispersal, we examined a series of P. aeruginosa mutants and found that mutants rhlA, rhlB, rhlI, and rhlR, defective in rhamnolipids production, had significantly reduced levels of SRB biofilm dispersal. Corroborating these results, purified rhamnolipids dispersed SRB biofilms, and rhamnolipids were detected in the P. aeruginosa supernatants. Hence, P. aeruginosa supernatants disperse SRB biofilms via rhamnolipids. To determine the genetic basis of how the P. aeruginosa supernatants disperse SRB biofilms, a whole transcriptomic analysis was conducted (RNA-seq); based on this analysis, we identified four proteins (DVUA0018, DVUA0034, DVUA0066, and DVUA0084) of the D. vulgaris megaplasmid that influence biofilm formation, with production of DVUA0066 (a putative phospholipase) reducing biofilm formation 5.6-fold. In addition, the supernatants of P. aeruginosa dispersed the SRB biofilms more readily than protease in M9 glucose minimum medium and were also effective against biofilms of Escherichia coli and Staphylococcus aureus.

A subclass of glycolipids produced by the bacterium Pseudomonas aeruginosa breaks down a corrosive bacterial biofilm that afflicts industries such as petroleum extraction. The P. aeruginosa molecules, known as rhamnolipids, have been previously used to break down other bacterial biofilms. Now, a team from the United States, led by Pennsylvania State University’s Thomas Wood, showed that extracts from P. aeruginosa dispersed 98% of Desulfovibrio vulgaris biofilm, which is problematic for many industries, and also dispersed Escherichia coli and Staphylococcus aureus biofilms. Genetic investigations showed rhamnolipids were responsible for the majority of the biofilm inhibition. Further analyses also revealed four D. vulgaris proteins that significantly affect biofilm formation. This research highlights a mechanism that may be used to reduce the impact of industry-affecting biofilms and deepens the understandings of the mechanisms of biofilm formation.

Pitfalls associated with evaluating enzymatic quorum quenching activity: the case of MomL and its effect on Pseudomonas aeruginosa and Acinetobacter baumannii biofilms

BACKGROUND

The enzymatic degradation of quorums sensing (QS) molecules (called quorum quenching, QQ) has been considered as a promising anti-virulence therapy to treat biofilm-related infections and combat antibiotic resistance. The recently-discovered QQ enzyme MomL has been reported to efficiently degrade different N-acyl homoserine lactones (AHLs) of various Gram-negative pathogens. Here we investigated the effect of MomL on biofilms formed by two important nosocomial pathogens, Pseudomonas aeruginosa and Acinetobacter baumannii.

METHODS

MomL was expressed in E.coli BL21 and purified. The activity of MomL on AHLs with hydroxyl substituent was tested. Biofilms of P. aeruginosa PAO1 and Acinetobacter strains were formed in 96-well microtiter plates. Biofilm formation was evaluated by crystal violet staining, plating and fluorescence microscopy. The effect of MomL on biofilm susceptibility to antibiotics was also tested. We further evaluated MomL in dual-species biofilms formed by P. aeruginosa and A. baumannii, and in biofilms formed in a wound model. The effect of MomL on virulence of A. baumannii was also tested in the Caenorhabditis elegans model.

RESULTS

MomL reduced biofilm formation and increased biofilm susceptibility to different antibiotics in biofilms of P. aeruginosa PAO1 and A. baumannii LMG 10531 formed in microtiter plates in vitro. However, no significant differences were detected in the dual-species biofilm and in wound model biofilms. In addition, MomL did not affect virulence of A. baumannii in the C. elegans model. Finally, the effect of MomL on biofilm of Acinetobacter strains seems to be strain-dependent.

DISCUSSION

Our results indicate that although MomL showed a promising anti-biofilm effect against P. aeruginosa and A. baumanii biofilms formed in microtiter plates, the effect on biofilm formation under conditions more likely to mimic the real-life situation was much less pronounced or even absent. Our data indicate that in order to obtain a better picture of potential applicability of QQ enzymes for the treatment of biofilm-related infections, more elaborate model systems need to be used.