A perspective on the safety of parabens as preservatives in wound care products

Antimicrobial and/or preservative ingredients incorporated in wound care products are subjected to certain safety restrictions. However, several of those agents, and paraben preservatives in particular, have been criticised. Conflicting reports on the potential of parabens to induce allergic contact dermatitis, and their assumed oestrogen‐like activity, raised public health concerns about their overall safety. Here, we seek to provide a balanced perspective on the most significant purported adverse health effects, and thereby allay the many misconceptions regarding the safety of parabens. Extensive and long‐term monitoring of paraben allergy frequencies illustrate that allergic reactions are quite uncommon, especially when compared with other antimicrobial and preservative agents. The estrogenic potential of parabens was illustrated to be far less potent than that of natural oestrogen receptor ligands, and the etiological significance of their presence in human tissue has not been established. The general consensus based on investigations by both the scientific community and regulatory agencies indicates that, with current safety regulations regarding their use in place, this effective and well‐documented group of preservatives should not warrant drastic measures to replace them. As such, despite the ongoing concern, it is indicated that, when used at typical concentrations, parabens are unlikely to affect human health.

Whey Protein Complexes with Green Tea Polyphenols: Antimicrobial, Osteoblast-Stimulatory, and Antioxidant Activities

Polyphenols are known for their antimicrobial activity, whilst both polyphenols and the globular protein β-lactoglobulin (bLG) are suggested to have antioxidant properties and promote cell proliferation. These are potentially useful properties for a tissue-engineered construct, though it is unknown if they are retained when both compounds are used in combination. In this study, a range of different microbes and an osteoblast-like cell line (human fetal osteoblast, hFOB) were used to assess the combined effect of: (1) green tea extract (GTE), rich in the polyphenol epigallocatechin gallate (EGCG), and (2) whey protein isolate (WPI), rich in bLG. It was shown that approximately 20-48% of the EGCG in GTE reacted with WPI. GTE inhibited the growth of Gram-positive bacteria, an effect which was potentiated by the addition of WPI. GTE alone also significantly inhibited the growth of hFOB cells after 1, 4, and 7 days of culture. Alternatively, WPI significantly promoted hFOB cell growth in the absence of GTE and attenuated the effect of GTE at low concentrations (64 µg/mL) after 4 and 7 days. Low concentrations of WPI (50 µg/mL) also promoted the expression of the early osteogenic marker alkaline phosphatase (ALP) by hFOB cells, whereas GTE inhibited ALP activity. Therefore, the antioxidant effects of GTE can be boosted by WPI, but GTE is not suitable to be used as part of a tissue-engineered construct due to its cytotoxic effects which negate any positive effect WPI has on cell proliferation.

Elucidation of the mechanism behind the potentiating activity of baicalin against Burkholderia cenocepacia biofilms

Reduced antimicrobial susceptibility due to resistance and tolerance has become a serious threat to human health. An approach to overcome this reduced susceptibility is the use of antibiotic adjuvants, also known as potentiators. These are compounds that have little or no antibacterial effect on their own but increase the susceptibility of bacterial cells towards antimicrobial agents. Baicalin hydrate, previously described as a quorum sensing inhibitor, is such a potentiator that increases the susceptibility of Burkholderia cenocepacia J2315 biofilms towards tobramycin. The goal of the present study is to elucidate the molecular mechanisms behind the potentiating activity of baicalin hydrate and related flavonoids. We first determined the effect of multiple flavonoids on susceptibility of B. cenocepacia J2315 towards tobramycin. Increased antibiotic susceptibility was most pronounced in combination with apigenin 7-O-glucoside and baicalin hydrate. For baicalin hydrate, also other B. cepacia complex strains and other antibiotics were tested. The potentiating effect was only observed for aminoglycosides and was both strain- and aminoglycoside-dependent. Subsequently, gene expression was compared between baicalin hydrate treated and untreated cells, in the presence and absence of tobramycin. This revealed that baicalin hydrate affected cellular respiration, resulting in increased reactive oxygen species production in the presence of tobramycin. We subsequently showed that baicalin hydrate has an impact on oxidative stress via several pathways including oxidative phosphorylation, glucarate metabolism and by modulating biosynthesis of putrescine. Furthermore, our data strongly suggest that the influence of baicalin hydrate on oxidative stress is unrelated to quorum sensing. Our data indicate that the potentiating effect of baicalin hydrate is due to modulating the oxidative stress response, which in turn leads to increased tobramycin-mediated killing.

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.

Screening a repurposing library for potentiators of antibiotics against Staphylococcus aureus biofilms

Staphylococcus aureus biofilms are involved in a wide range of infections that are extremely difficult to treat with conventional antibiotic therapy. We aimed to identify potentiators of antibiotics against mature biofilms of S. aureus Mu50, a methicillin-resistant and vancomycin-intermediate-resistant strain. Over 700 off-patent drugs from a repurposing library were screened in combination with vancomycin in a microtitre plate (MTP)-based biofilm model system. This led to the identification of 25 hit compounds, including four phenothiazines among which thioridazine was the most potent. Their activity was evaluated in combination with other antibiotics both against planktonic and biofilm-grown S. aureus cells. The most promising combinations were subsequently tested in an in vitro chronic wound biofilm infection model. Although no synergistic activity was observed against planktonic cells, thioridazine potentiated the activity of tobramycin, linezolid and flucloxacillin against S. aureus biofilm cells. However, this effect was only observed in a general biofilm model and not in a chronic wound model of biofilm infection. Several drug compounds were identified that potentiated the activity of vancomycin against biofilms formed in a MTP-based biofilm model. A selected hit compound lost its potentiating activity in a model that mimics specific aspects of wound biofilms. This study provides a platform for discovering and evaluating potentiators against bacterial biofilms and highlights the necessity of using relevant in vitro biofilm model systems.

Novel Potentiators for Vancomycin in the Treatment of Biofilm-Related MRSA Infections via a Mix and Match Approach

A library of 52 hamamelitannin analogues was synthesized and investigated for its ability to potentiate the effect of vancomycin toward Staphylococcus aureus biofilms. Several compounds were found to effectively increase the susceptibility of staphylococcal biofilms toward this glycopeptide. The most active analogue identified in this study showed an EC₅₀ value of 0.26 μM.

Novel hamamelitannin analogues for the treatment of biofilm related MRSA infections-A scaffold hopping approach

Antimicrobial research is increasingly being focused on the problem of resistance and biofilm formation. Hamamelitannin (HAM) was recently identified as an antimicrobial potentiator for conventional antibiotics towards Staphylococcus aureus. This paper describes the synthesis and biological evaluation of novel hamamelitannin analogues with alternative central scaffolds. Via a ligand-based approach, several interesting compounds with improved synthetic accessibility were identified as potentiators for vancomycin in the treatment of MRSA infections.

Design, synthesis and biological evaluation of novel hamamelitannin analogues as potentiators for vancomycin in the treatment of biofilm related Staphylococcus aureus infections

Staphylococcus aureus is a frequent cause of biofilm-related infections. Bacterial cells within a biofilm are protected from attack by the immune system and conventional antibiotics often fail to penetrate the biofilm matrix. The discovery of hamamelitannin as a potentiator for antibiotics, recently led to the design of a more drug-like lead. In the present study, we want to gain further insight into the structure-activity relationship (S.A.R.) of the 5-position of the molecule, by preparing a library of 21 hamamelitannin analogues.

Hamamelitannin Analogues that Modulate Quorum Sensing as Potentiators of Antibiotics against Staphylococcus aureus

The modulation of bacterial communication to potentiate the effect of existing antimicrobial drugs is a promising alternative to the development of novel antibiotics. In the present study, we synthesized 58 analogues of hamamelitannin (HAM), a quorum sensing inhibitor and antimicrobial potentiator. These efforts resulted in the identification of an analogue that increases the susceptibility of Staphylococcus aureus towards antibiotics in vitro, in Caenorhabditis elegans, and in a mouse mammary gland infection model, without showing cytotoxicity.

Dressings Loaded with Cyclodextrin-Hamamelitannin Complexes Increase Staphylococcus aureus Susceptibility Toward Antibiotics Both in Single as well as in Mixed Biofilm Communities

Bacteria reside within biofilms at the infection site, making them extremely difficult to eradicate with conventional wound care products. Bacteria use quorum sensing (QS) systems to regulate biofilm formation, and QS inhibitors (QSIs) have been proposed as promising antibiofilm agents. Despite this, few antimicrobial therapies that interfere with QS exist. Nontoxic hydroxypropyl-β-cyclodextrin-functionalized cellulose gauzes releasing a burst of the antibiotic vancomycin and the QSI hamamelitannin are developed, followed by a sustained release of both. The gauzes affect QS and biofilm formation of Pseudomonas aeruginosa and Staphylococcus aureus in an in vitro model of chronic wound infection and can be considered as candidates to be used to prevent wound infection as well as treat infected wounds.

D-enantiomeric peptides that eradicate wild-type and multidrug-resistant biofilms and protect against lethal Pseudomonas aeruginosa infections

In many infections, bacteria form surface-associated communities known as biofilms that are substantially more resistant to antibiotics than their planktonic counterparts. Based on the design features of active antibiofilm peptides, we made a series of related 12-amino acid L-, D- and retro-inverso derivatives. Specific D-enantiomeric peptides were the most potent at inhibiting biofilm development and eradicating preformed biofilms of seven species of wild-type and multiply antibiotic-resistant Gram-negative pathogens. Moreover, these peptides showed strong synergy with conventional antibiotics, reducing the antibiotic concentrations required for complete biofilm inhibition by up to 64-fold. As shown previously for 1018, these D-amino acid peptides targeted the intracellular stringent response signal (p)ppGpp. The most potent peptides DJK-5 and DJK-6 protected invertebrates from lethal Pseudomonas aeruginosa infections and were considerably more active than a previously described L-amino acid peptide 1018. Thus, the protease-resistant peptides produced here were more effective both in vitro and in vivo.

Phenotypic characterization of an international Pseudomonas aeruginosa reference panel: strains of cystic fibrosis (CF) origin show less in vivo virulence than non-CF strains

Pseudomonas aeruginosa causes chronic lung infections in people with cystic fibrosis (CF) and acute opportunistic infections in people without CF. Forty-two P. aeruginosa strains from a range of clinical and environmental sources were collated into a single reference strain panel to harmonise research on this diverse opportunistic pathogen. To facilitate further harmonized and comparable research on P. aeruginosa, we characterized the panel strains for growth rates, motility, virulence in the Galleria mellonella infection model, pyocyanin and alginate production, mucoid phenotype, LPS pattern, biofilm formation, urease activity, and antimicrobial and phage susceptibilities. Phenotypic diversity across the P. aeruginosa panel was apparent for all phenotypes examined, agreeing with the marked variability seen in this species. However, except for growth rate, the phenotypic diversity among strains from CF versus non-CF sources was comparable. CF strains were less virulent in the G. mellonella model than non-CF strains (P = 0.037). Transmissible CF strains generally lacked O-antigen, produced less pyocyanin and had low virulence in G. mellonella. Furthermore, in the three sets of sequential CF strains, virulence, O-antigen expression and pyocyanin production were higher in the earlier isolate compared to the isolate obtained later in infection. Overall, this full phenotypic characterization of the defined panel of P. aeruginosa strains increases our understanding of the virulence and pathogenesis of P. aeruginosa and may provide a valuable resource for the testing of novel therapies against this problematic pathogen.

Quorum sensing inhibitors as anti-biofilm agents

Biofilms are microbial sessile communities characterized by cells that are attached to a substratum or interface or to each other, are embedded in a self-produced matrix of extracellular polymeric substances and exhibit an altered phenotype compared to planktonic cells. Biofilms are estimated to be associated with 80% of microbial infections and it is currently common knowledge that growth of micro-organisms in biofilms can enhance their resistance to antimicrobial agents. As a consequence antimicrobial therapy often fails to eradicate biofilms from the site of infection. For this reason, innovative anti-biofilm agents with novel targets and modes of action are needed. One alternative approach is targeting the bacterial communication system (quorum sensing, QS). QS is a process by which bacteria produce and detect signal molecules and thereby coordinate their behavior in a cell-density dependent manner. Three main QS systems can be distinguished: the acylhomoserine lactone (AHL) QS system in Gram-negative bacteria, the autoinducing peptide (AIP) QS system in Gram-positive bacteria and the autoinducer-2 (AI-2) QS system in both Gram-negative and -positive bacteria. Although much remains to be learned about the involvement of QS in biofilm formation, maintenance, and dispersal, QS inhibitors (QSI) have been proposed as promising antibiofilm agents. In this article we will give an overview of QS inhibitors which have been shown to play a role in biofilm formation and/or maturation.

Composites of gellan gum hydrogel enzymatically mineralized with calcium-zinc phosphate for bone regeneration with antibacterial activity

Gellan gum hydrogels functionalized with alkaline phosphatase were enzymatically mineralized with phosphates in mineralization medium containing calcium (Ca) and zinc (Zn) to improve their suitability as biomaterials for bone regeneration. The aims of the study were to endow mineralized hydrogels with antibacterial activity by incorporation of Zn in the inorganic phase, and to investigate the effect of Zn incorporation on the amount and type of mineral formed, the compressive modulus of the mineralized hydrogels and on their ability to support adhesion and growth of MC3T3-E1 osteoblast-like cells. Mineralization medium contained glycerophosphate (0.05 m) and three different molar Ca:Zn ratios, 0.05:0, 0.04:0.01 and 0.025:0.025 (all mol/dm³ ), hereafter referred to as A, B and C, respectively. FTIR, SAED and TEM analysis revealed that incubation for 14 days caused the formation of predominantly amorphous mineral phases in sample groups A, B and C. The presence of Zn in sample groups B and C was associated with a drop in the amount of mineral formed and a smaller mineral deposit morphology, as observed by SEM. ICP-OES revealed that Zn was preferentially incorporated into mineral compared to Ca. Mechanical testing revealed a decrease in compressive modulus in sample group C. Sample groups B and C, but not A, showed antibacterial activity against biofilm-forming, methicillin-resistant Staphylococcus aureus. All sample groups supported cell growth. Zn incorporation increased the viable cell number. The highest values were seen on sample group C. In conclusion, the sample group containing the most Zn, i.e. group C, appears to be the most promising. Copyright © 2015 John Wiley & Sons, Ltd.

Injectable self-gelling composites for bone tissue engineering based on gellan gum hydrogel enriched with different bioglasses

Hydrogels of biocompatible calcium-crosslinkable polysaccharide gellan gum (GG) were enriched with bioglass particles to enhance (i) mineralization with calcium phosphate (CaP); (ii) antibacterial properties and (iii) growth of bone-forming cells for future bone regeneration applications. Three bioglasses were compared, namely one calcium-rich and one calcium-poor preparation both produced by a sol-gel technique (hereafter referred to as A2 and S2, respectively) and one preparation of composition close to that of the commonly used 45S5 type (hereafter referred to as NBG). Incubation in SBF for 7 d, 14 d and 21 d caused apatite formation in bioglass-containing but not in bioglass-free samples, as confirmed by FTIR, XRD, SEM, ICP-OES, and measurements of dry mass, i.e. mass attributable to polymer and mineral and not water. Mechanical testing revealed an increase in compressive modulus in samples containing S2 and NBG but not A2. Antibacterial testing using biofilm-forming meticillin-resistant staphylococcus aureus (MRSA) showed markedly higher antibacterial activity of samples containing A2 and S2 than samples containing NBG and bioglass-free samples. Cell biological characterization using rat mesenchymal stem cells (rMSCs) revealed a stimulatory effect of NBG on rMSC differentiation. The addition of bioglass thus promotes GG mineralizability and, depending on bioglass type, antibacterial properties and rMSC differentiation.

Thiazolidinedione derivatives as novel agents against Propionibacterium acnes biofilms

AIMS

The aim of the present study was to determine the effect of two thiazolidinedione derivatives on Propionibacterium acnes biofilm formation in vitro and to assess their effect on the susceptibility of P. acnes biofilms towards antimicrobials.

METHODS AND RESULTS

The compounds were shown to have a moderate to strong antibiofilm activity when used in subinhibitory concentrations. These compounds do not affect P. acnes attachment but lead to increased dispersal of biofilm cells. This dispersal results in an increased killing of the P. acnes biofilm cells by conventional antimicrobials.

CONCLUSION

The antibiofilm effect and the effect on biofilm susceptibility of the thiazolidinedione-derived quorum sensing inhibitors were clearly demonstrated.

SIGNIFICANCE AND IMPACT OF THE STUDY

Propionibacterium acnes infections are difficult to treat due to the presence of biofilms at the infection site and the associated resistance towards conventional antimicrobials. Our results indicate that these thiazolidinedione derivatives can be promising leads used for the treatment of P. acnes infections and as anti-acne drugs.

Biofilm inhibitory and eradicating activity of wound care products against Staphylococcus aureus and Staphylococcus epidermidis biofilms in an in vitro chronic wound model

AIMS

Although several factors contribute to wound healing, bacterial infections and the presence of biofilm can significantly affect healing. Despite that this clearly indicates that therapies should address biofilm in wounds, only few wound care products have been evaluated for their antibiofilm effect. For this reason, we developed a rapid quantification approach to investigate the efficacy of wound care products on wounds infected with Staphylococcus spp.

METHODS AND RESULTS

An in vitro chronic wound infection model was used in which a fluorescent Staph. aureus strain was used to allow the rapid quantification of the bacterial burden after treatment. A good correlation was observed between the fluorescence signal and the bacterial counts. When evaluated in this model, several commonly used wound dressings and wound care products inhibited biofilm formation resulting in a decrease between one and seven log CFU per biofilm compared with biofilm formed in the absence of products. In contrast, most dressings only moderately affected mature biofilms.

CONCLUSION

Our model allowed the rapid quantification of the bacterial burden after treatment. However, the efficacy of treatment varied between the different types of dressings and/or wound care products.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our model can be used to compare the efficacy of wound care products to inhibit biofilm formation and/or eradicate mature biofilms. In addition, the results indicate that treatment of infected wounds should be started as soon as possible and that novel products with more potent antibiofilm activity are needed.

Phenotypic and genotypic characterisation of Burkholderia cenocepacia J2315 mutants affected in homoserine lactone and diffusible signal factor-based quorum sensing systems suggests interplay between both types of systems

Many putative virulence factors of Burkholderia cenocepacia are controlled by various quorum sensing (QS) circuits. These QS systems either use N-acyl homoserine lactones (AHL) or cis-2-dodecenoic acid ("Burkholderia diffusible signal factor", BDSF) as signalling molecules. Previous work suggested that there is little cross-talk between both types of systems. We constructed mutants in B. cenocepacia strain J2315, in which genes encoding CepI (BCAM1870), CciI (BCAM0239a) and the BDSF synthase (BCAM0581) were inactivated, and also constructed double (ΔcepIΔBCAM0581, ΔcciIΔBCAM0581 and ΔcepIΔcciI) mutants and a triple (ΔcepIΔcciIΔBCAM0581) mutant. Subsequently we investigated phenotypic properties (antibiotic susceptibility, biofilm formation, production of AHL and BDSF, protease activity and virulence in Caenorhabditis elegans) and measured gene expression in these mutants, and this in the presence and absence of added BDSF, AHL or both. The triple mutant was significantly more affected in biofilm formation, antimicrobial susceptibility, virulence in C. elegans, and protease production than either the single or double mutants. The ΔBCAM0581 mutant and the ΔcepIΔBCAM0581 and ΔcciIΔBCAM0581 double mutants produced significantly less AHL compared to the WT strain and the ΔcepI and ΔcciI single mutant, respectively. The expression of cepI and cciI in ΔBCAM0581, was approximately 3-fold and 7-fold (p<0.05) lower than in the WT, respectively. The observed differences in AHL production, expression of cepI and cciI and QS-controlled phenotypes in the ΔBCAM0581 mutant could (at least partially) be restored by addition of BDSF. Our data suggest that, in B. cenocepacia J2315, AHL and BDSF-based QS systems co-regulate the same set of genes, regulate different sets of genes that are involved in the same phenotypes and/or that the BDSF system controls the AHL-based QS system. As the expression of the gene encoding the C6-HSL synthase CciI (and to a lesser extent the C8-HSL synthase CepI) is partially controlled by BDSF, it seems likely that the BDSF QS systems controls AHL production through this system.

Antiseptic cyclodextrin-functionalized hydrogels and gauzes for loading and delivery of benzalkonium chloride

Prevention and management of wound infections receive a lot of attention, since the presence of micro-organisms interferes with the wound-healing process. The aim of this work was to use cyclodextrins (CDs) to endow hydrogels and gauzes with the ability to take up antiseptics and sustain their delivery for several hours. Benzalkonium chloride (BzCl) can form inclusion complexes with cross-linked CDs that regulate the release through an affinity-driven mechanism. Grafting of CDs to cotton gauzes using citric acid as the linker, at 190 °C and for 15 min, led to grafting yields of about 148%, much larger than those obtained at 180 °C or with shorter reaction times. Microbiological tests revealed that the BzCl-loaded networks can inhibit the growth of Staphylococcus epidermidis and Escherichia coli both on agar plates and in liquid medium. Furthermore, the antiseptic-loaded gauzes were able to inhibit biofilm formation by Staphylococcus aureus RN1HG pMV158GFP when applied in early stages of biofilm formation and could reduce the number of living cells in preformed biofilms grown in a chronic wound biofilm model. These findings highlight the role of CDs as main components of hydrogels and gauzes for the efficient delivery of antiseptics.

Synthesis and evaluation of thiazolidinedione and dioxazaborocane analogues as inhibitors of AI-2 quorum sensing in Vibrio harveyi

Two focused libraries based on two types of compounds, that is, thiazolidinediones and dioxazaborocanes were designed. Structural resemblances can be found between thiazolidinediones and well-known furanone type quorum sensing (QS) inhibitors such as N-acylaminofuranones, and/or acyl-homoserine lactone signaling molecules, while dioxazaborocanes structurally resemble previously reported oxazaborolidine derivatives which antagonized autoinducer 2 (AI-2) binding to its receptor. Because of this, we hypothesized that these compounds could affect AI-2 QS in Vibrio harveyi. Although all compounds blocked QS, the thiazolidinediones were the most active AI-2 QS inhibitors, with EC(50) values in the low micromolar range. Their mechanism of inhibition was elucidated by measuring the effect on bioluminescence in a series of V. harveyi QS mutants and by DNA-binding assays with purified LuxR protein. The active compounds neither affected bioluminescence as such nor the production of AI-2. Instead, our results indicate that the thiazolidinediones blocked AI-2 QS in V. harveyi by decreasing the DNA-binding ability of LuxR. In addition, several dioxazaborocanes were found to block AI-2 QS by targeting LuxPQ.

A quorum sensing-disrupting brominated thiophenone with a promising therapeutic potential to treat luminescent vibriosis

Vibrio harveyi is amongst the most important bacterial pathogens in aquaculture. Novel methods to control this pathogen are needed since many strains have acquired resistance to antibiotics. We previously showed that quorum sensing-disrupting furanones are able to protect brine shrimp larvae against vibriosis. However, a major problem of these compounds is that they are toxic toward higher organisms and therefore, they are not safe to be used in aquaculture. The synthesis of brominated thiophenones, sulphur analogues of the quorum sensing-disrupting furanones, has recently been reported. In the present study, we report that these compounds block quorum sensing in V. harveyi at concentrations in the low micromolar range. Bioluminescence experiments with V. harveyi quorum sensing mutants and a fluorescence anisotropy assay indicated that the compounds disrupt quorum sensing in this bacterium by decreasing the ability of the quorum sensing master regulator LuxR to bind to its target promoter DNA. In vivo challenge tests with gnotobiotic brine shrimp larvae showed that thiophenone compound TF310, (Z)-4-((5-(bromomethylene)-2-oxo-2,5-dihydrothiophen-3-yl)methoxy)-4-oxobutanoic acid, completely protected the larvae from V. harveyi BB120 when dosed to the culture water at 2.5 µM or more, whereas severe toxicity was only observed at 250 µM. This makes TF310 showing the highest therapeutic index of all quorum sensing-disrupting compounds tested thus far in our brine shrimp model system.

Synthesis and evaluation of the quorum sensing inhibitory effect of substituted triazolyldihydrofuranones

Acylhomoserine lactone (AHL) analogues in which the amide function is replaced by a triazole group were synthesized and evaluated for their effect on quorum sensing (QS) and biofilm formation in Burkholderia cenocepacia and Pseudomonas aeruginosa. In addition, the influence of the length of the acyl-mimicking chain was investigated. The compounds showed selectivity between two different AHL QS systems. 3-(1H-1,2,3-Triazol-1-yl)dihydrofuran-2(3H)-ones, in which the 4-substituent best resembled the acyl chain of the native AHL molecule exhibited significant QS agonistic and antagonistic activities. Replacing this aliphatic substituent by a phenyl-containing moiety resulted in active inhibitors of QS. The most active compounds showed biofilm inhibitory as well as biofilm eradicating activities in both test organisms.

Eradication of Propionibacterium acnes biofilms by plant extracts and putative identification of icariin, resveratrol and salidroside as active compounds

Propionibacterium acnes is a Gram-positive bacterium that plays an important role in the pathogenesis of acne vulgaris. This organism is capable of biofilm formation and the decreased antimicrobial susceptibility of biofilm-associated cells may hamper efficient treatment. In addition, the prolonged use of systemic antibiotic therapy is likely to lead to the development and spread of antimicrobial resistance. In the present study we investigated whether P. acnes biofilms could be eradicated by plant extracts or their active compounds, and whether other mechanisms besides killing of biofilm cells could be involved. Out of 119 plant extracts investigated, we identified five with potent antibiofilm activity against P. acnes (extracts from Epimedium brevicornum, Malus pumila, Polygonum cuspidatum, Rhodiola crenulata and Dolichos lablab). We subsequently identified icariin, resveratrol and salidroside as active compounds in three of these extracts. Extracts from E. brevicornum and P. cuspidatum, as well as their active compounds (icariin and resveratrol, respectively) showed marked antibiofilm activity when used in subinhibitory concentrations, indicating that killing of microbial cells is not their only mode of action.

Biofilm inhibition and drug-eluting properties of novel DMAEMA-modified polyethylene and silicone rubber surfaces

Poly(2-(dimethylaminoethyl) methacrylate) (pDMAEMA) was grafted to low density polyethylene (LDPE) and silicone rubber (SR) in order to make them less susceptible to microbial biofilm formation. The direct grafting of DMAEMA using γ-rays was an efficient and fast procedure for obtaining modified materials, which could be quaternized in a second step using methyl iodide. Raman spectroscopy showed that the grafting occurred only at the surface of the LDPE, but both at the surface and in the bulk of the SR. Consequently, the grafted chains caused changes in the surface-related features of the LDPE (water contact angle and viscoelastic behavior in the dry state) and in the bulk-related properties of the SR (swelling and viscoelasticity in the swollen state). The microbiological assays revealed that the grafted DMAEMA reduced Candida albicans biofilm formation (almost no biofilm on SR), while the quaternized surfaces inhibited C. albicans and Staphylococcus aureus biofilm by more than 99% compared to pristine materials. Modified LDPE and SR were capable of holding considerable amounts of nalidixic acid, an anionic antimicrobial drug, and sustained the release for several hours. In addition, the grafted materials were cytocompatible (fibroblast cell survival > 70%). In conclusion, these materials have the ability to inhibit microbial biofilm formation and at the same time act as drug-eluting systems, and for that reason may hold great promise for anti-biofouling applications.

AI-2 quorum-sensing inhibitors affect the starvation response and reduce virulence in several Vibrio species, most likely by interfering with LuxPQ

The increase of disease outbreaks caused by Vibrio species in aquatic organisms as well as in humans, together with the emergence of antibiotic resistance in Vibrio species, has led to a growing interest in alternative disease control measures. Quorum sensing (QS) is a mechanism for regulating microbial gene expression in a cell density-dependent way. While there is good evidence for the involvement of auto-inducer 2 (AI-2)-based interspecies QS in the control of virulence in multiple Vibrio species, only few inhibitors of this system are known. From the screening of a small panel of nucleoside analogues for their ability to disturb AI-2-based QS, an adenosine derivative with a p-methoxyphenylpropionamide moiety at C-3′ emerged as a promising hit. Its mechanism of inhibition was elucidated by measuring the effect on bioluminescence in a series of Vibrio harveyi AI-2 QS mutants. Our results indicate that this compound, as well as a truncated analogue lacking the adenine base, block AI-2-based QS without interfering with bacterial growth. The active compounds affected neither the bioluminescence system as such nor the production of AI-2, but most likely interfered with the signal transduction pathway at the level of LuxPQ in V. harveyi. The most active nucleoside analogue (designated LMC-21) was found to reduce the Vibrio species starvation response, to affect biofilm formation in Vibrio anguillarum, Vibrio vulnificus and Vibrio cholerae, to reduce pigment and protease production in V. anguillarum, and to protect gnotobiotic Artemia from V. harveyi-induced mortality.

Cinnamaldehyde and cinnamaldehyde derivatives reduce virulence in Vibrio spp. by decreasing the DNA-binding activity of the quorum sensing response regulator LuxR

BACKGROUND

To date, only few compounds targeting the AI-2 based quorum sensing (QS) system are known. In the present study, we screened cinnamaldehyde and substituted cinnamaldehydes for their ability to interfere with AI-2 based QS. The mechanism of QS inhibition was elucidated by measuring the effect on bioluminescence in several Vibrio harveyi mutants. We also studied in vitro the ability of these compounds to interfere with biofilm formation, stress response and virulence of Vibrio spp. The compounds were also evaluated in an in vivo assay measuring the reduction of Vibrio harveyi virulence towards Artemia shrimp.

RESULTS

Our results indicate that cinnamaldehyde and several substituted derivatives interfere with AI-2 based QS without inhibiting bacterial growth. The active compounds neither interfered with the bioluminescence system as such, nor with the production of AI-2. Study of the effect in various mutants suggested that the target protein is LuxR. Mobility shift assays revealed a decreased DNA-binding ability of LuxR. The compounds were further shown to (i) inhibit biofilm formation in several Vibrio spp., (ii) result in a reduced ability to survive starvation and antibiotic treatment, (iii) reduce pigment and protease production in Vibrio anguillarum and (iv) protect gnotobiotic Artemia shrimp against virulent Vibrio harveyi BB120.

CONCLUSION

Cinnamaldehyde and cinnamaldehyde derivatives interfere with AI-2 based QS in various Vibrio spp. by decreasing the DNA-binding ability of LuxR. The use of these compounds resulted in several marked phenotypic changes, including reduced virulence and increased susceptibility to stress. Since inhibitors of AI-2 based quorum sensing are rare, and considering the role of AI-2 in several processes these compounds may be useful leads towards antipathogenic drugs.