Carvacrol as potential quorum sensing inhibitor of Pseudomonas aeruginosa and biofilm production on stainless steel surfaces

Rheological Behavior, Antimicrobial and Quorum Sensig Inhibition Study of an Argentinean Oregano Essential Oil Nanoemulsion

In this study, Argentinean oregano essential oil (OEO) nanoemulsions (NEs) were developed. Four NEs were prepared: a control (CNE), EONE1 (10.6 mg EO/g NE), EONE2 (106 mg EO/ g NE), and EONE3 (160 mg EO/g NE) and tested for antimicrobial activity against Staphylococcus aureus ATCC 13565, Listeria monocytogenes Scott A, Pseudomonas aeruginosa ATCC 14213, and Escherichia coli O157:H7 using a broth microdilution assay and quorum sensing inhibition in a model using Chromobacterium violaceum ATCC 12472, where the production of violacein was quantified. The chemical composition of the EO was determined by gas chromatography-mass spectrometry. The average particle size (nm) and polydispersity index were monitored over 14 days at two different storage temperatures (4 and 23°C). A rheological behavior study was carried out using a dynamic shear rheometer, and flow curves, as well as viscoelastic properties, were determined. E. coli and L. monocytogenes were the most sensitive microorganisms to EONE (MIC of 2 and 5 mg/ml for EOEN3). Sub-MICs for NE were found at lower concentrations than those for pure EO. A significant reduction in violet pigment intensity and colorless coloration (p < 0.05) were observed at different NE concentrations concerning the control sample. The flow behavior index (n) decreased, and the consistency index (k) increased when the EO concentration was increased. CNE, EONE1, and EONE2 showed liquid-like behavior (G' < G″) in the low-frequency region, whereas a solid-like behavior (G' > G″) was observed in the high-frequency region, presenting a viscoelastic behavior, appearing as a wormlike micellar solution. For EONE3, a strong increase in both moduli was observed with increasing OEO concentration. The G' was about one order of magnitude higher than the G″ over the whole frequency range, indicating the presence of a gel-like structure. The incorporation of EOs into an NE increased their stability, lowering the particle size, leading to a wormlike micelle with higher viscosity. Moreover, this NE had good antimicrobial activity and novel quorum-sensing inhibition activity. The results of this study indicated that Argentinean OEO NE could be used in a food system as a natural and stable antimicrobial agent.

Combined Application of Bacteriophages and Carvacrol in the Control of Pseudomonas syringae pv. actinidiae Planktonic and Biofilm Forms

Pseudomonas syringae pv. actinidiae (Psa) is the causative agent of the bacterial canker of kiwifruit (Actinidia spp.). Phage therapy has been suggested as a viable alternative approach to controlling this disease, but its efficacy is limited by the emergence of phage-resistant mutants. Carvacrol is an essential oil that may be useful for the control of Psa. Combination therapies can be used to overcome resistance development. Here, the combination of phages (single phage suspensions of phages PN05 and PN09, and a cocktail of both phages) and carvacrol was investigated in controlling Psa planktonic and biofilm forms in vitro. The phage therapy alone (with phages PN05 and PN09), and the carvacrol alone (minimum inhibitory concentration 2.0 mg/mL), inhibited Psa growth, but the combined effect of both therapies was more effective. The phages alone effectively inhibited Psa growth for 24 h, but Psa regrowth was observed after this time. The carvacrol (2.0 mg/mL) alone prevented the biofilm formation for 48 h, but did not destroy the pre-formed biofilms. The combined treatment, phages and carvacrol (2.0 mg/mL), showed a higher efficacy, preventing Psa regrowth for more than 40 h. In conclusion, the combined treatment with phages and carvacrol may be a promising, environment-friendly and cost-effective approach to controlling Psa in the kiwifruit industry.

Antivirulence properties and related mechanisms of spice essential oils: A comprehensive review

In recent decades, reduced antimicrobial effectiveness, increased bacterial infection, and newly emerged microbial resistance have become global public issues, leading to an urgent need to find effective strategies to counteract these problems. Strategies targeting bacterial virulence factors rather than bacterial survival have attracted increasing interest, since the modulation of virulence factors may prevent the development of drug resistance in bacteria. Spices are promising natural sources of antivirulence compounds owing to their wide availability, diverse antivirulence phytochemical constituents, and generally favorable safety profiles. Essential oils are the predominant and most important antivirulence components of spices. This review addresses the recent efforts of using spice essential oils to inhibit main bacterial virulence traits, including the quorum sensing system, biofilm formation, motility, and toxin production, with an intensive discussion of related mechanisms. We hope that this review can provide a better understanding of the antivirulence properties of spice essential oils, which have the potential to be used as antibiotic alternatives by targeting bacterial virulence.

Inhibition of Glucosyltransferase Activity and Glucan Production as an Antibiofilm Mechanism of Lemongrass Essential Oil against Escherichia coli O157:H7

The resistance of Escherichia coli O157:H7 to disinfection is associated with its ability to form biofilms, mainly constituted by glucans produced by glucosyltransferases. Citral and geraniol, terpenes found in the essential oil of Cymbopogon citratus (EO), have proven antibacterial activity against planktonic E. coli; however, no information was found about their efficacy and mode of action against E. coli biofilms. Therefore, the inhibitory effect of C. citratus EO, citral, and geraniol on glucans production and glucosyltransferase activity as anti-biofilm mechanism against E. coli was evaluated. EO, citral, and geraniol inhibited the planktonic growth of E. coli (minimal inhibitory concentration or MIC= 2.2, 1.0, and 3.0 mg/mL, respectively) and the bacterial adhesion (2.0, 2.0, and 4.0 mg/mL, respectively) on stainless steel. All compounds decreased the glucans production; citral and geraniol acted as uncompetitive inhibitors of glucosyltransferase activity (The half maximal inhibitory concentrations or IC₅₀ were 8.5 and 6.5 µM, respectively). The evidence collected by docking analysis indicated that both terpenes could interact with the helix finger of the glucosyltransferase responsible for the polymer production. In conclusion, C. citratus EO, citral, and geraniol inhibited glucosyltransferase activity, glucans production, and the consequent biofilm formation of E. coli O157:H7.

Plant-Derived Inhibitors of AHL-Mediated Quorum Sensing in Bacteria: Modes of Action

Numerous gram-negative phytopathogenic and zoopathogenic bacteria utilise acylated homoserine lactone (AHL) in communication systems, referred to as quorum sensing (QS), for induction of virulence factors and biofilm development. This phenomenon positions AHL-mediated QS as an attractive target for anti-infective therapy. This review focused on the most significant groups of plant-derived QS inhibitors and well-studied individual compounds for which in silico, in vitro and in vivo studies provide substantial knowledge about their modes of anti-QS activity. The current data about sulfur-containing compounds, monoterpenes and monoterpenoids, phenylpropanoids, benzoic acid derivatives, diarylheptanoids, coumarins, flavonoids and tannins were summarized; their plant sources, anti-QS effects and bioactivity mechanisms have also been summarized and discussed. Three variants of plant-derived molecules anti-QS strategies are proposed: (i) specific, via binding with LuxI-type AHL synthases and/or LuxR-type AHL receptor proteins, which have been shown for terpenes (carvacrol and l-carvone), phenylpropanoids (cinnamaldehyde and eugenol), flavonoid quercetin and ellagitannins; (ii) non-specific, by affecting the QS-related intracellular regulatory pathways by lowering regulatory small RNA expression (sulphur-containing compounds ajoene and iberin) or c-di-GMP metabolism reduction (coumarin); and (iii) indirect, via alteration of metabolic pathways involved in QS-dependent processes (vanillic acid and curcumin).

The antibacterial properties of phenolic isomers, carvacrol and thymol

Most of the antibacterial activities of essential oils from the Lamiaceae herbaceous plant family thyme and oregano are attributed to their bioactive isomeric monoterpenoid constituents, carvacrol and thymol. Commercially available antibiotics of thymol or carvacrol have not yet been developed but health products have incorporated thymol into their formulations for their antimicrobial properties. Carvacrol and thymol are generally considered safe for consumption and they have been used in dental applications, approved as food flavorings and have been considered as antibacterial additives in food and feed. Many studies have demonstrated that carvacrol and thymol are potent antibacterial agents against both Gram-positive and Gram-negative bacteria. The most frequently reported mechanism of antibacterial action of both isomers involves the disruption of bacterial membrane leading to bacterial lysis and leakage of intracellular contents resulting in death. Other proposed mechanisms of antibacterial action include the inhibition of efflux pumps, prevention in the formation and disruption of preformed biofilms, inhibition of bacterial motility, and inhibition of membrane ATPases. In addition, both isomers have been found to act additively or synergistically with conventional antibiotics important in overcoming the problem of bacteria resistance in food and disease.

AHLs Regulate Biofilm Formation and Swimming Motility of Hafnia alvei H4

The aim of this study was to evaluate the role of N-acyl homoserine lactones (AHLs) in the regulation of swimming motility of Hafnia alvei H4 and its biofilm formation on 96-well plate, glass and stainless-steel surfaces. The luxI gene, which codes for an enzyme involved in AHL synthesis, was deleted to generate a luxI mutant (ΔluxI). The mutant produced no AHL, and the relative expression of the luxR gene was significantly (P < 0.05) decreased. In addition, qRT-PCR analysis showed that the relative expression of the luxR gene in ΔluxI was stimulated by the presence of exogenous AHLs (C4-HSL, C6-HSL, and 3-o-C8-HSL) added at concentrations ranging from of 50–250 μg/ml. Among the three AHLs, C6-HSL had the strongest effect. The ability of ΔluxI to form biofilm on 96-well plate, glass and stainless-steel surfaces was significantly reduced (P < 0.05) compared with the wild type (WT), but was increased when provided with 150 μg/ml C4-HSL, whereas C6-HSL and 3-o-C8-HSL had no effect. Scanning electron microscopy analysis of the biofilm revealed less bacteria adhering to the surface of stainless-steel and fewer filaments were found binding to the cells compared with the WT. Furthermore, ΔluxI also exhibited significant (P < 0.05) decrease in the expression of biofilm- and swimming motility-related genes, flgA, motA and cheA, consistent with the results observed for biofilm formation and swimming motility. Taken together, the results suggested that in H. alvei H4, C4-HSL may act as an important molecular signal through regulating the ability of the cells to form biofilm, as well as through regulating the swimming motility of the cell, and this could provide a new way to control these phenotypes of H. alvei in food processing.

Virulence of Pseudomonas aeruginosa exposed to carvacrol: alterations of the Quorum sensing at enzymatic and gene levels

The main goal of this study was to evaluate the inhibition of Pseudomonas aeruginosa virulence factors and Quorum Sensing during exposure to carvacrol. P. aeruginosa (ATCC 10154) was exposed to carvacrol determining changes in biofilm development, motility, acyl-homoserine lactones (AHL) synthesis and relative expression of lasI/lasR. Docking analysis was used to determinate interactions between carvacrol with LasI and LasR proteins. P. aeruginosa produced 60% lower AHLs when exposed to carvacrol (1.9 mM) compared to control, without affecting cellular viability, indicating a reduction on the LasI synthase activity. AHL-C12, C6, and C4 were detected and related to biofilm development, motility, and pyocyanin production, respectively. The presence of carvacrol reduced the expression of lasR, without affecting lasI gen. Moreover, computational docking showed interactions of carvacrol with amino acids in the active site pocket of LasI (-5.6 kcal mol^-1) and within the binding pocket of LasR (-6.7 kcal mol^-1) of P. aeruginosa. These results demonstrated that virulence of P. aeruginosa was reduced by carvacrol, by inhibiting LasI activity with the concomitant reduction on the expression of lasR, biofilm and swarming motility. This study provides relevant information about the effect of carvacrol against quorum sensing to inhibit virulence factors of P. aeruginosa at enzymatic and gene levels. These findings can contribute to the development of natural anti-QS products, which can affect pathogenesis.

Synthesis, In Silico, and In Vitro Evaluation of Long Chain Alkyl Amides from 2-Amino-4-Quinolone Derivatives as Biofilm Inhibitors

Infection from multidrug resistant bacteria has become a growing health concern worldwide, increasing the need for developing new antibacterial agents. Among the strategies that have been studied, biofilm inhibitors have acquired relevance as a potential source of drugs that could act as a complement for current and new antibacterial therapies. Based on the structure of 2-alkyl-3-hydroxy-4-quinolone and N-acylhomoserine lactone, molecules that act as mediators of quorum sensing and biofilm formation in Pseudomonas aeruginosa, we designed, prepared, and evaluated the biofilm inhibition properties of long chain amide derivatives of 2-amino-4-quinolone in Staphylococcus aureus and P. aeruginosa. All compounds had higher biofilm inhibition activity in P. aeruginosa than in S. aureus. Particularly, compounds with an alkyl chain of 12 carbons exhibited the highest inhibition of biofilm formation. Docking scores and molecular dynamics simulations of the complexes of the tested compounds within the active sites of proteins related to quorum sensing had good correlation with the experimental results, suggesting the diminution of biofilm formation induced by these compounds could be related to the inhibition of these proteins.

Potassium 2-methoxy-4-vinylphenolate: a novel hit exhibiting quorum-sensing inhibition inPseudomonas aeruginosa viaLasIR/RhlIR circuitry

Effect of potassium 2-methoxy-4-vinylphenolate on quorum sensing inPseudomonas aeruginosa.

Comparison of Single and Combined Use of Catechin, Protocatechuic, and Vanillic Acids as Antioxidant and Antibacterial Agents against Uropathogenic Escherichia Coli at Planktonic and Biofilm Levels

The objective of this study was to evaluate the effect of combining catechin, protocatechuic, and vanillic acids against planktonic growing, adhesion, and biofilm eradication of uropathogenic Escherichia coli (UPEC), as well as antioxidant agents. The minimum inhibitory concentrations (MIC) of protocatechuic, vanillic acids and catechin against the growth of planktonic bacteria were 12.98, 11.80, and 13.78 mM, respectively. Mixing 1.62 mM protocatechuic acid + 0.74 mM vanillic acid + 0.05 mM catechin resulted in a synergistic effect acting as an MIC. Similarly, the minimum concentrations of phenolic compounds to prevent UPEC adhesion and biofilm formation (MBIC) were 11.03 and 7.13 mM of protocatechuic and vanillic acids, respectively, whereas no MBIC of catechin was found. However, combinations of 1.62 mM protocatechuic acid + 0.74 mM vanillic acid + 0.05 mM catechin showed a synergistic effect acting as MBIC. On the other hand, the minimum concentrations to eradicate biofilms (MBEC) were 25.95 and 23.78 mM, respectively. The combination of 3.20 mM protocatechuic acid, 2.97 mM vanillic acid, and 1.72 mM catechin eradicated pre-formed biofilms. The antioxidant capacity of the combination of phenolics was higher than the expected theoretical values, indicating synergism by the DPPH^•, ABTS, and FRAP assays. Effective concentrations of catechin, protocatechuic, and vanillic acids were reduced from 8 to 1378 times when combined. In contrast, the antibiotic nitrofurantoin was not effective in eradicating biofilms from silicone surfaces. In conclusion, the mixture of phenolic compounds was more effective in preventing cell adhesion and eradicating pre-formed biofilms of uropathogenic E. coli than single compounds and nitrofurantoin, and showed antioxidant synergy.

Bactericidal Property of Oregano Oil Against Multidrug-Resistant Clinical Isolates

Development of non-antibiotic alternatives to treat infections caused by multidrug-resistant (MDR) microbes represents one of the top priorities in healthcare and community settings, especially in the care of combat trauma-associated wound infections. Here, we investigate efficacy of oregano oil against pathogenic bacteria including MDR isolates from the combat casualties in vitro and in a mouse burn model. Oregano oil showed a significant anti-bacterial activity against 11 MDR clinical isolates including four Acinetobacter baumannii, three Pseudomonas aeruginosa, and four methicillin-resistant Staphylococcus aureus (MRSA) obtained from combat casualties and two luminescent strains of PA01 and MRSA USA300, with a MIC ranging from 0.08 mg/ml to 0.64 mg/ml. Oregano oil also effectively eradicated biofilms formed by each of the 13 pathogens above at similar MICs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that oregano oil damaged bacterial cells and altered the morphology of their biofilms. While efficiently inactivating bacteria, there was no evidence of resistance development after up to 20 consecutive passages of representative bacterial strains in the presence of sublethal doses of oregano oil. In vivo study using the third-degree burn wounds infected with PA01 or USA300 demonstrated that oregano oil, topically applied 24 h after bacterial inoculation, sufficiently reduced the bacterial load in the wounds by 3 log₁₀ in 1 h, as measured by drastic reduction of bacterial bioluminescence. This bactericidal activity of oregano oil concurred with no significant side effect on the skin histologically or genotoxicity after three topical applications of oregano oil at 10 mg/ml for three consecutive days. The investigation suggests potentials of oregano oil as an alternative to antibiotics for the treatment of wound-associated infections regardless of antibiotic susceptibility.

Biocidal effect of thymol and carvacrol on aquatic organisms: Possible application in ballast water management systems

Ballast water is essential for maintaining the balance and integrity of a ship. However, exchanging ballast water resulted in discharging water of different origins in vessel recipient ports, and this may have caused ecosystem disturbance or aquatic pollution. The ballast water management (BWM) system is essential for the purification and disinfection of the ballast water that is taken up. Because current BWM systems widely use biocides for the treatment of aquatic organisms, the biocides may result in unintended toxicity of the discharged ballast water. In this study, we suggested thymol and carvacrol as chemical biocides for BWM systems and investigated their effectiveness using Artemia salina and Escherichia coli. Thymol and carvacrol showed biocidal effects in our study. A combination of these substances showed a synergistic increase in the biocidal effects. Moreover, carvacrol naturally degrades after disinfection, which indicates that natural substances may be promising candidates to increase the efficacy and reduce unwanted side effects of the BWM system.

Antibacterial, antibiofilm and antiquorum sensing effects of Thymus daenensis and Satureja hortensis essential oils against Staphylococcus aureus isolates

AIMS

The present study was conducted to investigate the effects of Thymus daenensis and Satureja hortensis essential oils (EOs) on the planktonic growth, biofilm formation and quorum sensing (QS) of some Staphylococcus aureus isolates (strong biofilm producers).

METHODS AND RESULTS

Minimum inhibitory concentration (MIC) of the EOs, inhibition of biofilm formation as well as disruption of preformed Staph. aureus biofilms were assessed. The antibiofilm activity of the EOs was determined using microtitre plate test (MtP) and scanning electron microscope (SEM). The QS inhibitory activity was also examined on the pregrown biofilms by gene expression analysis using quantitative real-time RT-polymerase chain reaction (PCR) of hld gene (RNAIII transcript). Moreover, tetrazolium-based colorimetric assay (MTT) was performed to detect cytotoxic effects of these EOs on the Vero cell line. Finally, the major components of the tested EOs were determined using Gas Chromatography-Mass Spectrometry (GC-MS). The MICs of T. daenensis and S. hortensis EOs against planktonic cells of the isolates were 0·0625 and 0·125 μl ml^-1 respectively. The minimum bactericidal concentrations for both of the EOs was 0·125 μl ml^-1 . The MtP test showed a significant inhibitory effect of the EOs on the biofilm formation and disruption at sub-MIC concentrations. These results were confirmed by SEM. Real-time PCR revealed a significant down-regulation of hld gene following treatment with MIC/2 concentration of S. hortensis EO. GC-MS analysis showed that carvacrol, terpinene and thymol were the major components of the applied EOs.

CONCLUSIONS

As selected EOs did not show significant cytotoxic effects even up to tenfold of MIC concentration, the applied EOs seem to be good candidates for preventing of biofilm formation of Staph. aureus cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present study introduced T. daenensis and S. hortensis EOs as new antibiofilm, and S. hortensis EO as anti-QS herbal agents with natural origin against Staph. aureus.