One of the most impressive processes in microbiology is the ability of bacteria to communicate with each other via signal molecules (48). This type of bacterial communication coordinates the accumulation and responses to small molecules called AIs (7,8,49,50). The process known as QS allows the bacterial community to coordinate the gene expression, leading to the activation of specific phenotypes in the population. The most common processes that are under QS control, used by bacteria as a survival strategy, are bioluminescence, biofilm formation and dispersal, virulence factors expression, motility, pigment synthesis, sporulation, conjugation, symbiosis, antibiotic production (5–8).
During the antibiotic century, the revolution of better human health was a good scenario. Unfortunately, this development has led to an increase in bacterial resistance. Nowadays, it is necessary the discovery of new targets for inhibiting microbial pathogenicity, without stimulating microbial resistance (7). One of the most novel anti-virulence strategies is to interrupt the cascade of the QS system (51,52). Each step of the QS signaling cascade, could be a good target and result in the inhibition of pathogenicity (53). Some of the most attractive biomolecules, which could be used for this purpose, are natural QSIs (7,51). Similar inhibitors, that can mediate bacterial QS, are described in different marine algae, fungi, corals, tunicates, cyanobacteria (54–57), bacterial (7,58,59) and mammalian cells (60). Most were isolated from plant cells (61–65).
Bearing this in mind, our major interest is focused on QSIs isolated from plants, including their medicinal and anti-QS properties found in C. violaceum.
The plant kingdom is one of the most popular with their species and families, whose metabolite products have broad biological activities. It is well known about the antimicrobial activities of different plant extracts (12,13,66–68), essential oils (22,69), fractions, and their constituent, but their efficacy against QS systems are poorly understood. During the last few years, it has been found that plant extracts can act as inhibitors of QS pathways. Such active metabolites can be extracted from different parts of plant tissues such as roots, stems, leaves, bark, fruits, flowers, seeds, and green pod (70–73). Major groups of these compounds can be identified as QSIs including cyclic compounds, phenolic derivatives, nitrogen cyclics, furanones, lactones, cinnamaldehydes, alkaloids, phenolics, saponins, tannins, and terpenoids (74,75). Their functionality is different - they can inhibit bioluminescence, fluorescence, biofilm formation, pigment production, block enzyme activity, and inhibit a variety of reporters (7,12,13). These abilities depend on their chemical structure and stability. To interfere with signal acceptance, QSIs must be competitive and non-competitive molecules that prevent binding the signal by its similar receptor. It is essential to know that for competitive molecules to bind to a receptor, they must be structurally similar to the original signal molecules. Non-competitive binding molecules will bind to a site different from the signal-binding site on the receptor. Several scenarios have been known using plants molecules or metabolites as QSIs: a) homologically masking the QS signal and disrupting the bacterial communication; b) interfering with different enzymes; c) preventing the accumulation of signals; d) blocking the main receptors (22,74).
4.1. Quorum sensing inhibitory potential by plants
In the environment, plants are constantly exposed to a wide range of stress conditions. These stress factors that affect plants are temperature changes, nutrient deficiency, drought, salinity, UV radiation, lack of oxygen, pesticides, pollutants, and anthropogenic activities. Apart from environmental stress, some species such as bacteria, fungi, viruses, nematodes, and insects can cause distress. Plants have been facing most of their attackers for more than a million years. Living with their natural enemies in reciprocal evolutionary interaction, they have been learning and developing mechanisms for resisting stress and attacks. Due to this reason, plants reveal an “immune system” comparable to that of animals by biosynthesizing active compounds and secondary metabolites as a protection against infections or induce in response to pathogen attacks. Besides improving the defense against both biotic and abiotic stresses, most of the secondary metabolites have therapeutic activities: anticancer, antioxidant, antidiabetic, immunosuppressive, antifungal, anti-inflammatory, antimalarial, anti-oomycete, antibacterial, anti-fever, anti-diabetic, insecticidal, anti-biofilm and antiviral agents (9,10,12,13,68).
Lately, one of the most interesting QSIs applications is their ability to block signaling molecules produced by bacteria and consequently obstruct the bacterial virulence factors by disrupting their QS system. For this reason, the bacterial QS system is an excellent target for novel QSIs. Scientific evidence proves that the identification of the binding conformation of QSIs into the binding sites of main proteins, by molecular docking analysis, provides new information about their antagonistic characteristics (76). QSIs have been reported in many plants including medicinal plants like Syzygium cumini, Pimenta dioica, Psidium guajava, Medicago truncatula, Lotus corniculatus, Pisum sativum, Moringa oleifera, Vernonia blumeoides, Tecoma capensis, and many others (7,75,77). Their acetone, methanol, and water extracts have been proven to possess quorum sensing inhibitory activity against C. violaceum.
Our review represents summarized information on the plant QSIs, comprehensively studied in C. violaceum. C. violaceum is Gram-negative bacteria, easily cultivated on laboratory media like Blood agar, MacConkey agar, and Nutrient agar. It produces smooth violet colonies whose color comes from a violet antioxidant pigment known as violacein. The increased interest in research communities about C. violaceum is related to its phenotypic characteristics: violacein production, elastase production, biofilm formation, and cyanide production controlled from the QS system by the signal molecules - AHLs.
Many years ago, plants have been studied for their medical values (as digestives, diuretics, expectorants, and sedatives), and also for antioxidant and antimicrobial activities, which further develop the basis of modern phytotherapy. The main interests in their biological functions and mode of action for regulating bacterial communications escalated during the last years. Studies include tests on crude or ethanol, methanol, acetone, ethyl acetate, dichloromethane, hexane and water extracts, essential oils, and phytochemicals partially purified, enriched, or pure fractions. All these plant products could suppress the production of pigment violacein, biofilm formation, motility, and microbial activity in
C. violaceum (
Table 1).
Koh and Tham (78) screened ten Chinese medicinal plants, Prunus armeniaca, Prunella vulgaris, Nelumbo nucifera, Panax notoginseng (root and flower), Punica granatum, Areca catechu, and Imperata cylindrical for their QS activity. Seven of the extracts inhibit QS in bioreporter strain C. violaceum CV026. Part of the tested compounds have the potential to suppress violacein synthesis and six of them formed a clear zone, indicating antimicrobial activity. These results could be compared to other aqueous extracts from Ananas comosus, Musa paradiciaca, Manilkara zapota, Ocimum sanctum, Camellia sinensis L., Nymphaea tetragona and Quercus cortex whose active components were responsible only for inhibition of synthesis of pigment violacein in C. violaceum CV026 and ATCC 12472 (50,82,103). Important observations were discovered about methanol extracts from herbal plants like Pisum sativum, Trigonella foenum graecum, Myristica cinnamomea, Kigelia africana, Melicope lunuankenda, Cuminum cyminum, Costus speciosus, that proved to be inhibitors of violacein production (50,79,81,83,93,95). Bioscreening of ethanol extracts from Egypt`s ornamental and medicinal plants and such collected from Jordan, such as Adhatoda vasica, Bauhinia purpurea L., Lantana camara L., Myoporum laetum, Piper longum L., and Taraxacum officinale, Laurus nobilis L., Populus alba L., Populus nigra L., Lavandula angustifolia, Rosmarinus officinalis L., Sonchus oleraceus L., Tecoma capensis Thunb. Lindl., Jasminum sambac Ait., reveals anti-microbial activity against C. violaceum (71,87). In contrast with this, ethanol extracts from Cinnamomum zeylanicum, Ocimum basilicum, and Scutellaria baicalensis Georgi, demonstrate violacein inhibition on C. violaceum CV12472 and QS inhibition on C. violaceum CV026 (90). Similar results with ethanol extracts, but obtained from Acer monspessulanum subsp. Monspessulanum were reported by Ceylan et. al. (89). The authors determined the violacein inhibition in C. violaceum CV 12472, CV026, and the anti-QS activity of ethanol extracts. Fatima also used the same bioreporter strains to detect the QS regulatory role of ethanol seed extracts from leguminous plants Pisum sativum and Trigonella foenum graecum (78). Eight fractions, including phenolic (gallic acid, ellagic acid, epicatechin, rutin) from green pods of Acacia nilotica have been studied for their capacity to inhibit pigment production in C. violaceum 12472, as two of them can be classified as QSIs with the potential to regulate violacein production, without influencing bacterial growth. Other phenolic plant extracts of Rubus rosaefolius also have shown a similar effect on pigmentation and biofilm formation (72,97). Polyphenolic extracts from Rosa rugosa have been the focus of Zhang et. al. (101) research, with their anti-biofilm and QS inhibitory potentials. The authors proved high pigment reduction without changes in microbial growth. Indian medicinal plants, flowers seeds, barks, and fruits from Punica granatum, Syzygium cumini, Embelia ribes, Phyllanthus emblica, Terminalia bellirica, Terminalia chebula, Punica granatum, Mangifera indica, Acacia arabica, Terminalia arjuna, Thespesia populnea, Casuarina equisetifolia, were screened for the anti-QS activity where tannin-rich extracts and punicalagin influence QS mechanisms as decreased on violacein synthesis. Shukla and Bhathena (100) qualify this phenomenon in the presence of subinhibitory concentrations of tannin extracts (97,102).
The ethyl acetate fractions and eugenol of Syzygium cumini L. and Pimenta dioica L. displayed significant anti-QS activity by inhibiting the pigment production by C. violaceum (85,88,91). Extracts from different plants like Rhizophora annamalayana (bark), Astilbe rivularis, Fragaria nubicola, Osbeckia nepalensis, Dionysia revolute, Eucalyptus camaldulensis, Green tea, Amomum tsaoko, Punica granatum, and Saraca asoca barks (stem), were found to possess QS activities, but most of them against violet pigmentation of Chromobacterium (50,82,91,94,96,104).
Essential oils (EOs) have been produced in more aromatic species and stored in various plant organs, e. g., flowers, leaves, wood, roots, rhizomes, fruit seedling, and seeds. They are secondary metabolites from plant sources, characterized by natural multicomponent systems composed mainly from terpenes (monoterpenes, sesquiterpenes, and diterpenes), oxygenated compounds, which are mainly phenols, alcohols, aldehydes, ketones, esters, oxides, and hydrocarbons. Essential oils and their constituents are important for biomedical or pharmaceutic purposes due to their bactericidal, virucidal, fungicidal, analgesic, sedative, anti-inflammatory, spasmolytic, and local anesthetic properties (105,106).
Among the plant products, essential oils are most popular for their widespread use in ethnomedicine. Some of them, isolated from three species of the genus Piper growing in Colombia, Piper bredemeyer, Piper bogotense, and Piper brachypodon interfere the pigment production and proved minor effect against bacterial growth in C. violaceum CV026 as well (84). Likewise, the four Eos prepared from Cinnamomum verum, Origanum majorana, Thymus vulgaris, and Eugenia caryophyllata were evaluated as QSIs, where disruption of pigmentation production is with a lower percentage only for marjoram oil (92). Many scientists reported different EOs manifesting inhibition of violacein production, identified in Elletaria cardamomum, Eucalyptus radiate, Origanum vulgare, Melaleuca alternifolia, and Mentha suaveolens (24,98,99). Interestingly, among some EOs, like limonene from Citrus lemon, terpinen-4-ol, and pinene from Juniperus communis, and tea tree oil from Melaleuca alternifolia, that identified as QSIs of the purple pigment in C. violaceum, only cis-cis-p-menthenolide from Mentha suaveolens altered biofilm matrix during biofilm formation (98,99,107).