Antioxidant activity, antimicrobial and effects in the immune system of plants and fruits extracts

ABSTRACT

Nowadays, medicinal plants, and fruits and vegetables with antioxidant and antimicrobial activity are increasingly studied. In this review, the impact of the medicinal plant extracts on the immune system was discussed. The first part described the phenomenon of oxidative stress, followed by the mechanisms involved in the antioxidant activity and finally discussed on the evaluation of the antioxidant activity of plants and fruits. The second part revised the works that discuss about the antimicrobial activity of plants and fruits extracts, emphasizing in the factors that influence the evaluation of such characteristic. Finally, in the third part,  studies on the effect of the plant extracts over the immune response were discussed, describing the results of the enhancer effects, as well as those of the suppressing ones. This work allowed us to discern the necessity of a wider research on the chemical composition of the natural extracts, in order to establish a correlation with the effects observed in health.

KEYWORDS:

• Oxidative stress
• antioxidants
• antimicrobial activity
• immune response

Introduction

Humans generate knowledge and develop technologies. With them, they synthesize a wide variety of compounds to improve their quality of life. Such compounds (e.g. drugs or insecticides), although solve health problems, cause damage both to the consumers and to the environment. However, humans, in order to solve health and environmental problems, have turned their attention to the use of natural alternatives, such as plant extracts.

Plant extracts have the advantage of having a biological origin, are biodegradable and demonstrate a positive impact on human health and the environment (Bravo et al. 2000). The use of plant extracts to heal diseases has been documented since ancient times: the first apothecaries emerged in the ancient civilizations of Mesopotamia (4000–5000 BC), particularly the Babylonians; also Egyptians, Chinese and Indians made use of this resource (Jácome 2003).

Among the various properties that medicinal plants, vegetables and fruits possess, antimicrobial and antioxidant properties are chosen as the subject of this review. Furthermore, this paper documents the effect of extracts of such products on the immune system.

Plants with antioxidant activity

Although the human body has an antioxidant defense system, often this system is not sufficient to neutralize the multiple attacks that bombard the body day by day. In order to maintain a balance between oxidants and antioxidants in the body, typically substances acting on reactive oxygen species is used in the form of food supplements. However, in recent years, use of some synthetic antioxidants has been restricted because of their toxic and carcinogenic effects (Frankel et al. 1995).

Therefore, alternative medicine has been used, such as extracts or herbal teas and fruit with antioxidant properties. These properties are being increasingly studied by the scientific community. Various studies show that some medicinal plants have more potent antioxidant activity than some fruits and vegetables (Song et al. 2010).

Consumption of fruits and vegetables with antioxidant capacity, as well as medicinal herbs and plants that have this property can provide optimal health and nutritional outcomes. This is based on that quality nutrition may have beneficial effects in the prevention of some chronic diseases prevalent in society (Weisburger 2000).

Vegetables are an alternative of quality nutrition, as they have natural substances and enzymatic systems that retard or prevent oxidation and protect against free radical attack than are normally produced in the metabolism. These are endowed with a series of enzymes and small molecules which react with free radicals. For example, superoxide dismutases catalyze the dismutation of superoxide anion (O₂₋) to hydrogen peroxide (H₂O₂) and O₂, while the nonspecific peroxidases and catalases destroy H₂O₂.

However, most of the antioxidant capacity of fruits and vegetables is provided by its content of vitamin E, C and carotenoids, as well as different polyphenols (Ko et al. 2005; Zugic et al. 2014) (Table 1) which are essential for life. Polyphenols as antioxidants can protect cells against oxidative damage and therefore reduce the risk of various degenerative diseases associated with oxidative stress caused by free radicals (Scalbert et al. 2005; Huang et al. 2010). Among the more important phenolic compounds are flavonoids, which, besides its demonstrated antioxidant activity, has been attributed with a wide range of therapeutic effects, such as cardiotonic, anti-inflammatory, hepatoprotective, antineoplastic and antimicrobial activity. (Narayana et al. 2001).

Table 1. Natural and synthetic antioxidant molecules.

Substance	Source
Ascorbic acid	Green vegetables, seeds, grapefruit, kiwi, lemon, asparagus, avocados, beet, broccoli, Brussels, sprouts, cantaloupe, currant, cabbage, mango, fresh, mustard, onion, orange, papaya, parsley, peas, pineapple, radish, spinach, strawberry, tomato, fresh turnip, watercress, chestnut, eggplant, leek, celery, endive, potatoes, garlic and apple.
Tocopherols (α, γ)	Spinach, walnut, peanut, cereals, wheat germ, vegetable oils packaged cold, watercress, parsley and pollen
β-carotene	Fish liver oil, liver, alfalfa, apricots, asparagus, beets, broccoli, cantaloupe, carrots, chard, fresh dandelion, garlic, cabbage, mustard, papaya, parsley, peach, red pepper, sweet potato, spinach, spirulina, pumpkin, turnip, watercress, lettuce, plums, olives, beets, beetroot, tomatoes and almonds
Lycopene	Tomatoes, watermelon, guava
Europeina	Grapes, red wine
Olein	Olive oil
Flavonoids	Alfalfa sprouts, spinach, lemon, onion, chiles (apple, serrano, poblano, jalapeno green tree), nopal, broccoli, guava, strawberry, blackberry, red apple, red grape, orange, black pepper, wine, beer, tea green, black tea, soy. Plants: Cranberry, gingko biloba, milk thistle
17β-estradiol	Hormones, steroids
N-acetyl-cysteine	Chemical synthesis, breakdown product
Pyridoxamine (B6)	Meat, offal, fish, milk, eggs, whole grain cereals, potatoes, sweet potatoes, spinach, green beans, wheat germ, rice husks, avocado, hazelnuts, almonds, peanuts, bananas, brewer's yeast

Food with vegetal origin contains a variety of hydroxylated flavonoids and other phenolic compounds in amounts, ranging from traces to several grams per kilogram of fresh weight. From above derives the importance of the study of the antioxidant properties of vegetables used in human nourishment. The daily intake of polyphenols in individuals who have a diet rich in these foods is approximately 1 g/day. It has been observed that there is a wide variation in bioavailability between polyphenols observed as between individuals (Scalbert et al. 2005). In general, bioavailability is in function of the solubility (Eastwood 1999) and therefore the polyphenols present different absorption levels in biological media (Bourne et al. 2000).

Some plants with antioxidant activity due to the polyphenols are: cat tail, ginseng, gingko biloba, eleu- therococcus, anamú, grapes, eucalyptus, mandarin, grapefruit, lemon, orange, rosemary, agrimony, tangerine, calendula and oat (Abad-Garcia et al. 2012) (Table 1). They have properties relevant to human health functioning as antiradicals, antimutagenics, anticarcinogenics, ratardants of senescence; antiatherogenics and antimicrobials (Reyes et al. 2009).

It is recognized that some plants have positive antioxidant effects when they were tested against reactive oxygen compounds in biological systems. Many subproducts of plants such as peanut hulls, red grape, carob seeds, citrus peel and some seeds, as well as malt root extracts have also been exploited because of their antioxidant potential (Bonnely et al. 2000).

Evaluation of antioxidant activity

Although there are different methods to evaluate the antioxidant activity, either in vitro or in vivo, in vitro methods allow us to have an approximate idea of what happens in complex situations in vivo. The most used methods are ABTS (2,2′-azino-bis [3-ethylbenzothiazoline]-6-sulfonic acid) and DPPH (2,2-diphenyl-1-picrilhidrazil). Both methods have excellent stability under certain conditions although they exhibit differences. DPPH is a free radical obtainable directly without prior preparation, while ABTS has to be generated after a reaction which may be chemically (manganese dioxide, potassium persulfate, 2,2′-azobis [2-amidino propane]), enzymatic (peroxidase, myoglobin) or also electrochemical. With ABTS, the activity of lipophilic and hydrophilic compounds can be measured, which makes it very useful in the simultaneous study of various plants (Castañeda et al. 2008).

Moreover, in a study conducted on 56 plants to determine the antioxidant properties and phenolic content, the 4 plants that showed a higher concentration of phenolic compounds were Dioscorea bulbifera (59.43 ± 1.03 mg GAE [gallic acid equivalents]/g), Eriobotrya japonica (31.47 ± 0.48 mg GAE/g), Tussilago fárfara (30.03 ± 0.19 mg GAE/g) and Ephedra sínica (27.70 ± 0.89 mg GAE/g).

D. bulbifera is a plant used in the prevention of tumors, diabetes and leprosy, and the more important compounds that give the antioxidant activity to the plant are (+)-catechin and myricetin; E. japonica has anticarcinogenic, inflammatory, hypoglycemic and hypolipidemic effects; T. fárfara has antimicrobial activity and is used in bronchitis treatments and asthmatic disorders; finally, E. sínica is employed for the treatment of multiple symptoms of cold and allergy, in body weight control, in the treatment of acute liver failure and to alleviate the inflammatory responses (Song et al. 2010).

After water and coffee, tea is the most consumed beverage in the world (Mantilla 2003). This last drink can be consumed in various varieties, black tea, oolong tea, white tea and green tea. A cup of green tea provides about 150 mg of flavonoids, most released during the first minutes of infusion.

Guava consumption reduces oxidative stress, and the risk of diseases caused by free radicals and hypercholesterolemia (Rahmat et al. 2004). However, the flavonoid content in the leaves of guava is greater than in the fruit; hence, the leaves could be used to extract flavonoids (Vargas et al. 2006) and also to suggest its use in less traditional ways as extracts or infusions.

Another study reported that the antioxidant capacity of a glass of wine equals 7 orange juices or 20 apple juices (Neira 2004).

It is known that the antioxidant properties of plants and fruits are related to not only the total amount of antioxidants, but also the presence of certain compounds. You could say that the total antioxidant content of fruits, vegetables, juices and beverages can be interpreted as the combined action of endogenous antioxidants.

The antiradical action of some fruit juices or beverages can be due to (i) differences in the content and the composition of bioactive (ii) diversity in structural features of the potential of phenolic antioxidants and (iii) a synergy of bioactive with other components present in each of the juices or drinks, and (iv) the differences in the kinetic behavior of possible antioxidants. All these factors may contribute to the efficiency of the antioxidant activity of juices and beverages (Ramadan 2008) (Table 2).

Table 2. Antioxidant capacity by DPPH method.

Sample radical	Extract	%Uptake of free (F.R.)
Vitamin C (control)
Ascorbic acid	*	92.92
Cinnamom bark
(Cinnamomum zeylanicum) 10%	Ethanol	97.59^a
Lagarto leaf
(Calophyllum brasiliense) 20%	Methanol	99.76^b
Camu-camu, fruit
(Myrciaria dubia) 10%	Methanol	98.09^b
Muña, leaf
(Minthostachys mollis) 10%	Aqueous	92.41^b
Hiporuro, leaf
(Alchornea castaneifolia) 10%	Methanol	75.96^b
Lagarto, leaf
(C. brasiliense) 10%	Aqueous	110.56^c
Yacón, leaf
(Smallanthus sonchifolius) 10%	Ethanol	103.19^d
MACA KOKEN, hypocotyl
(Lepidium peruvianum) 10%	Aqueous	95.55^d
MACA AMAZON, hypocotyl
(Lepidium meyenii) 10%	Methanol	88.21^d

^a1 µg/mL.

^b50 µg/mL.

^c100 µg/ mL.

^d200 µg/ mL.

Antimicrobial activity

The use of plant extracts in alternative medicine has increased in recent years (Duraipandiyan et al. 2013), so that the health-care system has evolved among the Hispanic community in North America since Spanish colonialism. Initially the natives of American countries used plants to treat their illnesses, this goes back to Spanish colonialism and has its roots in the Spain occupied by Arabs, Greek humoral medicine and Aztec mythology (Van Wyk et al. 1997). Among the properties that are ascribed to plants is the antimicrobial activity. The ways to obtain the plant extracts, methods to evaluate the antimicrobial activity, as well as test microorganisms are diverse (Table 3).

Table 3. Methods for obtain extracts of plants and for measuring its antimicrobial activity.

Plant	Extraction method	Method for measuring the antimicrobial activity	Microorganisms studied	Reference
Ephedra breana Nolana sedifolia Fabiana imbricata	Maceration complete, methanolic extract, ethanolic fraction	Agar diffusion	Botrytis cinerea	Vio-Michaelis et al. (2012).
Ficus enormis Myracrodruon urundeuva Patagonula americana Piptocarpha rotundifolia Psidium cattleianum Maytenus ilicifolia	Ethanolic extract	Agar dilution method	Fusobacterium nucleatum Porphyromonas gingivalis	Gaetti-Jardim et al. (2011)
Costus speciosus	Extraction with hexane, chloroform, ethyl acetate and methanol	Disc diffusion method	Bacteria: Bacillus subtilis, Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, Klebsiella pneumoniae, Proteus vulgaris, Pseudomonas aeruginosa and Erwinia sp. Fungi: Trichophyton rubrum, Trichophyton mentagrophytes, Trichophyton simii, Epidermophyton floccosum, Scopulariopsis sp. Aspergillus niger, B. cinerea, Curvularia lunata, Magnoporthe grisea and Candida albicans	Duraipandiyan et al. (2013)
Virola sebifera	Ethanol maceration	Agar diffusion	S. aureus	Velasco et al. (2005)
Acacia seyal, Combretum hartmannianum, Capparis deciduas, Erythrina latissima, Kigelia africana	Extract with ethanol and dichloromethane	Microdilution assay	B. subtilis, S. aureus, E. coli and K. pneumoniae	Eldeen and Van Staden, (2007)
Cestrum nocturnum	Solvent extraction and fractionation by column chromatography	Agar diffusion	Rhizopus stolonifer	Barrera-Necha and Bautista-Baños, (2008)
Euphorbia caracasana, Euphorbia cotinifolia	Maceration and fractionation with n-hexane and dichloromethane	Agar diffusion	S. aureus ATCC 25923, E. faecalis ATCC 29212, E. coli ATCC 25992, K. pneumoniae ATCC 23357 and P. aeruginosa ATCC 27853	Rojas et al. (2008)
Bidens tripartita	Hydrodistillation, maceration and heating. Solvent fractionation	A broth microdilution and agar diffusion	B. subtilis ATCC 6633, Micrococcus luteus ATCC 9391, S. aureus ATCC 6538, E. coli ATCC 25922, E. coli ATCC 35218, K. pneumoniae ATCC 700603, P. aeruginosa ATCC 27853, C. albicans ATCC 10231, Candida parapsilosis ATCC 22019, Aspergillus fumigatus, Aspergillus terreus	Tomczykowa et al. (2008)
Burchellia bubalina	Sequential extraction with petroleum ether, dicrlorometano, ethanol and water.	Agar diffusion	B. subtilis ATCC 6051, S. aureus ATCC 12600, E. coli ATCC 11775, K. pneumoniae ATCC 13883	Amoo et al. (2009)
Solanecio mannii, Monodora myristica, Albizia gummifera, Glyphaea brevis	Extraction with cyclohexane and ethyl acetate.	Agar diffusion and broth dilution.	Gram positive bacteria, Gram negative bacteria, C. albicans, Candida krusei	Mbosso et al. (2010)
Duabanga grandiflora, Acalypha wilkesiana	Maceration	Disk diffusion plate by casting, spreading groove seeding disc, agar dilution and by turbidimetry.	E. coli and S. aureus	Othman et al. (2011)

The results of the evaluation of the antimicrobial activity differ substantially from each other. Among the factors influencing these differences are those that may be attributable to the substances under study, the method in which the activity is assayed and in the protocols variations between different research groups (Boorn et al. 2010). So, many reports of the antimicrobial activity of substances are difficult to compare because of the differences in methodology. In some cases the aqueous extract is the one that has higher antimicrobial activity (Velasco et al. 2005) and in others is the ethanolic extract (Tolosa & Cañizares 2002). Some authors have reported contradictory results from the antimicrobial activity of the extract of the same plant (Bommarito et al. 1998; Suárez et al. 2007). This suggests that the effectiveness of plant extracts may depend on the test microorganism, the method of extract obtainment and the method of evaluation of the antimicrobial activity.

The antimicrobial activity depends upon several factors. The kind of extraction and separation conditions of the fractions influence the antimicrobial activity of extracts (Rubilar et al. 2011). These show selectivity for inhibiting the growth of microorganisms (Gaetti-Jardim et al. 2011) since some extracts may inhibit the growth of pathogenic bacteria and show no activity against innocuous bacteria or against fungus (Shene et al. 2009), which is attributed to different mechanisms of growth or reproduction that have the groups of microorganisms.

Nevertheless, it has also been observed that the oil extracted from the leaves of certain plants may not have significant bacteriostatic effects, but demonstrate strong antifungal activity (Tomczykowa et al. 2008). They can also be more effective against Gram positive bacteria than against Gram negative, which is probably due to its thin murein layer that prevents the entry of inhibitors (Rabe & Van Staden 1997).

The antimicrobial activity of the plants is due to the presence of polyphenolic compounds, which are in larger quantities in the crude extracts of leaves than on fruit extracts, while the former has more antimicrobial activity (Rubilar et al. 2011). Such activity depends on intrinsic factors (genetic characteristics, concentration of components in the plant, part of the plant used and plant stage in which the cut is made in the same, extrinsic factors – environmental conditions), factors such as treatment methods and how to evaluate the antimicrobial activity, and synergistic factors, antagonistic and additives – result of the interaction of various compounds in the medium.

Results of many research papers on the antimicrobial activity of extracts are not directly comparable because there are no standardized methods for measuring antimicrobial activity. Al-Bakri and Afifi (2007) reported that some methods are comparable when estimating the antimicrobial activity of the substances, in a general way, but there is not a strong linear correlation between them (Al-Bakri & Afifi 2007). Among the agar-based assays to measure the antimicrobial activity, Othman et al. (2011) propose the disk diffusion method, in which agar is poured on plate, which presents more reproducible results (Othman et al. 2011). However, Boorn et al. (2010) reported that the agar diffusion method is not the most appropriate to evaluate the activity of complex natural substances (Boorn et al. 2010). Among the limitations of the agar diffusion method, it is considered that low levels of antimicrobial activity may not be detectable (Allen et al. 1991), and also because non-polar compounds may not diffuse properly in aqueous-based agar (Griffin et al. 1999).

Despite the large number of papers published on the antimicrobial activity of extracts of natural products, standard methods to measure antimicrobial activity and in-depth details regarding the chemical characterization of these extracts are still missing, which, if provided, will help us to study the groups of compounds resoponsible for the above-said activity, isolate them and apply them with more specific antimicrobial purposes.

Immune system effects

In addition to the antimicrobial activity of plant extracts, studies concerning the effects therefore on the immune system have become more relevant in recent years. The interest of the scientific community to find new drugs with potential effects on the modulation of the immune response is strongly associated with the findings on the participation of the immune system in the development of cancer (León & Faxas 2004) and other conditions.

Therefore, the discovery of active substances that regulate the immune response could lead to significant advances in the treatment of immunologically mediated diseases, including cancer, allergies, autoimmune phenomena, immunodeficiencies, organ rejection and infectious diseases.

Plants with activity on immune system

Conventional medicine employs scientifically established treatments to treat conditions associated with abnormal function of the immune system. However, there are diseases whose symptoms are not completely beaten down by conventional medicine. In these cases, the patients appeal to the use of plant infusions as additional or alternative therapy. Although the use of the products listed is not scientifically based, there are several investigations that seek to describe the mechanisms of action of plant extracts on immune system components.

So far potentiating effects have been described (Table 4) and suppressors (Table 5) of the immune response induced by extracts of plants, so that the applications of these in current therapeutic become relevant.

Table 4. Natural products with potentiating effect of the immune response.

Product	Effect	Reference
Andrographis paniculata	Increases lymphocyte proliferation Increases the production of IL-2 and TNF-	Kumar et al. (2004) Rajagopal et al. (2003)
Echinacea purpurea, Echinacea angustifolia and Echinacea pallida	Activates macrophages, increases the phagocytosis and the production of cytokine Activates NK cells, improves the humoral response and increases the number of lymphocytes and monocytes	Sharma et al. (2006) Yakoot and Salem (2011)
Ginseng Panax quinquefolius	Activates macrophages, NK cells and lymphocytes and increase the production of cytokines and antibodies	Pannacci et al. (2006) Scaglione et al. (1990)
Piper longum	Increases leukocyte counts Increases the number of plasma cells and the amount of circulating antibodies	Sunila and Kutan (2004) Pradeep and Kuttan (2004)
Tea of eight herbs	Increased production of Th1 and Th2 cytokines	Hong et al. (2005)
Vitamin C	Activates neutrophils, NK cells and monocytes Increases lymphocyte proliferation and chemotaxis.	Wintergerst et al. (2006)

Table 5. Natural product with immunosuppressive effect.

Product	Effect	Reference
Fish oil	Production IL-10, suppression of COX-2, IL-2 and IL-6	Prescott and Stenson (2005) Babcock et al. (2002)
B. pinnatum	Reduces inflammation	Domínguez and Bacallao (2002)
Onion, apple (quercetin)	Inhibits endocytosis and dendritic cell migration Inhibits production of IL-1β, TNF-α, IL-6, IL-12 and ERK phosphorylation	Huang et al. (2010)
Citric fruits (vitamin C)	Treg cell generation. Inhibiting the activation of NF-kB	Tan et al. (2005)
Royal jelly	Inhibition of TNF-α e IL-6	Kohno et al. (2004)
Moringa citrifolia L.	Reduces inflammation	Rodríguez et al. (2005)
P. leiophylla	Reduces inflammation associated with decreased neutrophil migration	Márquez et al. (2008)

Potentiating effects of the immune response

In conditions such as immune deficiencies and cancer, immune response decreases; that is why it is so desirable to increase the same in these patients. Also in some infectious disorders, such as tuberculosis, it is important to ensure that the immune response can be potentiated to eliminate microbial antigen. In all these conditions, stimuli and effector mechanisms of the immune response are different. However, they have a common factor, the need for the use of drugs which allow the increase of the immune response facing certain antigens. Historically adjuvants have been used, for example vaccines to enhance immune response against specific antigens. The classic example of this is Freud's complete adjuvant, used in the production of antisera in situations where limited quantities of antigen are available or when this present low immunogenicity (Morris et al. 1999). However, their use has high toxicity, so that science seeks alternatives with equal effect and fewer side effects.

Currently the alternative medicine is the one that has contributed to the study of the action mechanisms of the active principles of plants and substances employed in ethno pharmacy as modulators of the immune response. Table 4 details the more relevant scientific findings related with the possible therapeutic potential of plant extracts regarding the potentiating effect.

Not all effects of the products listed in Table 4 have been studied in pathological models. However, they produce a clear visible change in the immune response; either increasing the humoral or cellular immune response, or by inducing an increase in the inflammatory response by increased secretion of pro-inflammatory cytokines, as in the case of Andrographis paniculata (Rajagopal et al. 2003; Kumar et al. 2004).

The effects of Echinacea, ginseng and vitamin C on immune system cells have caught attention. These findings explain, at least in part, the effects of these products in cancer treatment.

Suppressive effects of the immune response

Unlike in cancer, in autoimmune processes, allergies and organ rejection phenomena, there is a not desirable exacerbated immune response, so these treatments are directed to reduce this response. In these phenomena, the immune response is exacerbated against different types of antigens -autoantigens, innocuous antigens, alloantigens, respectively- so they are used as nonspecific antigen immunosuppressive drugs with good results. However, so far no treatment has been found to cure these diseases, so therapies with fewer adverse reactions, and equal or better therapeutic effects are searched.

Alternative medicine has generated scientific research in pursuit of immunosuppressive treatments; Table 5 summarizes some of the major findings in this respect.

Márquez et al. (2008), Rodríguez et al. (2005) and Domínguez and Bacallao (2002) describe animal models of inflammation and have tested the effect of Pinus leiophylla, Bryophyllum pinnatum and Morinda citrifolia respectively. These investigations described the potential therapeutic effects of these species; however, they do not describe the mechanisms of action.

So far, there are relatively few investigations that are identifying the active principle of the plant extracts and the exact mechanism of action of the same. Such as in the example of the research made by Huang et al. (2010), which examines the effect specifically on dendritic cells quercetin, finding inhibition of endocytosis by these cells (Huang et al. 2010). Furthermore the authors of this study also describe a decreased production of pro-inflammatory cytokines and chemokines. As a result of studies, the suppressive effect of natural extracts on the immune response is closely related to the decreased production of pro-inflammatory cytokines.

Currently, investigations are beginning to explore the effects of natural products on the number or function of regulatory T cells, which are intimately involved in the modulation of immune response. Such is the case of vitamin C found in citrus fruits, which is found to be a potential inducer, generating regulatory T cells (Tan et al. 2005). However, the findings of Tan et al. (2005) contrast those of Wintergerst et al. (2006) describing a leukocyte activation.

Conclusions

It is evident that plant extracts, and generally natural products, are potential sources of drug with antioxidant and antimicrobial activity as well as immune-response mediators. The effect of these products on health and their mechanisms of action must be studied. This will allow us to find the best form of administration of substances and the conditions in which they could deliver better benefits.

Disclosure statement

No potential conflict of interest was reported by the authors.