Keywords flavonoids - nutraceuticals - antibacterial - antiviral - antifungal
Introduction
Flavonoids are a category of naturally and extensively occurring polyphenolic organic
compounds that consist of a benzo-gamma-pyrone structure that are almost universally
present in plants.[1 ] Polyphenols are secondary metabolites that are biosynthesized by phenylpropanoid
pathway through shikimic acid.[2 ] They are synthesized in plants in reaction to stressful environmental changes such
as microbial infections. Their biological properties and their chemical nature are
dependent on their molecular structure and chemical factors, e.g., their structural
class, degree of hydroxylation, polymerization, etc. Flavonoids have numerous potential
health benefits in humans because of their antioxidant properties. These antioxidant
properties are mediated by the functional hydroxyl groups, which scavenge free radicals
and chelate metal ions. This chelation of metal ions plays a role in preventing radical
generation, which, in turn, prevents damage to target biomolecules.[1 ] They are present in stems, roots, bark, flowers, fruits, vegetables, cereals, legumes,
wine, and tea.[3 ] Flavonoids are extracted through plant products through many methods. Conventional
methods include maceration, boiling, percolation, soaking, and hydro-distillation.
The advanced techniques utilize microwaves, ultrasound, pressurized liquids, and supercritical
fluids to extract flavonoids.[4 ]
There have been numerous studies that describe the protective role of flavonoids against
a wide spectrum of human disorders and diseases including infectious diseases, degenerative
conditions, and malignancies.[1 ] Infections are the most common type of disease processes in humans and are caused
by pathogenic microorganisms that include bacteria, viruses, fungi, and parasites.
An infection occurs when microbes enter the body and begin multiplying, whereas the
actual disease usually occurs in a small fraction of infected people whose bodies
exhibit cellular destruction and disrupt normal functions. COVID-19 is one such viral
infection caused by SARS-CoV-2. It was first reported as pneumonia of unknown cause,
in a hospital in Wuhan, China, in December 2019. Human saliva and throat swab have
been used extensively for diagnostic tests of SARS-CoV-2.[5 ]
The oral cavity is a known portal of entry of the virus due to the presence of ACE2-expressing
cells.[6 ] It primarily affects the upper respiratory tract and causes a multitude of debilitating
symptoms. Because there is no definitive treatment for this viral infection, we must
rely on our immune system instead. Hence, immunity plays a crucial role in the current
pandemic situation. The body's immunity helps fight and overcome the disease in its
incubation period. The risk of developing COVID-19 and its complications is greater
in people with a compromised immune system or increased inflammatory response. The
disease severity depends not only on the viral load but the host response as well.
In severe infections, an acute immune reaction ensues by the release of multiple cytokines
concurrently, which results in multiorgan failure or death. This is known as a cytokine
storm. Flavonoids can act as strong immunity boosters that can help prevent and cure
such severe diseases. In the longer run, mucormycosis in patients with a history of
COVID-19 have proven to be a burden on healthcare system. Prolonged ventilation, inadequate
glycemic control, and excessive use of corticosteroids along with depressed immunity
have been shown to be the possible reasons for the mucormycosis outbreak.[7 ]
Our immune system swings into action in response to an infection. The various components
of the immune system such as leukocytes, antibodies, complement system, and cytokines
work toward fighting the pathogens.[8 ] Thus, the competency and functioning of the immune system is the major deciding
factor in the occurrence, duration, and severity of an infection. The treatment of
infectious diseases is solely based on chemotherapeutic agents, primarily antibiotics.
Adjunctive therapy includes palliative and supportive care such as rest, diet, and
increased fluid intake. Antibiotics cause several adverse effects, the worst being
a degradation of our immunity and consequential increased susceptibility to various
diseases. Antiviral agents have been tested and used in various viral infections but
none of them have been successful in curing the patient before the incubation period.
It is therefore prudent that our approach in dealing with infectious diseases shifts
toward prevention. Improving our immunity can shorten the recovery period and reduce
the intensity and effect of the microbes on various organs of our body. The ever-evolving
nature of the coronavirus has only intensified our need to strengthen our immune system.
Flavonoids not only help fend off infections and reinforce our immune system; however,
their use significantly decreases the incidence of antibiotic resistance.[9 ] As a result, these compounds are now being largely being researched and successfully
used as nutraceuticals. Previous researches have depicted that biocompatibility and
bio-safety is also an important requirement of the materials used for medical and
dental applications whether in case of the treatment of disease or any other invasive
procedure.[10 ]
[11 ]
[12 ]
Classification of Flavonoids
Classification of Flavonoids
Flavonoids are secondary plant metabolites, which exhibit a wide range of chemical
forms and ecological significance.[1 ]
[2 ] Flavonoids are found mainly in spermatophytes, gymnosperms, and angiosperms, along
with some bryophytes, pteridophytes, and algae. They are the third largest cluster
of naturally occurring products with 10,000 flavonoids recorded so far that are classified
based on their biosynthetic origin.[1 ]
[2 ]
[3 ] Chemically, flavonoids can be defined as an aromatic nucleus-containing compounds.
This nucleus contains three rings, namely, A, B, and a C ring (heterocyclic). They
are considered one of the most extensive group of phytochemicals because of their
structural diversity. Flavonoids are usually classified according to the position
of attachment of ring B to ring C. Mostly, the B ring is attached to position 2 of
the C ring, but it can also bind to position 3 or 4. The pattern of glycosylation
and structural characteristics are deciding factors of their biological properties.
A detailed classification of flavonoids is described in [Fig. 1 ].[13 ]
Fig. 1 Schematic representation of flavonoids classification.
Mechanism of Action
Flavonoids are chemotaxonomic compounds with numerous biological activities. Since
ancient times, flavonoids have been used in various concoctions as the primary active
ingredient to treat human diseases.
They have antioxidant and anti-inflammatory properties and exhibit effective action
against microbial infection. Hence, flavonoids display antibacterial, antiviral, antifungal,
and chemopreventive actions. Furthermore, studies have exhibited synergy between certain
flavonoids and existing pharmaceutical products. Flavonoids deactivate or interfere
with the microbial adhesions by forming hydrophobic, covalent or hydrogen bonding
with protein complexes ([Table 1 ]).[14 ]
[15 ]
[16 ]
[17 ]
[18 ]
[19 ]
[20 ]
[21 ]
[22 ]
[23 ]
[24 ]
[25 ]
[26 ]
[27 ]
[28 ]
[29 ]
[30 ]
[31 ]
[32 ]
[33 ] A few lipophilic flavonoids act by causing the disruption of microbial membranes.
A detailed description of the antibacterial mechanism of flavonoids is depicted in
[Fig. 2 ]. Many studies have shown flavonoids to cause the inhibition of many enzymes linked
to virus growth.[1 ] They display inhibitory action against HIV, Dengue, Polio Virus, Herpes Simplex
Virus, and Sindbis virus. Flavonoids also prevent viral polymerase binding of viral
capsid proteins. These properties were suggested as antiviral modes of action[34 ]
[35 ]
[36 ] ([Fig. 3 ]). The antifungal action of flavonoids have been described in [Fig. 4 ].
Fig. 2 Antibacterial action of flavonoids.
Fig. 3 Antiviral action of flavonoids.
Fig. 4 Antifungal action of flavonoids.
Table 1
Target organism of flavonoids
Source
Category
Target microorganism
Reference
Pelargonium sidoides extract
polymeric proanthocyanidins
Anti-adhesive property
[14 ]
Cranberry
A-Type proanthocyanidins
UTI–E. coli
[15 ]
Cranberry
Type A oligomers
UTI–Streptococcus mutans
[16 ]
Cranberry
Proanthocyanidins
Decrease inflammatory mediators
[17 ]
Tea
Epigallocatechin Gallate
Helicobacter pylori inhibition
[18 ]
Tea
Epigallocatechin Gallate
E. coli inhibition of virulence factors such as formation of biofilm and Bacterial motility
of the swarm
[19 ]
Tea
Epigallocatechin Gallate
Flu virus-prevents binding of viral virulent particles to the target receptor cells
by binding to the viral haemagglutinin
[20 ]
Tea
Adenovirus and Enterovirus-inhibition
[21 ]
[22 ]
Tea
Candida albicans–inhibition in superinfection of oral cavity, intestine, vagina
[23 ]
Rhamnetin, Myricetin, Morin, Quercetin
Chlamydia pneumoniae inhibition (decreases infectivity by 50%) in acute respiratory tract infections
[24 ]
Propolis
Galangin, izalpinin, and rhamoncitrin
M. gypseum , T. mentagrophytes , T.
rubrum (bacterial and fungal dermatitis–external topical use)
[25 ]
Cranberry
Proanthocyanidin
E. coli , S. mutans , S. aureus -inhibition
[26 ]
Influenza A virus and type 1 HSV
[27 ]
Raspberry, Cloudberry, Strawberry
Ellagitannins
Certain strains of Salmonella , Helicobacter , Staphylococcus , E. coli , Clostridium , Campylobacter and Bacillus (gram-negative intestinal bacteria)
[28 ]
Mango Kernel
Gallotannins
Bacillus subtilis,
Clostridium botulinum
B. cereus , C. jejuni , L. monocytogenes,
S. aureus , E. coli , and Salmonella enterica (food borne bacteria)
[29 ]
Ocotea odorifera
Ellagitannin
Candida parapsilos
[30 ]
Ellagitannins
HIV infection
[31 ]
[32 ]
HSV-1, HSV-2, and EBV
[33 ]
Antibacterial Action
The antibacterial action of flavonoids has been well researched and documented. Plants
extracts used since centuries have been studied and screened for antibacterial properties.
Certain preparations containing a high concentration of flavonoids have shown antibacterial
activity. Among these, specific structures or classes of flavonoids have been isolated
such as apigenin, epigallocatechin gallate, galangin, luteolin, quercetin glycosides,
kaempferol derivatives. These phytochemical compounds have a wide range of inhibitory
actions against bacterial cells. They are of three broad types: (i) inhibition of
nucleic acid synthesis: Flavonoids that displayed this activity were robinetin, myricetin,
and epigallocatechin. The B ring on the flavonoids has been suggested to play a role
in the hydrogen bonding of the nucleic acid bases that explains the inhibitory effect
on DNA and RNA synthesis. The antibacterial action of quercetin has been attributed
to the inhibition of DNA gyrase. (ii) Inhibition of cytoplasmic membrane function:
sophoraflavanone G has shown an antibacterial activity by reducing the fluidity of
the membrane of the bacterial cells. Epigallocatechin gallate, a flavonoid found commonly
in green tea has exhibited strong antibacterial activity. It may disrupt the lipid
bilayer by disrupting them or causing fusion of the cell membrane layers. Galangin
has also shown to cause cell membrane damage while quercetin caused an alteration
of membrane potential and increased permeability of the inner cell membrane. (iii)
Inhibition of energy metabolism: Flavonoids hamper the energy metabolism in bacterial
cells by interrupting their oxygen consumption and by inhibiting NADH-cytochrome c
reductase.[34 ]
Anti-Viral Action
The importance of antiviral activity of flavonoids has risen in the past few years,
particularly due to the increased frequency of viral infections. Numerous in vitro studies have shown significant antiviral properties of flavonoids such as viral enzyme
inhibition, cytopathic effect, and virion number reduction. Flavonoids have also shown
potential inhibitory activity against coronaviruses, including the current pandemic
outbreak (COVID-19), Herpes Simplex Virus-1, Hepatitis A, B, and C viruses, and influenza
virus. The underlying molecular mechanisms of flavonoids involve the inhibition of
viral replication, increased interferon-γ levels, inhibition of virion production,
and virion assembly.[35 ] Flavonoids exhibit an inhibitory activity against SARS-CoV-2 by preventing the entry
of the virus into cells and their replication by targeting and binding to essential
proteins involved in the viral life cycle. These phytocompounds also showed remarkable
immunomodulatory activity including the inhibition of several inflammatory cytokines,
thus curbing the possibility of a cytokine storm.[36 ]
There are several studies showing the antiviral effects of flavonoids. Apigenin, luteolin,
quercetin, and quercetagetin have been found to exhibit an antiviral activity against
HCV. Flavonoids such as epigallocatechin gallate and chamaejasmin have the potential
to affect the Dengue virus replication, and even pinocembrin targets the molecular
machinery used by the Zika virus to replicate its own genome.[35 ]
Anti-Fungal Action
Fungal infections affect millions of people worldwide every year and can even be fatal.
These diseases are more prevalent in people with impaired immune systems such as patients
of AIDS, cancer, diabetes, and organ transplantation. COVID-19 patients have also
reported to develop fungal illnesses such as mucormycosis. There exist very few therapeutic
agents for fungal infections compared to bacterial infections. Additionally, drug
resistance in microbes is growing every day due to the elevated use of such antifungal
agents. Medicinal plants with bioactive compounds are thus being applied comprehensively
as an alternative and safer treatment modality. Flavonoids with antifungal activity
have been screened from plants by employing spore germination, broth dilution and
disk diffusion. They have shown to be effective against various species of pathogenic
fungi such as C. albicans, C. krusei, C. glabrata, T. deformans, and A. flavus . Several modes of action have been exhibited by flavonoids in inhibiting their growth.
These include plasma membrane disruption that causes reduction in fungal cell size,
leakage of intracellular components, and cell death. Flavonoids can also cause cell
wall deformations and mitochondrial dysfunction through various pathways. Furthermore,
fungal cell division, nucleic acid and protein synthesis, and efflux pumps are inhibited
by certain flavonoids as well. These flavonoids are effective even in synergetic combinations
with existing antifungal drugs.[37 ]
Flavonoid Sources
Understanding various sources of flavonoids is very essential for their health-promoting
effects. Flavonoids are available richly in plant and human diet products. Black rice
being the rich source of it, flavonoids are also seen in blackberries, blueberries,
propolis, honey, red beans, nuts, tea, and wine. Natural sources of flavonoids and
their classes and subclasses is essential in understanding and implementing these
products in daily dietary supplements ([Table 2 ]).
Table 2
Natural sources of flavonoids
Natural source
Plant product
Flavonoid class
Flavonoid subclass
Fruits
Apples
Flavonols
Quercetin, rutin, fisetin, kaempferol
Grape seeds/grapes
Flavonol
Quercetin, rutin, kaempferol
Flavonone
Narigenin
Anthocyanin
Anthocyanin
Citrus fruits
Flavonol
Quercetin,
rutin
Flavonone
Eriodictyol, hesperidin
Flavononol
Taxifolin
Flavone
Luteolin
Berries
Strawberries
Flavonol
Fisetin
Cranberries, blueberries
Anthocyanidin
Peonidin
Raspberries, blackberries
Flavonol
Kaempferol
Vegetables
Red pepper
Flavonols
Quercetin, myrecetin, Rutin
Cucumber
Flavonols
Fisetin, kaempferol
Broccoli
Flavonol
Kaempferol
Flavone
Luteolin
Lettuce, brussel sprouts, green beans, squash, spinach
Flavonol
Kaempferol
Carrots, Celery, green pepper
Flavone
Luteolin
Cereal
Rice bran
Flavone
Tricin
Legumes
Soyabean, fava beans
Isoflavone
Genistein
Chickpea
Isoflavone
Biochanin
Red clover
Isoflavone
Biochanin, genistein
French bean seeds
Flavonone
Abyssinones
Vetch
Flavone
Diosmetin
Nuts
Flavonol
Myricetin
Tubers
Onion
Flavononol
Fisetin
Sweet potato
Anthocyanidin
Peonidin
Miscellaneous
Milk, chocolate
Flavone
Apigenin
Tea/coffee
Flavonol
Rutin, myricetin,
Flavone
Luteolin
Catechin
Theaflavin
Red wine
Flavonol
Myricetin
Olive oil
Flavone
Luteolin
Nutraceuticals - Commercially Available in The Market
Nutraceuticals - Commercially Available in The Market
Natural sources of therapeutic agents have a higher preference over synthetic agents.
There is an ongoing search for natural agents, which can protect body from various
infections and diseases. This has given rise to various supplements rich in flavonoids
available for use. Details of flavonoid supplements available for use is enlisted
in [Table 3 ].[38 ]
[39 ]
[40 ]
[41 ]
[42 ]
[43 ]
[44 ]
[45 ]
Table 3
List of nutraceuticals-flavonoids available for use
Flavponoid type
Dosage
Use
Reference
Rutin
750 mg/day
Anticarcinogenic, neuroprotective, antioxidant, cardioprotective
[38 ]
Diosmin + hesperidin
1000 mg/day (3 months)
Chronic venous diseases, hemorrhoids
[39 ]
Hidrosmin
600 mg/day
Chronic venous diseases
[40 ]
Flavoxate
100-200 mg 3-4/day
Muscle spasms of the urinary tract
[41 ]
Genistein
30-60 mg/day
Anticancer, antimicrobial, anti-inflammatory, reduce symptoms of menopause
[42 ]
Silibinin
200-400 mg/day
Hepatoprotective
[43 ]
Galangin
Antibacterial, antiviral, antitumor, antidiabetic
[44 ]
Quercetin
500-1000 mg/day
Immunity booster, anti-inflammatory, anti-allergy
[45 ]
Conclusion
Flavonoids are potential agents in the prevention of ailments and diseases. In the
present scenario with ongoing COVID-19 pandemic, the emphasis has been on the prevention
of disease and reducing the risk of transmission.[46 ] While various agents, such as melatonin derivatives with antioxidant and immunomodulatory
properties have been assessed for their therapeutic potential against SARS-CoV-2,
the arrival of newer virulent strains has driven the attention of the researchers
towards the use of natural agents in building immunity.6 Flavonoids with their nutraceutical and therapeutic action and minimal or no adverse
effects, have gained significant role in the field of health and pharmacy. This review
paper highlights the modes of action of flavonoids against infectious agents with
their natural sources.[47 ]
