Functional Foods: A Promising Strategy for Restoring Gut Microbiota Diversity Impacted by SARS-CoV-2 Variants

1. Introduction

Since ancient times, functional or therapeutic foods have been used to treat and prevent many ailments. Many plants and herbs contain bioactive compounds such as tannins, polyphenols, flavonoids, terpenoids, etc., known to possess therapeutic or preventive properties [1]. Additionally, using probiotics as food supplements can help fight infections and support the body’s natural defense mechanism. Preventative measures against Covid19 have gained popularity, and various functional foods effectively strengthen the immune response to fight infection. The nutritional status of an individual is a key factor governing the extent of severity of SARS-COV-2 infection. Thus, supplementation of vitamins and minerals is helpful in the prevention and treatment [2]. Several functional foods of therapeutic value are beneficial in this regard. Several vitamins, like Vitamin C, D, and E, have been shown to promote an immune response against viral infections[3].

Furthermore, they function as potent antioxidants and help to reduce the levels of ROS and cellular stress. Minerals in the form of micro-minerals or macro-minerals are essential for many biological functions [4]. Supplementation of some minerals like Zinc has an immune-protective effect as it promotes autophagy, cytokine production, anti-viral function, and enhancement of anti-viral drugs’ efficacy. Apart from functional foods, plants derived small molecules or herbal formulations are found to be effective in reducing the detrimental effects of COVID-19 infection. Many of these have been a part of traditional medicine in various cultures. Isolation and characterization of their bioactive molecules have proved their protective effect in various clinical conditions. Notably, incorporating functional foods and herbal formulations as a therapeutic strategy against COVID-19 infection has benefits in terms of prevention through strengthening the immune system and diminishing the extent of infection.

The emergence of the SARS-CoV-2 virus, which causes the COVID-19 pandemic, has severely impacted public health and the world economy [5]. As the virus mutates, it produces different variants that vary in their ability to spread and the severity of the infections they cause [6,7]. Unlike the original strain, these variants are also affecting younger individuals. Additionally, the SARS-CoV-2 virus is more contagious than the SARS-CoV virus.[8]. The genetic makeup of SARS-CoV-2 has undergone considerable changes leading to variations compared to the first identified genetic sequence of Wuhan-Hu1 [9]. The identification and nomenclature of genetic sequence mutations are carried out using Phylogenetic Assignment of Named Global Outbreak (PANGO). The nomenclature employs Greek letters, i.e., Alpha, Beta, Delta, etc., to denote variants and a series of letters and numbers to assign lineages, i.e., BA.1.1, B.A.2, etc. [10]. Mutations in the SARS-CoV-2 receptor binding domain which has given rise to new SARS-CoV-2 variants, including Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), Delta (B.1.617.2), Cluster 5, B.1.1.207, B.1429/B.427, B.1.525, Omicron (B.1.1.529), Mu (B.1.621) and LAMBDA (C.37).

Based on their impact on public health, the emerging SARS-CoV-2 variants have been categorized by WHO into three groups: variants of concern (VOC), Variants of interest (VOI), and Variants under monitoring (VUM) [11]. The mutations in SARS-CoV-2 can be either deleterious or neutral, resulting in resistance to existing treatment strategies and showing higher infectivity or no apparent effect, respectively [12]. The viral phenotype is modified via a small set of mutations conferring an advantage regarding their pathogenicity and transmissibility. The antigenicity of mutant SARS-CoV2 modulates the immune recognition properties leading to their escape from the host immune response. SARS-CoV2 variants with higher accumulation of such mutations have shown enhanced transmissibility and are categorized under VOC. It has been shown that nearly 90% of plasma antibodies act against the spike receptor binding domain (RBD) [13].

Patients infected with COVID-19 commonly display respiratory symptoms, such as fever, fatigue, cough, shortness of breath, and abnormal chest X-ray results. Additionally, many COVID-19 patients experience gastrointestinal issues, such as nausea or vomiting, diarrhea, abdominal pain, and loss of appetite [14]. Studies have found that the balance of microbiota in the respiratory and gastrointestinal systems of hospitalized COVID-19 patients is disturbed. This disruption may make patients more susceptible to secondary infections of the respiratory and gastrointestinal tracts, which contribute to the severity of COVID-19 [15]. Therefore, the microbiota may play a crucial role in SARS-CoV-2 infection. The human body contains ten times more bacterial cells in its microbiota than human tissue cells and a hundred times more bacterial cells than human genes. These bacteria inhabit all areas of the human body, including the respiratory and gastrointestinal tracts, and are selectively allowed to colonize. The microbiota performs several vital functions in the human body, including the decomposition of indigestible proteins and carbohydrates, nutrient absorption and digestion, vitamin production, and the regulation of host immunity. The microbiota is also linked to various diseases and significantly impacts human health [16,17,18].

The management of Covid-19 infection is a major challenge in light of its serious implications on systemic functions and adverse effects on immunity. Apart from must be implemented to augment the body’s natural defense mechanism. In this regard, natural herbs and functional foods provide an excellent way to minimize the effect of the SARS-CoV-2 virus through various mechanisms like anti-viral functions, immune-boosting functions, antioxidant functions, and maintenance of gastrointestinal microbiota homeostasis.

There have been suggestions that probiotics could help reduce the spread of COVID-19, but there is currently a lack of evidence to support this [19]. However, this also highlights the need for further research and a better understanding of the microbiome. One major challenge is the potential for misinterpretation of data and unrealistic expectations for how quickly complex research can be translated into practical applications by society, scientists, and healthcare professionals.

So far, the use of probiotics in COVID-19 has mainly been supported by indirect evidence [20]. The current understanding is that the gut microbiome, which refers to the combined genetic material of the various microorganisms in the human gastrointestinal tract, can regulate and be regulated by viruses entering the body, leading to positive or adverse effects. Furthermore, certain beneficial bacteria in the gut can help defend against potential pathogens by interacting with human cells and promoting healthy immune function [21]. This suggests that consuming dietary microbes like probiotics or prebiotics could positively impact immune function during infections [22].

This review provides an overview of recent research on how COVID-19 affects the gut microbiota, including the specific bacteria involved and their impact on immunity and disease severity. Additionally, it discusses what is currently known about the potential for probiotics, prebiotics, and nutritional interventions to reduce susceptibility to COVID-19. Finally, the review examines several studies that have used plant-based probiotics to target coronavirus and considers implications for future research design and the potential role of precision nutrition in COVID-19.

2. What qualifies a food as "functional"?

Functional foods result from advancements in nutritional science that emerged during the same period as the baby boomer generation. Experiences of food rationing during wartime highlighted the importance of population-based strategies to prevent dietary deficiencies [23]. To address this, free school milk was introduced, and vitamins and minerals were added to certain foods to ensure people met their recommended intake. This led to the acceptance of fortified breakfast cereals as a healthy part of the lifestyle for a new generation of Australians. Thus, a new era of "functional foods" began, which refers to foods that provide sustenance and positively impact consumers' health.

Functional food science is the result of collaboration between different fields of science and the needs of the general public. It combines food science, nutrition, and medicine principles to create foods that straddle the boundary between food and pharmaceuticals. Researchers in this field study food's various components and potential health benefits. Functional food scientists utilize biomarkers or "indicators" present in the body to track changes in health and the body's ability to maintain equilibrium. This technique helps assess the effects of functional foods on health and determine the appropriate and safe dosage of these foods. Functional foods can be classified based on their intended health benefits or the ingredients that provide those benefits [24]. These classifications are not mutually exclusive; many functional foods may fit into multiple categories. Here are some common classifications:

• Nutraceuticals: Foods or food products that contain bioactive compounds (such as vitamins, minerals, or antioxidants) that have medicinal properties.
• Fortified/Enriched Foods: Foods that have additional nutrients added to them to increase their nutritional value (such as vitamin-D fortified milk or iron-enriched cereal).
• Probiotics: Foods that contain live beneficial bacteria that help improve gut health and boost the immune system.
• Prebiotics: Foods that contain non-digestible fibers that promote the growth of beneficial gut bacteria.
• Dietary Supplements: Products that contain one or more ingredients (such as vitamins, minerals, or herbs) intended to supplement the diet.
• Functional Beverages: Beverages that contain added nutrients, such as vitamins or minerals, or bioactive compounds, like antioxidants.
• Whole Foods: Certain foods like fruits, vegetables, nuts, and whole grains have inherent health benefits and can be considered functional foods when consumed as part of a healthy diet.

In the 1980s, functional food emerged in Japan as food scientists provided evidence of their products' beneficial effects on health [25]. These foods were given a special label, FOSHU or Food for Specific Health Uses, upon approval. Other countries and scientific organizations attempted to develop their definitions for functional foods, which resulted in high sales but also led to confusion among consumers about what the term functional food means. The Functional Food Center (FFC) has defined "functional food" as natural or processed foods that contain biologically active compounds, whether known or unknown [26]. When consumed in defined, and effective, these foods have clinically proven and documented health benefits for the prevention, management, or treatment of chronic diseases. This definition is distinctive in recognizing the importance of "bioactive compounds," which are biochemical molecules that enhance health through physiological mechanisms. Moreover, the definition emphasizes that bioactive compounds must be consumed in non-toxic amounts, as there are upper limits beyond which they can become hazardous. The main goal of the FFC is to establish a uniform definition of functional foods that will enhance the credibility of functional food research. Additionally, it is important for all the centre aims to promote the commercialization of functional foods, enhance global collaboration and communication, and ultimately improve the health of the population.

3. SARS-CoV-2 infection in the gastrointestinal tract

Angiotensin-converting enzyme 2 (ACE2) is present in several body tissues, such as the cardiovascular system, kidneys, testis, and the gastrointestinal tract, including the colon, cecum, duodenum, ileum, and jejunum. This implies that SARS-CoV-2 can affect the gastrointestinal tract, leading to irregular intestinal secretion, malabsorption, and other gastrointestinal symptoms [27,28]. The gut virome can affect the host's health or disease state by influencing the immunophenotype rather than acting as a pathogen. The detection of SARS-CoV-2 RNA in the stool of COVID-19 patients suggests that the virus can spread through the fecal-oral route [29]. The virus was found in various parts of the gastrointestinal tract, and the nucleocapsid protein was found in the stomach, rectum, and duodenum glandular epithelial cells. In some cases, viral RNA was still detectable in feces even after respiratory tests were negative, indicating that the virus was still replicating in the gastrointestinal tract, and fecal-oral transmission could continue even after respiratory clearance [30,31,32,33,34].

3.1. Gastrointestinal complications in COVID-19

COVID-19 patients have been found to have bowel wall thickening with hyperemia, mesenteric thickening, fluid-filled large bowel, and sometimes ischemia and pneumatosis, as shown by imaging techniques [35]. In addition to fever and respiratory symptoms, COVID-19 patients commonly experience digestive symptoms such as nausea, anorexia, vomiting, and diarrhea. Some COVID-19 patients experience only digestive symptoms, while others may have these symptoms before developing respiratory symptoms. Systematic reviews and meta-analyses have shown that gastrointestinal symptoms occur in 2-21% of COVID-19 patients, with the most common being diarrhea (8-17%), nausea or vomiting (4-20%), and loss of appetite (2-21%) [36]. Dysregulation of intestinal ion transporters due to SARS-CoV-2 infection may cause infectious diarrhea and malabsorption, leading to inflammation and gastrointestinal symptoms. Patients infected with SARS-CoV-2 commonly experience diarrhea along with fever and cough. Animal studies have shown that changes in the ACE2 enzyme can lead to susceptibility to diarrhea and inflammation in the colon. In a retrospective analysis of 84 SARS-CoV-2 patients in Wuhan Union Hospital, viral RNA was detected in the fecal samples of all patients with diarrhea. A statistical analysis of 35 studies showed that 15% of COVID-19 patients experienced digestive symptoms like diarrhea, vomiting, nausea, and lack of appetite [37]. Gut dysbiosis is thought to be the primary cause of diarrhea in COVID-19 patients. However, further research is needed to understand the mechanisms underlying viral infections causing diarrhea and the relationship between respiratory and gastrointestinal symptoms [38] (Figure 1).

3.2. Alteration of gut microbiota upon SARS-CoV-2 infection

Table 1 demonstrates that the human gastrointestinal tract and respiratory tract host microbiota, which is a community of microorganisms numbering in the trillions. This microbiota is crucial for maintaining metabolic homeostasis and facilitating immune maturation [39]. A "healthy" microbiome interacts functionally and continuously with the host organism through surface antigens and microbial metabolites. This interaction provides necessary molecular signals to fine-tune immune responses in various body parts, including the respiratory system, and protect against disease [40]. The gut microbiota can influence the maturation, development, and function of immune cells and the activation of peripheral immune cells, including cellular and humoral immunity [41]. Disrupting gut barrier integrity and releasing proinflammatory cytokines into the circulatory system activate innate and adaptive immune cells, leading to systemic inflammation. The gut microbiota is disrupted by entering inflammatory cells, such as neutrophils and lymphocytes, into the intestinal mucosa [42].

The composition of gut microbiota changes after viral infection. The severity of the disease is linked with an increase in the concentration of inflammatory cytokines and blood markers, such as C-reactive protein, lactate dehydrogenase, aspartate aminotransferase, and g-glutamyl transferase [43]. A viral infection can cause intestinal wall permeability and malabsorption by enterocytes. Patients infected with SARS-CoV-2 had elevated fecal calprotectin levels, indicating intestinal inflammation [44]. A recent systematic review showed that COVID-19 patients had a depletion in the genera Ruminococcus, Alistipes, Eubacterium, Bifidobacterium, Faecalibacterium, Roseburia, Fusicathenibacter, and Blautia, as well as the enrichment of Eggerthella, Bacteroides, Actinomyces, Clostridium, Streptococcus, Rothia, and Collin-sella as shown in Figure 2 [45].

Respiratory dysbiosis can also influence the digestive tract through immune control, and the composition and function of the gut microbiome can affect the respiratory tract through the underlying mucosal immune system. [46]. In patients with COVID-19, changes in the gut microbiome are associated with disease severity and poor prognosis. Specifically, there are increases in certain bacteria, such as Bacteroides, Parabacteria, Clostridium, and reductions in others, such as Roseburia, Eubacterium, and Faecalibacterium. The Ascomycota phylum indicated higher relative abundance and decreased diversity and richness in the fungal gut microbiota of severe/critical COVID-19 patients. [47,48]. Additionally, patients with severe SARS-CoV-2 infection had significantly lower bacterial diversity and lower relative abundances of certain beneficial bacteria in the gut microbiome. Therefore, correcting the gut microbiota during the COVID-19 pandemic may help improve population immunity and protect public health [49].

The severity of COVID-19 is linked to the relative abundance of certain bacteria in the gut microbiome. While Faecalibacterium prausnitzii, which contains anti-inflammatory characteristics, exhibited an inverse association with disease severity, Coprobacillus, Clostridium ramosum, and Clostridium hathewayi each showed positive correlations with disease severity. [50]. Moreover, fecal samples with high SARS-CoV-2 infectivity contained more opportunistic pathogenic bacteria (such as Collinsella species and Morganella morganii) and fewer bacteria that produce short-chain fatty acids (like Parabacteroides merdae and Lachnospiraceae bacterium) compared to samples with low SARS-CoV-2 viral infectivity [44].

F. prausnitzii is a type of bacteria that lives in the human body without causing harm and has been found to have anti-inflammatory effects in clinical studies [51,52]. By secreting bioactive chemicals, it can prevent intestinal epithelial cells from activating nuclear factor-kB and producing IL-8. Additionally, F. prausnitzii can stimulate the secretion of IL-10, which inhibits the production of proinflammatory cytokines and enhances the suppressive activity of Tregs in the mucosa [53]. This bacterium is an important contributor to the body's defense mechanisms, and its depletion has been observed in COVID-19 patients. During hospitalization, the abundance of certain intestinal microorganisms showed a reverse trend compared to the load of SARS-CoV-2 in fecal samples of patients. Coprobacillus was found to upregulate the expression of ACE2, while Bacteroides thetaiotaomicron, Bacteroides dorei, and Bacteroides massiliensis were found to downregulate ACE2 expression in the gut of patients. Overall, these findings suggest that harmful bacteria and the absence of beneficial bacteria in the gut are closely related to the severity of COVID-19 [54,55].

3.3. Loss of gut microbial diversity and associated risk factors in COVID-19 patients

Risk factors and comorbidities associated with COVID-19 include dementia, Alzheimer's disease, male gender, smoking, hypertension, or asthma [55]. All these risk factors and comorbidities are associated with changes in the gut microbiome. These changes lead to alterations in the immune system and increase the susceptibility to suffering from the adverse effects of CoV [56]. The microbial diversity reduces as aging occurs, which causes severe inflammatory conditions. So, SARS-CoV-2 infection seems more perilous in older people and people with diseased conditions. Although the evolution of gut microbiota during human life is similar to older people's gut microbiota, infant gut microbiota also shows decreased diversity [57]. The microbiota in infants remains unstable in the initial years until it changes to adult microbiota [58]. In a healthy individual, the gut microbiota is believed to be stable throughout life, and its diversity remains high [59]. In old age or diseased individuals, gut microbiome diversity diminishes, and gut dysbiosis rises, associated with mental impairment, inflammation, and depression [60]. In older adults, it was also identified that the gut microbiota contains reduced levels of specific bacterial groups. Nonetheless, bacterial groups such as Lactobacillus, Bacteroides, and Clostridium show recurrent alterations [61].

It has been stated that more COVID-19 fatalities are associated with older individuals due to the following reasons (1) lessened microbial diversity, (2) weak immune system, and (3) systemic inflammation. Notably, gut microbiota diversity decreases with aging, so COVID-19 has become a fatal infection in most elderly patients [17]. Besides older age, obesity is also correlated with changes in intestinal microbiota, which serves as one of the risk factors for the severity of SARS-CoV-2 infection. Intriguingly, 25% of the patients who died from COVID-19 infection in the United States were found to be obese [62]. Adipose tissue serves an essential role in systemic immune activation and widespread COVID-19. Adipocytes show a heightened level of ACE2 expression in obese individuals, and eliminating existing inflamed adipose tissue reduces the viral entry and systemic viral spread [63].

Moreover, adipose tissue shows irregulated lymphoid and myeloid responses in individuals with obesity, which is correlated with improper regulation of cytokine profiles [64]. Furthermore, obese individuals show increased levels of chemerin, a proinflammatory cytokine, and leukotriene, worsening the cytokine storm risk [65]. Additionally, COVID-19 patients with diabetes have a high chance of facing severe complications accredited to gut metabolite dysfunction and systemic inflammation [66]. Likewise, SARS-CoV-2-infected patients with cardiovascular diseases, hypertension, nervous disorders, and other diseases may worsen the complications of COVID-19 due to an imbalance in the intestinal flora and reduced microbial diversity [67]. Pre-existing diseases like cancer, cardiovascular disorders, lung problems, hyperglycemia, and diabetes increase the mortality risk during COVID-19 and influenza viral infections. These morbidities pave to gut dysbiosis, circadian dysregulation, and mitochondrial dysfunction [68]. There is proof that the microbiome in the gastrointestinal tract impacts the metabolic, digestive, and immune systems, host sleep, mental states, and circadian rhythms via the microbiome-gut-brain axis [69]. It was found that microorganisms and circadian genes are associated. Therefore, psychological status, good sleep duration, and quality are essential to cope with the COVID-19 pandemic. Nevertheless, various drugs exert adverse effects by destroying the gut and lung microbiome integrity to worsen the potentiality to overcome COVID-19 infection and other viral infections and pathogenicity [70].

4. Therapies in the management of gut-related symptoms in COVID-19 patients

COVID-19 patients with gastrointestinal symptoms, such as diarrhea, vomiting, etc., cannot be cured entirely with antidiarrheal and antiemetic drugs. Hence, regular monitoring of potassium levels and sufficient rehydration solutions must be given to save patients' lives [71]. Several therapeutic measures have been developed after the advent of Covid, based on various targets, as depicted in Figure 3. Interestingly, the SARS-CoV-2 infection can be eliminated initially using novel therapeutic approaches, including activated epithelial receptors. The immunomodulation of the host's innate immunity activates these epithelial receptors [72]. In addition, the intake of complex polysaccharides, proteins from plants, omega-3 fatty acids, polyphenols, and essential micronutrients are associated with increased beneficial bacterial quantity, stimulating SCFA production [73].

Probiotics are highly recommended to help alleviate gut-related symptoms like nausea and abdominal pain and increase gut microbial diversity [74]. In addition, the consumption of probiotics can help prevent severe COVID-19 clinical manifestations primarily related to the digestive tract [75]. Aged persons and comorbidities patients may benefit significantly as their gut microbiota is highly susceptible to gut dysbiosis [76].

4.1. Influence of prebiotics and probiotics on SARS-CoV-2 infection

The role of prebiotics in regulating intestinal microbiota is widely recognized. Prebiotics refer to special fibers that contain fermenting ingredients necessary for gut microbes. These fibers selectively stimulate beneficial indigenous probiotic bacteria' growth and/or activity. The use of prebiotics as a form of treatment has been linked to the improvement of gastrointestinal conditions like constipation and diarrhea, as well as non-gastrointestinal ailments such as decreasing the chances of osteoporosis, atherosclerotic cardiovascular diseases linked with dyslipidemia, insulin resistance, obesity, and conceivably type 2 diabetes [77]. Studies conducted on mice indicated that inulin, a dietary fiber, protects against influenza A by affecting specific immune and T cells' production. Traditional Chinese medicine has also been used to prevent, treat and aid in the recovery from COVID-19, with a particular treatment called the "Lung Cleansing and Detoxifying Decoction" showing promise in relieving symptoms in 90% of patients after a 3-day course [78]. Prebiotics, a dietary fiber that selectively stimulates the growth of beneficial gut bacteria, include carbohydrate polymers like oligosaccharides and polysaccharides. Polysaccharides have been found to possess immunomodulatory properties, which are significant in enhancing cellular immunity and fighting viral infections [79]. The growth and function of probiotics are closely linked to prebiotics.

Gut microbes, including probiotics, break down different types of prebiotics like fructan, glucan, and arabinoxylan. This process produces short-chain fatty acids (SCFAs) that have been shown to regulate the host immune system. SCFAs affect the host immune response by activating G protein-coupled receptors (GPCRs) through specific receptors with varying degrees of intensity [80]. T cells, macrophages, and dendritic cells' ability to differentiate or carry out particular functions may all be influenced by the butyric and propionic acids that the gut microbiota produces through prebiotic fermentation. (Figure 4).

Initially, it was believed that the positive effects of probiotics, defined as live microorganisms that provide health benefits to the host when administered in sufficient amounts, were due to their ability to improve the balance of microbes in the intestine. However, recent evidence suggests that probiotics can also offer advantages by regulating the immune system of the host [81]. Probiotics have been demonstrated to influence the T cell subsets [82], stimulate the production of anti-microbial peptides by Paneth cells [83], and trigger Th17 cell differentiation in the small intestine [84]. To identify and design effective treatments, it is crucial to comprehend the basic immunological mechanisms that underlie the clinical effects of SARS-CoV-2 infection. Certain probiotics have been found to have antiviral properties, including against other types of coronaviruses, according to laboratory research [85]. Researchers recently found that Paenibacillus bacteria naturally produce carboxypeptidases structurally and functionally similar to ACE2 [86]. Preventive intranasal administration of the probiotic Lacticaseibacillus rhamnosus GG was demonstrated to impact Toll-like Receptor (TLR4) signaling in a neonatal mouse model of influenza infection. [87]. Notably, LL-37, a human cathelicidin, is an anti-microbial peptide with broad-spectrum activity against viruses and bacteria [88] and can be effectively delivered by food-grade probiotics such as Lactococcus lactis expression system [89]. Using cathelicidin-expressing Lactococcus lactis as a probiotic delivery system may offer new opportunities for combatting SARS-CoV-2 infection through protective immunomodulatory effects. The WHO recently released new guidelines stating that mild to moderate COVID-19 patients should not receive antibiotic therapy or prophylaxis unless there is a suspected bacterial infection. However, using antibiotics in the early phase of COVID-19 can lead to a disrupted microbiota, increasing susceptibility to secondary infections. Therefore, using probiotics to reinforce the gut microbiota has been proposed as a possible way to reduce the risk of subsequent infections. However, additional clinical studies are required to determine the most appropriate strain(s), dose schedules, and timing of probiotic intervention for SARS-CoV-2 infection.

4.2. Probiotics modulate gut microbiota

The National Health Commission and National Administration of Traditional Chinese Medicine advised incorporating a probiotic diet and edible products to treat patients with COVID-19 [90]. Probiotics are evolved from a live culture that enhances the balance of gut microbiota diversity to specific effects with the capability of immune-modulation effects [91,92]. Many desirable therapeutic aspects are considered for designing a probiotic product, as shown in Figure 5. Dairy foods are present-day common food carriers to supply probiotics. New insights into selecting probiotic food products and bacterial selection potential toward probiotic nature should be carried out by analyzing the bacteria Field's genotypic and phenotypic characteristics [93].

Besides the analysis, a deeper study of host-gut microbial crosstalk is necessary for personalized treatment. The disclosure of metabolic activities of bacterial strains in the gut is the top way to determine the dynamics of the gut ecosystem [94,95]. The bacterial strains mostly belonging to the genera Lactobacillus and Bifidobacterium possess beneficial characteristics and act as potential sources for probiotics. Some of these strains showcase effective anti-inflammatory characteristics [96]. Probiotics are constituents of edible products that benefit the host when consumed properly and suitably. More probiotic effects are present in plant-based fibers, which promote beneficial bacteria such as Lactobacillus and Bifidobacterium species [97,98]. At the same time, they reduce the growth of harmful pathogens, e.g., Clostridium species. Intake of fiber-rich foods reduces fatality risk from respiratory and infectious diseases by 40% [99]. Whole grains have sufficient intestinal microbiota composition, which decreases (Tumour Necrosis Factor-α) TNF-α, C - reactive protein (CRP), and Interleukin-6 (IL-6) in the gut and circulation, which in turn reduces systemic inflammation and intestinal inflammation [100]. The fiber content in legumes, fruits, and vegetables fermented by intestinal flora and the beneficial metabolic compounds formed after fermentation have anti-inflammatory properties [101].

The probiotics listed in Table 2 are categorized by their mechanism of action within COVID-19 patients. The SCFAs, such as butyrate, acetate, and propionate, formed by the microbes in the gut, have anti-inflammation properties. SCFAs enhance the synthesis of anti-inflammatory cytokines and suppress the synthesis of proinflammatory cytokines by binding to the receptors of immune cells [102,103]. Nowadays, research has focused on investigating the correlation between gut bacterial molecular characteristics and their impacts on individual health conditions. Indeed, many significant challenges are stacked to elucidate the molecular bases for the interaction-mediated systemic effects [68]. Priority should be given to safety concerns in manipulating bacterial strains and probiotic products for preventive and therapeutic interventions [104].

4.3. Natural small molecules and functional foods against COVID-19 Infection

Various herbal formulations derived from plants of therapeutic value have been tested for their efficacy against covid infection. Phytochemicals comprise different bioactive primary and secondary metabolites with well-known potent pharmaceutical properties. Natural compounds such as quercetin, scutellarin, myricetin, etc., are effective against coronavirus infection and lead to further studies [107,108,109,110].

Echinacea purpurea is an herbal medicine used to treat respiratory infections by Native Americans [111,112]. It consists of compounds like chicoric acid, caffeic acid, polysaccharides, and alkylamides and is used as extracts, teas, sprays, and tinctures [113]. Echinacea purpurea has been reported to be effective against viral infections, especially enveloped viruses. Since SARS-CoV-2 is an enveloped virus, Echinacea purpurea can be useful as a therapeutic against covid infection.

Turmeric is well known for its medicinal properties due to the presence of bioactive compounds classified as curcuminoids, such as curcumin, demethoxycurcumin, and bisdemethoxycurcumin [114]. It has been reported to possess antioxidant, anti-inflammatory, and anti-cancer properties [115,116]. Curcumin is effective against covid induced anosmia (loss of smell) and ageusia (loss of taste). The possible mechanism for the action of curcumin in relieving anosmia and ageusia may involve binding to Mpro, a protease responsible for the production of proteins involved in viral replication. Furthermore, it can inhibit the complex formation between SARS-CoV-2 spike protein and ACE2 impeding viral entry [117].

Quinine alkaloids obtained from the bark of Cinchona trees have been used as anti-malarial drugs. Evidence gathered from several studies suggests its efficacy against viral infections and can be a potent candidate for treating covid infections [118,119].

Curcuma xanthorrhiza, commonly known as Java turmeric, originates from Indonesia. It is a food additive and medicine due to its anti-microbial, anti-inflammatory, hyperglycemic, anti-hypertensive, anti-cancer, and anti-cancer nephron-protective properties [120,121,122,123,124]. The bioactive ingredients in Xanthorrhizol consist of curcuminoids, xanthorrhizol, camphor, and volatile oils [125]. Xanthorrhizol has immunosuppressant properties and can inhibit proinflammatory cytokines, which can be effective against Covid associated cytokine release syndrome (CRS) [126]. However, clinical studies for its efficacy are not yet available, and it is suspected that immunosuppression can potentially cause more harm than benefit in case of Covid infection.

Polyphenolic compounds such as flavonoids, tannins, and rosemerininc acids are plant-derived bioactive molecules having an aromatic ring with one or more hydroxyl groups. Polyphenols have been reported for their efficacy against diseases like cancer and neurodegenerative disorders owing to their anti-inflammatory properties and ability to interact with multiple target proteins of etiological importance [127,128,129,130]. These compounds can interact and inhibit proteins involved in viral replication and can be beneficial as a treatment strategy against Covid infection [131,132,133,134].

Glycyrrhizin, a triterpene saponin derived from licorice roots and is known to have therapeutic value due to its anti-inflammatory, anti-microbial, and antioxidant functions. It can bind to ACE2 and has been reported to be effective in combining boswellic acid in treating Covid [135].

Quercetin is a polyphenol of the flavone class of compounds known to possess antioxidant, anti-inflammatory, immuno- protective, and anti-viral properties. It is naturally present in various vegetables and fruits, such as onions, berries, apples, cilantro, etc [136]. It can hinder the interactions of SARS-CoV-2 spike protein with ACE2, thus inhibiting the entry of the virus into the host [137].

Alkaloids represent a large group of secondary metabolites mainly derived from flowering plants, fungi, and bacteria. The broad class of alkaloid compounds includes quinolines, purines, imidazoles, indoles, etc. [138,139]. Alkaloids exhibit multiple therapeutic properties in the form of antioxidant, anti-viral, anti-fungal, and anti-bacterial functions. Various studies demonstrate its action against Covid infection as an individual drug molecule or in combination with other potential therapeutic molecules like caffeine, emetine, colchicine, etc. [139]. However, further studies are required to explore its role as a therapeutic molecule against Covid infection.

Functional food comprises vital components, including multivitamins, which function as cofactors in various biological pathways and ensure proper function [140]. Vitamin D plays a crucial role in immune functions. It is known to mediate the induction of anti-microbial peptides such as cathelicidin, LL37, and defensins [141]. It also induces monocyte differentiation and antibody formation, promotes macrophage-mediated phagocytosis, and inhibits lymphocyte proliferation. Further, it plays a role in anti-inflammation by reducing proinflammatory cytokine expression while enhancing anti-inflammatory mediators' production by macrophages [142]. It has been reported that Vitamin D decreases the expression of DPP4/CD26, a receptor involved in interaction with SARS CoV-2 spike protein. It also reduces sustained interferon-γ and the level of interleukin 6, which helps improve pneumonia and Covid patients [143].

Vitamin C is well known for its antioxidant functions and role in supporting innate and adaptive immunity [144]. It can reduce the susceptibility to infections and restores stress response functions. It also prevents the formation of neutrophil extracellular traps responsible for organ damage in Covid patients. Vitamin C relieves the complications of thrombotic or thromboembolic diseases, commonly observed in Covid patients [145,146].

Minerals are required for various important biological functions, and their optimum quantity should be supplied in the diet or supplements. Elements such as Zinc, Iron, Copper, and Selenium are required in micro-quantities, while Sodium, Potassium, Phosphorous, and Magnesium are needed in greater amounts [147,148]. Most trace elements function as co-factors in various enzymatic functions but act as a direct modulator by binding to proteins [149]. Zinc is known to possess anti-viral action by causing hindrance in viral attachment. It has also been associated with immune functions by playing a role in the differentiation and development of immune cells. A deficiency of Zinc is linked with hampering immune functions such as reduction in antibody formation, low cytokine production, thymus atrophy, and reduced natural killer cell activity [150].

Nutraceuticals include food components known to have therapeutic actions against various ailments like diabetes, cancer, and neurological disorders [2]. These are derived from natural herbs, dietary supplements, and processed food. Incorporating nutraceutical products into the diet improves overall health and protects against chronic illness [151]. Ginseng is a traditional medicine that has immunomodulatory functions and anti-inflammatory action. It is beneficial in respiratory diseases and relieves the common cold and flu. However, there is no current evidence to establish its benefits from Covid infection [152,153].

Omega-3 fatty acids have anti-inflammatory and immunomodulatory functions, which help reduce infection severity. They are generally required for cellular health and promote optimum cardiovascular, pulmonary, and endocrine functions [154,155].

5. Discussion and Conclusion

Although Covid-19 is primarily known for causing respiratory symptoms, it can also impact the digestive system by binding to ACE2 receptors and causing an imbalance in gut microbiota, known as dysbiosis. Dysbiosis is linked to Covid-19 in older individuals and those with the virus. Antibiotics can cause inflammation in the intestines, which can be significant. The immune response in the gut can be influenced by bacterial metabolites and fragments, thus affecting the presentation of COVID-19. Short-chain fatty acids (SCFA) in fiber-rich diets can protect against inflammation in the intestines. Probiotics can help improve gut dysbiosis by modulating the immune system. Personalized dietary strategies should be used in conjunction with traditional therapies. The gut microbiota can be influenced by diet and is malleable. Research could investigate the effects of COVID-19 variants on the gut microbiota and how bacteria, fungi, and phages create an environment in the stomach for COVID-19. Before and during SARSCoV-2 infection, gut dysbiosis can affect disease progression and clinical outcomes in a self-reinforcing cycle.

The composition of the gut microbiota in COVID-19 patients during their hospitalization is associated with the levels of cytokines, chemokines, and inflammation markers in their plasma. This indicates that the gut microbiota may influence the severity and course of disease by influencing the host's immune response. According to recent immunological studies of COVID-19 patients, the depletion of particular bacterial species was associated with increased TNF-, CXCL10, CCL2, and IL-10. This suggests that these bacteria play a role in preventing excessive inflammation. Several drugs used to treat COVID-19 may interfere with the normal functioning of the gastrointestinal (GI) tract and lead to various clinical symptoms that could further exacerbate the risk and severity of GI symptoms in Covid-19 patients, such as nausea, vomiting, gastroparesis, and gastric ulcers, through various molecular pathways and mechanisms.

The respiratory infection driven on by SARS-CoV-2 affects the gastrointestinal tract microbiome, whereas respiratory infections affect the respiratory tract bacteria and immune system. However, gastrointestinal infections are often neglected. The SARS-CoV-2 immune response is influenced by the SARS-CoV-2 microbiome, part of the GI tract microbiota. Supporting this theory, depleted gut commensals such B. adolescentis, F. prausnitzii, E. rectale, R. (Blautia) obeum, and D. formicigenerans have been associated to decreased host inflammatory response in several inflammatory-related disorders.F. prausnitzii can activate human colonic regulatory T cells that produce IL-10, which is an anti-inflammatory cytokine. E. rectale has been associated with reduced inflammation in Alzheimer’s disease, while B. adolescentis can block the activation of nuclear factor κB, a promoter of proinflammatory cytokines. A microbial-mediated immunological dysregulation is supported by the prevalence of Ruminococcus gnavus, Ruminococcus torques, Bacteroides dorei, and Bacteroides vulgatus in COVID-19 patients. Whereas B. dorei and B. vulgatus are correlated to inflammatory gut disorders like irritable bowel syndrome and ulcerative colitis, R. gnavus and R. torques have been linked to inflammatory bowel disease. It is currently unclear whether the inflammatory gut microorganisms that are more common in COVID-19 patients are actively involved in causing the disease or if they are thriving due to the absence of other beneficial gut microorganisms. The increased frequency of virus infections has allowed SARS-CoV-2 to mutate and develop new variants (such as alpha, beta, gamma, and delta) with higher transmission rates and can cause damage to the endothelial lining of blood vessels or disrupt the immune system. As a result, gut microbial dysbiosis may exacerbate the gastrointestinal injury. This raises questions about the potential relationship between microbial diversity and health and the emergence of COVID-19 variants. The current treatment for COVID-19 with numerous variants relies on symptomatic approaches, and considering gastrointestinal problems associated with this drug is vital. A novel potential strategy using probiotics, gut microbiota metabolites, and dietary products to enhance the gut microbiota composition in COVID-19 patients can be of great significance. Finally, the role of the lung microbiota may also be investigated. These investigations will help us understand SARS-CoV-2 variations and their effects on gut dysbiosis and probiotic regulation of the intestinal microbiota to improve COVID-19 patient health. Furthermore, the available therapeutics against Covid must include modes of preventive measures apart from treatment regimes.

Traditional herbal medicines and functional foods are promising to enhance innate and adaptive immune functions and provide extended protection against COVID. Many herbal formulations and functional foods have been studied for their efficacy against COVID, and they show potential to be developed further for safe and effective therapeutic use. Several functional foods have demonstrated antiviral activity, an important aspect of their potential benefits against COVID infection. Most functional foods have a potent immune-boosting effect, including small molecules like polyphenols, flavonoids, and curcumin, vitamins such as Vitamin C, E, and D, and elemental micronutrients such as zinc. Furthermore, functional foods have demonstrated health benefits in various chronic conditions, making them especially beneficial for patients with conditions like osteoporosis, obesity, diabetes, and other metabolic diseases, which make them more vulnerable to the adverse effects of SARS-CoV-2. However, the health benefits of functional foods against COVID require scientific claims and optimizations to establish their role in therapeutics. Additionally, there is a need for toxicological evaluation to determine the optimal dosage and safe administration of these functional foods.

6. Study highlights

There is no proof of SARS-CoV-2 mutations and how they impact gut dysbiosis, but many studies have suggested a connection between COVID-19 and an imbalance in the gut flora. We investigated the effects of the various SARS-CoV-2 variants (Alpha, Beta, Gamma, Delta, and Omicron) on gut microbiota dysbiosis in COVID-19 patients. Additionally, we explored dietary interventions, such as probiotics, that could alleviate gut dysbiosis in COVID-19 patients. While traditional herbal remedies and functional foods may offer some advantages, more research is necessary to understand how they work and any drawbacks. Therefore, further research is required to fully comprehend the interaction between the virus, host, and microbiome and develop more effective therapies.