The Possible Role of Probiotics in Combating COVID-19

Since first reported in Wuhan in the end of 2019, COVID-19 infection has spread globally. However, our battle against COVID-19 is still limited with treatment modalities, yet giving a big challenge for researchers to explore promising treatment candidate. Probiotics, based on previous studies of its role in immune system and respiratory tract infection, seem to be potential candidate. Probiotics work in a distinct way through connectivity between gut and the lung, since both have expression of angiotensin converting enzyme2 (ACE2) of which highly bound by COVID-19. At present, no guidelines formally include probiotics as part of COVID-19 treatment. Hence, this study aimed to review the link of probiotics and immune system and its potential role as part of COVID-19 treatment.


INTRODUCTION
A wave of pneumonia with undetermined cause initially detected in December 2019 in Wuhan so called COVID-19. 1 SARS-Cov-2, the virus, is a member of the family Coronaviridae with Betacoronavirus as its genera. It can be spread from person-to-person by respiratory droplets produced when infected people sneeze or cough. 2 Over 30,000 confirmed cases reported worldwide in early February 2020, 1 and WHO declared COVID-19 as a pandemic on March 2020. 3 In less than five months COVID-19 had infected over 2 million people worldwide. 4 The fast-paced infection pushes a great number of studies. Several trials consider corticosteroid, arbidol hydrochloride, chloroquine phosphate, and hydroxychloroquine to treat COVID-19. However, those treatments limited with small number of subjects and risk of adverse effects. 3 Another treatment taken into consideration is probiotics. Even though it is still early to suggest that probiotics might cure severe SARS-Cov-2 infections, it is undeniable that probiotics might have some beneficial effects in fighting COVID-19. [4][5][6] Probiotics are known for their anti-inflammatory effects in maintaining the balance of intestinal microecology and preventing secondary bacterial infections in COVID-19 patients. 5 The strains contained in probiotics change the balance between immunoregulatory and proinflammatory cytokines. 6 The action permits viral clearance while lowering the damage of immune response-mediated to the lungs. 6 Furthermore, several previous studies reported that probiotics may have a significant effect in reducing the incidence of ventilator-associated pneumonia, nosocomial infections and respiratory tract infections. 7-10 Therefore, their role in fighting COVID-19 is worthy of consideration.
This review aimed to elaborate the relationship between probiotics and the immune system and its role in COVID-19, including its safety aspects.

PROBIOTIC AND IMMUNE SYSTEM
Probiotics defined as microorganisms that may be beneficial to health when consumed in adequate amounts. 11 Human has various microorganisms residing in the intestine, mainly composed of Firmicutes, Bacteroides, and in lesser amount of Proteobacteria, Actinobacteria, Fusobacteria and Verucomicrobia .11,12 The main role of gut microbiota in immune system is immunomodulatory and antiinflammatory functions. 13 Probiotics regulate immune homeostasis by modulating the immune response and regulatory T cells. 13,14,15 Probiotics are able to decreasing intestinal inflammation by suppress TLRs expression, produce metabolites that inhibit TNF-α and inhibit NF-kB. 13,14,16 Probiotics might enhance immune system by stimulating production of secretory immunoglobulin A (sIgA), which has been known as the first defense line against pathogens .13,15

PROBIOTICS' ROLE IN RESPIRATORY TRACT INFECTION
There are various kind of probiotics, of which, Lactobacillus and Bifidobacterium are the most common and extensively studied. 14,17-23

Genus Lactobacillus
Lactobacillus rhamnosus has a good effect in preventing viral respiratory tract infection, decreasing viral-related lung damage, maintaining gut barrier integrity, alleviating allergic disease, lowering risk of rhinovirus infection. 19,22 L. plantarum and L. salivarius might enhance host response to influenza vaccine, decreasing inflammatory reactions, increasing production of anti-inflammatory cytokines. 19,24,25 L. acidophilus is reported to be beneficial in reducing fever, rhinorrhea, cough incidence and antibiotic prescription in pediatrics patients. 26 Previous studies also demonstrated its usage for treatment of travelers' diarrhea and Clostridium difficile associated infection, decreasing the length of stay of children with acute diarrhea in hospital, and relieving irritable bowel syndrome. 19,22 L. paracasei (L.casei 431) has a good effect to relieve manifestation of upper respiratory tract infection and acts as immuno-modulator. Moreover, it enhances the innate viral defense system and decreases inflammation in the host. L. paracasei was showed to enhance the immune system in healthy people using a vaccination model. 20 A study showed that consumption of yogurt fermented with L. delbrueckii increasing activity of NK cell and diminishing common cold risk in elderly people. It also may play role in preventing respiratory tract infections caused by influenza virus. 19,20 L. reuteri has immunomodulatory effect and showed to decrease short-term sick leave caused by respiratory or gastrointestinal infections. 19,20 Genus Bifidobacterium B. animalis might increase IgA specific antipoliovirus significantly. The combination of B. animalis and L. acidophillus showed to reduce fever, rhinorrhea, and cough incidence. 20,[26][27][28] Study by Rerksuppaphol demonstrated B. bifidum, together with L. acidophillus, significantlyreduced the syomptoms of common cold in schoolchildren. 29

PROBIOTICS IN COVID-19
The Gut-Lung Axis: Angiotensin-Converting Enzyme (ACE) 2 in Respiratory Tract and Gut The Renin-Angiotensin-Aldosterone system (RAAS) is a system that has long been known and is important for regulating sodium and blood pressure. 30,31 The sequence of events in the RAAS system is that the angiotensinogen produced by the liver is converted to angiotensin I by the action of renin produced by granular cells in the kidney. Angiotensin I is then converted to angiotensin II by angiotensin converting enzyme 1 produced by the capillaries of the lungs, angiotensin II then stimulates the release of aldosterone for sodium homeostasis. 32 Angiotensin II will later be broken down by angiotensin converting enzyme 2 (ACE2) into angiotensin 1-7. 33,34,35 ACE2 is now being a concern because of its association with the COVID-19. ACE2 is a membranebound peptidase with the majority of the protein comprising the extracellularly oriented NH2-terminal peptide domain including the catalytic location. 36 The highest levels of ACE2 expression is located in small intestine, testis, kidneys, liver, thyroid and adipose, and lowest level located in blood, spleen, bone marrow, brain, blood vessels, and muscle. Medium level of ACE2 expression are in lung, colon, liver, bladder, and adrenal gland expression levels. There is no different expression of ACE2 based on gender or age. 37 COVID-19 can be attached to ACE2 due to hydrophobic interactions and salt bridge formation with the spike protein of the virus. 38 Therefore, angiotensin II levels are increased compared to healthy people and directly proportioned to the viral load. There was evidence of ACE2 dysregulation and RAS imbalance, resulting in multiorgan damage from COVID-19 infection. 39 The attachment of COVID-19 causes excessive inflammation in the lungs, resulting fibrosis and pulmonary hypertension; meanwhile, it causes damage in gut blood barrier, gut dysbiosis and systemic inflammation. 40 To date, there is scarce of evidence of relationship between probiotic and COVID-19. Researchers all over the world are racing to conduct trials evaluating the benefit of probiotic in combating COVID-19. Gastrointestinal symptoms might be the presenting symptoms in COVID-19 patient as reported by Azwar. 41 Moreover, a review of 15 studies from China, Singapore, and WHO European Region by Schmulson reported that the prevalence of gastrointestinal symptoms in COVID-19 varied from 3.0% to 39.6%, with diarrhea as the most common symptoms (7.5%). 42 Ji, et. al. reported probiotic, without specifying the strain, as the only treatment in a pediatric COVID-19 patient presenting with mild diarrhea. 43 Horowitz reported several different probiotics, including acidophilus, lactobacillus, bifidobacterium, and Saccharomyces boulardii, along with azithromycin, hydroxychloroquine, zinc, vitamin C, alpha lipoic acid, glutathione, N-acetylcysteine in a case of recovered adult COVID-19 patient in New York City, USA. 44 A greater number of COVID-19 patients receiving probiotics as complementary treatment reported by Xu et. al. in Wuhan, China. It was reported that most cases of 62 patients in this study received probiotics in addition to antiviral, steroids, gamma globulin. 1 A report from 55 COVID-19 patients in Wuxi, China by Jiang et. al. also noted the use of probiotics as part of treatment in 47.3% of COVID-19 patients. In addition, probiotics was given more common in severe than non-severe patients (87.5% vs 40.4%, p=0.037). Unfortunately, both studies did not specify the strain, dose, dan duration of probiotics treatment given to the patients. It is hypothesized that viral replication associated with gastrointestinal immunity; hence, gut microbiota dysbiosis plays role in COVID-19. 6,45 There is still a great task in exploring the efficacy of probiotics as part of COVID-19 treatment; many studies attempted to correlate pathophysiology of COVID-19 disease and previous knowledge of mechanism action of some probiotics. 6,18,45 Not all probiotics seem to have the same treatment efficacy. Up to now, researchers worldwide are still actively exploring the most beneficial strain of probiotics and its dose; therefore, there is a wide variety of strain, dose, and treatment duration in each published study. A Lactobacilli and Bifidobacteria are the most common used of probiotics in many studies. There is a cytokine storm involving various pro-inflammatory cytokines such as interleukin-17 (IL-17), tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ). Blocking IL-17 decrease the inflammatory process; therefore, some Bifidobacterium strain having IL-17 inhibitory effect might be beneficial in treatment. 46 Also, Lactobacillus strain, such as Lactobacillus reuteri, Lactobacillus plantarum, Lactobacillus rhamnosus, and Lactobacillus acidophilus exhibited anti-inflammatory effect through suppressing TNF-α, IFN-γ production, IL-8, and IL-12. [47][48][49][50] To deepen understanding and knowledge of probiotics' role in fighting COVID-19, there are ongoing trials in Belgium, USA using Lactobacillus strain in COVID-19 patients. 51 Whether probiotics might play role as adjuvant in vaccine against COVID-19 or prophylaxis of COVID-19 infection is an interested topic to be further evaluated. Moreover, probiotic is beneficial in restoring gut microbiome composition due to antibiotic treatment, which is commonly used as part of treatment in COVID-19 patients. 4,51,52 At present, no specific therapeutic agents or vaccines for COVID-19 are available and most of the recommended drug are still under investigation. There are some published consensus and guidelines of COVID-19 treatment recommendation. However, probiotics have not been incorporated yet as part of treatment regimen since the rationale for using probiotics in treatment of COVID-19 is merely extrapolated from an indirect hypothetical evidence. 5,51,[53][54][55][56]

SAFETY ASPECT OF PROBIOTICS
Neither mortality nor serious adverse effects reported in the studies described in above. The subjects were varied from children to elderly, involving both men and women. Moreover, a study by Reid and Kirjaivanen reported Lactobacillus strains was safe to be used during pregnancy. 57

CONCLUSION
Probiotics supplementation might serve as a new hope in combating COVID-19. However, more studies involving greater number of subjects, similar strain and dose of probiotics are needed to establish the potency of probiotics as part of COVID-19 treatment.