Abstract
Keywords: Dysbiosis; Probiotics; SARS-CoV-2; COVID-19
Introduction
Probiotics are live microorganisms, which if administered
at adequate amounts, confer beneficial physiological effects [1].
Previous study underscored the positive impact (whether directly
or indirectly) of probiotics on the ACE enzymes [2]. During the
process of food fermentation, probiotics make bioactive peptides
which interfere with the ACE enzymes through blocking the active
sites [3,4]. The debris of the dead probiotic cells can also work
as inhibitors to ACE [1], suggesting that probiotics are possibly
potential blockers to the ACE receptors, which act as gateway for
SARS-CoV-2 to attack gastrointestinal cells. Imai and colleagues
reported that ACE blockers could be used to decrease respiratory
distress syndrome [5]. The prebiotics are defined as ‘substrates
that are selectively utilized by host microorganisms conferring a
health benefit’ [6]. Similar to probiotics, prebiotics can be orally
administered into microbially colonized body sites to reach the
intestine, or by a direct way to the skin or vaginal tract [6]. Prebiotics
include lactosucrose, oligosaccharides, isomaltooligosaccharides,
fructans, xylooligosaccharides, resistant starch, lactobionic acid,
galactomannan, arabinooligosaccharides, psyllium, polyphenols
and polyunsaturated fatty acids [6-8]. The health benefits of
prebiotics to the gastrointestinal tract such as stimulation of immune
system and inhibition of pathogens are because of their ability to
modulate the activity and composition of human microbiota [1].
Prebiotics, which enhance probiotics survivability and growth, may
have an excellent potential effect against COVID-19 [1]. Prebiotics
could block the ACE enzymes, which may have a direct effect on gastrointestinal symptoms caused by COVID-19 [1]. There are
many ongoing registered trials aiming to investigate the efficiency
of probiotics in treating COVID-19 patients [9].
Some COVID-19 patients showed intestinal microbial dysbiosis
characterized by decreased probiotics such as Lactobacillus and
Bifidobacterium. Prebiotic or probiotic supplementation, and
nutritional support has been recommended to re-normalize
the balance of intestinal microbiota and decrease the risk of
secondary infection due to bacterial translocation [10]. Probiotic
supplementation could be a promising strategy given previous
studies of the potential application of probiotics in treatment and
prevention of various viral infections [1,11,12]. The elderly and
disordered microbiota patients are the most susceptible groups to
COVID-19. Thus, it is suggested that probiotics supplementation
in those groups could increase the ability of the gastrointestinal
microbiota in modulation of immunity and help in prevention
of viral infections including COVID-19 [1]. Competition with
pathogens for nutrients, production of anti-microbial substances,
enhancement of the intestinal epithelial barrier, and adhesion
to the intestinal epithelium, and modulation of the host immune
system might explain clinical success of probiotics [13,14].
Saavedra and colleagues conducted randomized control trial of 55
infants and found that enteral supplementation with a combination
of Streptococcus thermophiles and Bifidobacterium bifidum
decreased the incidence of diarrhea and rotavirus shedding [15],
which may indicate interference with entry of the virus into cells
and/or inhibition of viral replication in the intestine. Although
probiotics were not administered to the respiratory tract, this
mechanism may play a role in lowering dissemination of SARS-CoV-2
through the gut. Therefore, direct inhibition may be impossible
at the respiratory tract. Having said that, lungs have their own
microbiota and a gut-lung connection has been previously reported
whereby microbe-microbe host-microbe, and immune interactions
could affect the course of respiratory diseases [14,16].
Growing evidence showed that the gut-lung axis plays a pivotal
role in the pathogenicity of viral and bacterial and infections, as
the intestinal microbiota could enhance the activity of alveolar
macrophage, thus having a prophylactic role in host defense against
pneumonia [17]. Respiratory tract infections such as influenza are
linked with a dysbiosis in the microbial communities of the both
gastrointestinal and respiratory tracts [18,19], which could alter
immune function and facilitate secondary bacterial infection [14].
Previous studies reported that COVID-19 could be associated with
intestinal dysbiosis leading to inflammatory reactions and poorer
response to pathogens [20,21], the case exists for probiotics
that could restore gut homeostasis [22]. Arroyo and colleagues
evaluated the efficacy of oral administration of Lactobacillus
fermentum CECT5716 or Lactobacillus salivarius CECT5713, two
lactobacilli strains isolated from breast milk, compared with the
efficacy of antibiotic therapy in treatment of lactational mastitis
[23]. They found that females took the probiotics improved more
and had reduced recurrence of mastitis than those who took the
antibiotic therapy.
The gut microbiome plays a pivotal role in systemic immune
responses, including those at distant mucosal sites such as the lungs
[24,25]. Administration of certain lactobacilli or bifidobacteria
helps in clearance of influenza virus from the respiratory tract
[24,26]. Probiotic strains increase type I interferon levels, the
activity and number of T cells, NK cells, antigen presenting cells, as
well as the levels of systemic and mucosal specific antibodies in the
lungs [24,27,28]. Growing evidence showed that probiotic strains
could regulate the dynamic balance between proinflammatory and
immunoregulatory cytokines that facilitate viral clearance with
minimum immune response-mediated lung damage [14]. This
seems be particularly important as a way to inhibit acute respiratory
distress syndrome, which is the most feared complication of
COVID-19.
Chong and colleagues reported that Lactobacillus plantarum
DR7 suppressed plasma pro-inflammatory cytokines (TNF-α,
IFN-γ,) in middle-aged adults, and enhanced anti-inflammatory
cytokines (IL-10, IL-4,) in young adults, along with decreased
levels of oxidative stress and plasma peroxidation [29]. This type
of modulation is considered to be very important, especially for
many COVID-19 patients, who have from cytokine storm. Orally
administered probiotic strains appear to involve the immune
response originating from the intestine, a main site of the body’s
defenses. Thus, probiotic strains, which could improve the integrity
of tight junctions, for example through butyrate augmentation, a
fuel for colonocytes, may in theory decrease SARS-CoV-2 invasion
[14]. Zuo and colleagues found that faecal samples with signature
of low-to-none SARS-CoV-2 infectivity had higher abundances of
short-chain fatty acid producing bacteria, Bacteroides stercoris,
Parabacteroides merdae, Lachnospiraceae bacterium 1_1_57FAA,
and Alistipes onderdonkii [30]. A recent study tested the impact
of short-chain fatty acids (acetate, propionate and butyrate) in the
infection by SARS-CoV-2 [31]. They found that short-chain fatty
acids did not change SARS-CoV-2 entry or replication in intestinal
cells. These metabolites had no effect on permeability of intestinal
cells and had only little effect on the synthesis of anti-viral and
inflammatory mediators. Although this may seem discouraging, we
propose that testing real short-chain fatty acid-producing bacteria
(not short-chain fatty acids only) may give good results. Testing
bacteria is different from testing metabolite, especially that there
are many current pieces of research that speak about virus-bacteria
interactions [32,33].
Ren and colleagues reported that faecal and oral microbial diversity
was remarkably decreased in confirmed COVID-19 patients
versus healthy controls [34]. They found that there was a reduction
in butyric acid-producing bacteria and an increase in lipopolysaccharide-
producing bacteria in COVID-19 patients in oral cavity.
Researchers reported that confirmed recovery COVID-19 patients
showed depletion in 47 lipid molecules, including sphingomyelin
(SM)(d40:4), SM(d38:5) and monoglyceride(33:5), and enrichment
of phosphatidylcholine(36:4p), phosphatidylethanolamine
(PE)(16:0p/20:5) and diglyceride(20:1/18:2) versus confirmed
COVID-19 patients. This is the first study that explores the alterations
in the human oral and gut microbiomes and lipidomics in
COVID-19 patients, which may be involved in the development and
progression of COVID-19 and could be also useful as an auxiliary
diagnostic tool. Previous clinical and experimental studies reported
that some probiotic strains have antiviral effects against common
respiratory viruses, including respiratory syncytial virus, rhinovirus,
influenza [12,28,35,36]. Although these mechanisms or effects
have yet to be tested on the SARS-CoV-2, this should not refute
considering this new line of investigation, especially when effects
of probiotics against other coronavirus strains such as transmissible
gastroenteritis virus have been reported [37-40]. Research is
urgently needed to assess the effect of probiotics and prebiotics
against SARS-CoV-2, which may lead to a better understanding of
the bacterial dynamics in the gastrointestinal tract.
Conflict of Interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
Consent Statement/Ethical Approval
Not required.
Funding Statement
This research received no specific grant from any funding agency.
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