Probiotics as a Weapon in the Fight Against COVID-19

In our previously published work, we support that probiotics could be used as an adjunctive treatment against COVID-19 (1) and other colleagues have also focused their attention on this subject (2, 3). Probiotics boost the immune system, enhance the mucosal barrier function and inhibit bacterial adherence and invasion capacity in the intestinal epithelium by being in a direct antagonism with pathogenic bacteria (1). The gut-lung axis is involved in the pathogenicity of bacterial and viral infections, as the intestinal microbiota boosts the alveolar macrophage activity, thus having a protective role in host defense against pneumonia (1). Along these lines, current clinical evidence connects gut, lung, and brain as an entity with communication mediated through complex neural, immunologic inflammatory, and neuroendocrine networks, the so called gut-brain-lung axis (4). There are indications in animals and humans that intestinal microbiota provides bacteria to the lungs, as abundance of Bacteroides sp. is observed in the lung following sepsis (5). Moreover, following sepsis, neurologic and cognitive outcomes are observed (4, 6). Without any doubt, the importance of the gut microbiome is stated (1). The composition of the gut microbiome may be used as a predictive tool of disease development and infection severity (1, 7, 8) . Pattern recognition receptors (PRRs) are of major importance for the developing of the innate immune response (9). Probiotics regulate the innate immune cells via interactions between cell wall components or metabolites with host PRRs (10). Yet, probiotic bacteria are activating Dendritic Cells (DCs) and macrophages boosting adaptive immune responses (B cell differentiation, T cell homing, Th17 cell stimulation) (11). The expression of Pattern Recognition Receptors (PRRs) is inflamed in the lung cells during inflammation processes. In this context, macrophages, monocytes and neutrophils are responding by increasing levels of PAMPs (Pathogen-Associated Molecular patterns) and DAMPs (Danger-Associated Molecular Patterns) (12). Besides that, the PRRs recognize DAMPs (Danger-Associated Molecular Patterns) as danger signals emitted by damaged or necrotic host cells which stimulate the pro-inflammatory response (6). Intruder's viral pathogens show a distinctive particular image of PAMPs giving a specific immune response (9). PAMPs are nucleic acids or glycoproteins indispensable for the pathogens' survival. PAMPs should be identified by PRRs leading to the expression of co-stimulatory molecules such as cytokines and chemokines which, in their turn, should activate the antigen presenting cells and specific adaptive immunity by eliminating pathogens (6, 13). It is of note that the most studied PRRs for pathogens' recognition are TLRs (Toll Like Receptors) which are membrane glycoproteins (12). The expression of Pattern Recognition Receptors (PRRs) is inflamed in the lung cells during inflammation processes. TLR4 signaling in pulmonary..