https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7217790/
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This is an excerpt of this long article.
The human gut microbiota consists of 1014 resident microorganisms which include bacteria, archae, viruses and fungi (Gill et al., 2006). Primarily, the gut bacteria in healthy individuals is dominated by four phyla Actinobacteria, Firmicutes, Proteobacteria, and Bacteroidetes (Villanueva-Millán et al., 2015). The colon harbours an extremely high density of bacteria in the families Bacteroidaceae, Prevotellaceae, Rikenellaceae, Lachnospiraceae and Ruminococcaceae (Hall et al., 2017). The gut microbiota plays a key role in health through its protective, trophic and metabolic actions.
While the microbes get a habitat and nourishment from the host, these microbes in turn help the host by regulating various host physiological functions, including dietary digestion, and imparting protective immunity against pathogens. Alterations of gut microbiota sometimes collectively called as “gut dysbiosis” have been shown to be associated with various diseases and disorders like IBD (Khan et al., 2019), type 2 diabetes (Gurung et al., 2020), depression (Zalar et al., 2018), cardiovascular disease (Tang et al., 2017).
Like the gut microbiota, there are evidences now that suggest the presence of distinct microorganisms in the lung (Bingula et al., 2017). In the gut Bacteroidetes and Firmicutes are predominant while Bacteroidetes, Firmicutes, and Proteobacteria preponderate in the lung (Zhang et al., 2020). Interestingly, the gut microbiota has been shown to affect pulmonary health through a vital cross-talk between the gut microbiota and the lungs which is referred to as the “gut-lung axis”(Keely et al., 2012). The gut-lung axis is supposed to be bidirectional, meaning the endotoxins, microbial metabolites can impact the lung through blood and when inflammation occurs in the lung, it can affect the gut microbiota as well (Dumas et al., 2018). This raises an interesting possibility that novel SARS-Cov2 might also have an impact on the gut microbiota. In fact, several studies have demonstrated that respiratory infections are associated with a change in the composition of the gut microbiota (Groves et al., 2020). One of the serious clinical manifestations of Covid-19 is pneumonia and progression to acute respiratory distress syndrome (ARDS) especially in elderly, immune-compromised patients (Lake, 2020). Numerous experimental and clinical observations have suggested that the gut microbiota plays a key role in the pathogenesis of sepsis and ARDS (Dickson and Arbor, 2017). Loss of gut bacteria diversity can lead to dysbiosis which may then be associated with many diseases (Mosca et al., 2016). In fact, elderly people have less diverse gut microbiota and beneficial microorganisms like bifidobacterium lose ground(Nagpal et al., 2018). Since, many elderly and immune-compromised patients progress to serious adverse clinical outcomes, it is therefore tempting to speculate that in Covid-19, there is a possible cross-talk taking place between the lung and the gut microbiota which might influence the outcome of the clinical manifestation.
2.1. Gut microbiota – role in immunity
The interactions of the host with the microbiota are complex, numerous and bidirectional. The gut microbiota are supposed to significantly regulate the development and function of the innate and adaptive immune system (Negi and Das, 2019). Intestinal commensals secrete antimicrobial peptides, compete for the nutrients and the habitat site thereby aiding in the state of homeostasis (Moens and Veldhoen, 2012). The gut microbiota and immune homeostasis seem to have a back and forth relationship and is also a field of great interest and intense research investigation in the field of infectious diseases. Also, gut microbiota derived signals are known to tune the immune cells for pro and anti-inflammatory responses thereby affecting the susceptibility to various diseases (Negi and Pahari, 2019). The immune gut homeostasis is orchestrated by the fine tuning of the regulatory balance of pro-inflammatory responses such as Th17 versus inflammatory regulatory T cells (Tregs) that is ultimately controlled by the commensal microorganisms(Round and Mazmanian, 2010). In circumventing a response to pathogenic infections like coronavirus, a healthy gut microbiome essentially could be pivotal in maintaining an optimal immune system to prevent an array of excessive immune reactions that eventually become detrimental to lungs and vital organ systems. In such circumstances, it becomes imperative to have a balanced immune response wherein an over reactive one or an under reactive one can equally be consequential to aggravate clinical complications like pneumonia and ARDS in a viral disease like Covid-19 (Fig. 1 ).
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