Small steps, big impact: The role of gut microbiota in health and disease
The gut microbiome is the totality of microorganisms – bacteria, viruses, protozoa and fungi – and their collective genetic material present in the gastrointestinal tract (GIT). The gut microbiota, on the other hand refers to all bacteria – commensal and pathogenic – residing in the GIT. Factors, such as mode of delivery, mode of feeding and type of diet, highly influence the composition of the gut microbiota.
Commensal bacteria, such as Bifidobacterium and Lactobacillus produce metabolites that make that gut less conducive for the growth of pathogenic bacteria. Additionally, they produce bacteriocin and bacteriocin-like substances that inhibit the replication and thus serving as first line of defense against pathogenic microorganisms. They also help regulate epithelial permeability through the modulation of tight junctions that are essential to upholding the physical intestinal barrier, and produce molecular signals that modulate the development of immune cells to fine-tune immune responses. A healthy balance in the microbiota composition is thus essential to the maintenance of gastrointestinal immunity. Factors, such as infection, diet/lifestyle, autoimmunity and inflammation, result in a distortion of this balance – called dysbiosis – which can then contribute to the development of disease.
Studies have shown that microbiome-induced immunity (through supplementation of probiotics) confers beneficial effects on upper respiratory illnesses, i.e. decrease in number of infections, fewer days of illness, reduced usage of antibiotics, lower rate of absences from school/daycare. These studies also suggest that probiotic supplementation conferred protection against various respiratory viruses, including rhinoviruses, coronaviruses and the human bocavirus. Evidence also suggest that the gut microbiome also plays a role in the disease course of COVID-19 infection, with elevated inflammatory cytokine levels observed in patients with intestinal dysbiosis. Further investigations in the role of the gut microbiota and its metabolic products in the context of COVID-19 may help identify new therapeutic targets to improve outcomes or provide protection against COVID-19.
In conclusion, the gut microbiome is important in shaping and regulating human immunity. An efficient and well-tolerant immunity is dependent on a symbiotic partnership between the gut microbiome and the immune system.
 Allam-Ndoul B, et al. Int J Mol Sci. 2020 Sep 3;21(17):6402.
 Lehtoranta L, et al. Nutrients. 2020 Oct 16;12(10):3163.
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