Lakhasly

The human microbiome has a widely recognised role in maintaining optimal human physiological functions.Furthermore, an association between the microbiome changes, impairment in immune system function and excessive hygiene could also be exploredThe microbiome relates to the compilation of different communities of bacteria, archaea, fungi and viruses whose composition is dynamic and influenced by environmental conditions.57 Disturbances to a healthy microbiome (dysbiosis) has been extensively linked with systemic diseases such as depression,58 Inflammatory Bowel Disease (IBD)59 and age-related neurological diseases such as Alzheimer's disease amongst others.60 Gut and oral microbiome dysbiosis have also been extensively linked with an increased risk of infection including microbial translocationMN7 and bacteraemiaMN8.8,61,62 Two well-established opportunistic infections caused by microbiome dysbiosis are these induced by Clostridium dificile (colitis) and Candida albicans (candidiasis).63,64 Additionally, the gut microbiome contributes to the maintenance of the intestinal wall.A study conducted in a Hindlimb Unloading (HU), Dextran Sulphate Sodium (DSS) colitis induced mouse model in simulated microgravity conditions revealed an overall predisposition to colitis.67 When compared to the ground control mice, the mice exposed to microgravity had an associated early onset of colitis, a fourfold increase of segmented filamentous bacteria, more than a twofold decrease in regulatory T cells (Treg) absolute numbers as well as a reported twofold increase in colonic IL-1?It needs to be highlighted that, so far human based microbiome studies during spaceflight have a very limited sample size (n = 4-9) and therefore statistical inference is difficult.

The human microbiome has a widely recognised role in maintaining optimal human physiological functions. The microbiome relates to the compilation of different communities of bacteria, archaea, fungi and viruses whose composition is dynamic and influenced by environmental conditions.57 Disturbances to a healthy microbiome (dysbiosis) has been extensively linked with systemic diseases such as depression,58 Inflammatory Bowel Disease (IBD)59 and age-related neurological diseases such as Alzheimer’s disease amongst others.60 Gut and oral microbiome dysbiosis have also been extensively linked with an increased risk of infection including microbial translocationMN7 and bacteraemiaMN8.8,61,62 Two well-established opportunistic infections caused by microbiome dysbiosis are these induced by Clostridium dificile (colitis) and Candida albicans (candidiasis).63,64 Additionally, the gut microbiome contributes to the maintenance of the intestinal wall. Gut dysbiosis can contribute to disruption of the gut barrier resulting in hyperpermeability of the gut walls or gut leakiness.65 This hyperpermeable state causes gastrointestinal (GI) tract dysfunction and can allow for translocation of both microbes and microbial products. Movement of microbes out of the gut and into systemic circulation has been associated with disorders including celiac disease.66 As with the human immune system, the human microbiome has been observed to be susceptible to space-related environmental stressors. A study conducted in a Hindlimb Unloading (HU), Dextran Sulphate Sodium (DSS) colitis induced mouse model in simulated microgravity conditions revealed an overall predisposition to colitis.67 When compared to the ground control mice, the mice exposed to microgravity had an associated early onset of colitis, a fourfold increase of segmented filamentous bacteria, more than a twofold decrease in regulatory T cells (Treg) absolute numbers as well as a reported twofold increase in colonic IL-1β and circulating neutrophils. The overall results of this and other studies utilising murine models68 suggest that both the rodent gut microbiome and the innate immune system, adversely respond to simulated microgravity contributing to a pro-inflammatory shift in the gut. The association of these findings with human physiology warrants further investigation. Studies examining the impact of spaceflight directly on the astronaut’s microbiome have also been attempted. A change in abundance of certain genera and species in the skin, nose, tongue and gastrointestinal tract of astronauts, which is usually a sign of dysbiosis, has been reported.33,69 Some signs of increased frequency of antimicrobial resistance (AMR) markers during spaceflight across five body areas including the skin and the mouth has been noted in one study.33 The AMR markers mostly related to tetracycline and erythromycin resistance. It needs to be highlighted that, so far human based microbiome studies during spaceflight have a very limited sample size (n = 4–9) and therefore statistical inference is difficult. An overall link between microbiome changes and the observed immune system impairment during spaceflight seems highly likely and larger studies are required when human space presence is extensive and larger cohorts are available. Furthermore, an association between the microbiome changes, impairment in immune system function and excessive hygiene could also be explored