Previous research has shown that a primary function of the gut microbiome is providing colonization resistance against many orally acquired pathogens; several exogenous factors, such as antibiotic treatments, could reduce host resistance to infections. The potential for manipulating the gut microbiome in the context of travellers’ health has not been previously explored in depth in a scientific review.
A 2016 review, led by Dr. Mark S. Riddle from the Enteric Diseases Department at the Naval Research Center in Silver Spring (USA) and Dr. Bradley A. Connor from the Department of Medicine at Weill Cornell Medical College and The New York Center for Travel and Tropical Medicine in New York (USA) explores the role of the human gut microbiome in travellers’ health.
First of all, the review explores the role of gut microbiome in host resistance to enteric infections, also termed colonization resistance. Both density and complexity gradient of commensal microbiota found in a matrix of intestinal mucous, which are highest at the lumen and gradually decrease closer to the surface of the enterocytes, prevent and limit pathogen colonization and growth through several mechanisms. Both direct mechanisms (production of bacteriocins, competition for nutrients, cross-feeding, and conversion of host derived metabolites) and indirect mechanisms (production of immunomodulatory molecules and stimulation of hematopoiesis) work in parallel to provide resistance to acute enteric infections.
The applicability of the mechanisms of colonization resistance is explored in the context of common causes of travellers’ diarrhoea including Shigella spp., Salmonella typhimurium and Campylobacter jejuni. Interestingly, the authors hypothesized that the traveller’s unique gut microbiome may predispose him or her to enteric infection, to a greater or lesser extent. For instance, travellers from Sweden to high-risk destinations for enteric infection are more susceptible to infection with Campylobacter if their pre-travel microbiome has a lower number of bacterial species. These data show that specific structure and composition features of the gut microbiome could induce protection against common travellers’ infections.
The researchers presented a study comparing the gut microbiome of individuals who developed travellers’ diarrhoea (TD) associated with Enterotoxigenic Escherichia coli (ETEC), norovirus or mixed pathogens, and TD with no pathogen identified, to healthy travellers. The study found a dysbiotic profile of a high Firmicutes:Bacteroidetes ratio in the travellers who developed diarrhoea, regardless of etiologic agent or presence of a pathogen. Besides this, the bacterial composition of the gut microbiome of the healthy travellers showed also a gut dysbiosis that was similar to those in the diarrhoeal groups. Further comparison of the healthy traveller microbiome to those from healthy subjects who were part of the Human Microbiome Project also revealed a significantly higher Firmicutes:Bacteroidetes ratio in the healthy travellers. These results show that travel itself may alter the composition of the gut microbiome in travellers regardless of the development of travellers’ diarrhoea.
On the other hand, observational studies (here; here) across multiple traveller populations have shown that travelling, in particular to regions of Asia but also to destinations such as the Netherlands, among those who developed TD and/or took antibiotics while travelling, is a risk factor for acquiring extended spectrum beta-lactamase resistant bacteria. Although it seems that the acquisition of multi-drug-resistant (MDR) organisms is transient, studies evaluating different drug classes on MDR acquisition and long-term health consequences are needed.
Finally, the authors discussed possible ways to manipulate the gut microbiome for prevention or treatment of TD:
- Diet changes: by using a gnotobiotic mouse model to evaluate short-term dietary changes on the gut microbiota and intestinal transit times, it was found that even short-term changes in eating patterns familiar to travellers can have long-term effects; their impact on transit time may persist upon returning to the pre-travel diet.
- Probiotics and prebiotics in TD prevention: probiotics including Saccharomyces boulardii (study here) and Lactobacillus GG (study here) have been shown to have a role in the prevention of TD, with greater effectiveness in travellers to the developing world compared to other regions. Prebiotics including a galacto-oligosaccharide mixture in travellers with low and high risk of TD were also shown to yield a significant reduction in diarrhoea in the prebiotic group as compared to those who consumed placebo.
- Probiotics and prebiotics in TD treatment: there are insufficient data to recommend either probiotics or prebiotics products for treatment of acute travellers’ diarrhoea.
In conclusion, travelling is associated with changes to the gut microbiome and recent research suggest that the host microbiome may be an important factor involved in protecting travellers from acute enteric infections. Although diet and probiotics and prebiotics have all been reported to be effective in preventing TD, further studies in humans are needed in order to elucidate how the host gut microbiome may protect travellers from enteric infections.
Riddle MS, Connor BA. The traveling microbiome. Curr Infect Dis Rep. 2016; 18(9):29. doi: 10.1007/s11908-016-0536-7.