Two months abroad rewires travelers' gut bacteria, raising cancer risk questions

Background: colorectal cancer (CRC) rates are rising in developing countries like Nepal, though the incidence remains 5-10% of that in Europe. The human microbiota significantly influences CRC development, shaped by diet and lifestyle. Travelling abroad can alter microbial composition due to lifestyle adjustments. The study aimed to delineate the nature, extent, and pace of changes in intestinal and oral microbiota among Swedish residents after a two-month stay in Nepal, with a particular focus on changes associated with risk of CRC development. Methods: eight study participants provided fecal and saliva samples before departing from Sweden, before departing from Nepal, as well as two and four weeks after returning to Sweden. The microbiota was analysed using deep sequencing with the Illumina MiSeq platform targeting the V3-V4 region of the 16S rRNA gene. Denoising was performed using DADA2 to generate observational taxonomic unit (OTU) composition. Taxonomy annotation was conducted based on SILVA. Results: after the stay in Nepal, there was a rise in intestinal abundance of genus Escherichia-Shigella across all travellers. Two weeks after returning to Sweden, all but one traveller showed detectable Escherichia-Shigella levels, one exhibiting an exceptionally high amount (36.7%). Four weeks post-return, Escherichia-Shigella persisted in 50% of participants. Following sojourn in Nepal, travellers exhibited a reduction in oral abundance of families Fusobacteriaceae and Campylobacteraceae. Conclusions: when conducting microbiome studies, it is essential to consider the influence of international travel, as it can lead to substantial microbiota alterations. Identified microbiota changes could potentially be utilised as risk markers in future studies of CRC.