June 11, 2021 -- Travelers returning home from foreign destinations are bringing back more than tourist souvenirs and memories. Their guts may be harboring new microbial hitchhikers, some of which bear gene sequences that make antibiotics less effective.
These new findings, published online June 7 in Genome Medicine, stand out because the researchers captured the fecal microbiome profile of the travelers before they departed and immediately upon their return. The before-and-after comparison is "the most novel part here," Jonathan Eisen, PhD, professor and microbiologist at the University of California, Davis, Medscape Medical News. Eisen was not involved in the study. "They're getting a snapshot in time of where pools of antimicrobial resistance organisms or genes are."
For this work, researchers profiled stool samples from 190 people traveling from their home bases in the Netherlands to areas in Northern and Eastern Africa and Southern and Southeastern Asia. The time that the visitors spent at their destination was at least a week but no more than 3 months, and they had not taken any antibiotics within the 3 months before their trips.
Immediately upon their return from their journeys, which were mostly taken for leisure, the same individuals provided an additional stool sample. Analyses of those samples showed many new microbiota arrivals along with their traveling human hosts, including 56 gene sequences associated with antibiotic resistance.
People can pick up new microbes in their gut in several ways, including by eating and drinking and using restroom facilities in the places they visit. Anyone who travels is considered a "sink" for acquiring new microbial companions, and a new place they visit is a "source" of them. In this study, the Dutch travelers were "sinks."
Those who visited the same destinations tended to pick up similar new microbes, suggesting specificity of these bacteria and their genes to geographic sources. The investigators tested the ability of some of the hitchhiking gene sequences to confer antibiotic resistance against E.coli bacteria and found that they did.
The study did not examine whether these acquired microbes are associated with the risk of developing human disease said study author Gautam Dantas, PhD, a professor of pathology and immunology at Washington University School of Medicine in St. Louis, Mo. He and his colleagues also did not evaluate what happens to the new gut residents after travelers settle in back at home. "We do not know how transient the acquisition is," Dantassaid."Who knows, maybe one week later, half of these things are gone, maybe a year later all of them are gone…we don't have that answer yet."
The concern with even the transient presence of new resistance genes in a person's gut relates to the way bacteria tend to swap sequences around. A bacterial cell carrying such a gene might not survive long, but if it transfers that gene to native microbes in a person's gut, the sequence itself could become a permanent resident. At some point, "the sequence could eventually hop into a bug that could cause disease," Dantas said, disarming the usual effects of antibiotic treatment.
That hypothetical outcome of compromised antibiotic effect remains to be demonstrated, Dantas said. He emphasized that the current findings suggest that international travel carries increased risk of taking the first step in that direction: picking up resistance companions.
He also pointed out that the results are unexpected with adult microbiomes. "Our general understanding is that the adult microbiome is generally stable, so a change is impressive," he said.
Eisen added that although sampling for resistant microbes and genes is pretty thorough in some countries, including the United States, "we don't have a lot of antimicrobial resistance data from other places." Studies such as these showing how travelers can be a "sink" for these microbes and sequences when they travel to "sources" highlight the need for global surveillance, he said.
"We know that we have a global problem with antimicrobial resistance and that movement of people contributes to the spread," Eisen said. "It's a no-brainer that we need a better understanding of how people carry these from one place to another, and this work gets at a piece of that picture."