TRAVEL DISRUPTS THE MICROBIOME —
EVEN WITHOUT DIARRHOEA

Short-term travel significantly increases intra-individual gut microbiota fluctuation even in travellers who do not develop diarrhoea, with greater compositional changes occurring post-travel compared to baseline. Contributing factors include environmental changes, altered mood, altered diet, and exposure to new microbial environments.¹ Disruption can also begin before arrival: long-haul flights – particularly eastward travel – introduce circadian challenges through altered meal timing, sleep disruption, and shifts in daylight exposure that can weaken microbiome stability early in a trip.²

The microbiome changes observed are substantial. Travel to the tropics is associated with reduced α-diversity, decreased species richness, and broad compositional shifts. Among the most striking findings is an approximately 40-fold post-travel bloom of Escherichia/Shigella, alongside significant increases in other oxygen-tolerant taxa such as Haemophilus and Bilophila, and concurrent depletion of obligate anaerobes within the Ruminococcaceae family – a group associated with butyrate production and gut barrier integrity. Indeed, diarrhoeagenic E. coli (DEC) was detected by qPCR in 89% of post-travel stool samples in one study cohort.³ Travellers also show enrichment of Escherichia/Shigella relative to non-travellers at the population level, with a low-diversity, Escherichia/Shigella-dominated microbiota profile occurring more frequently among those who travel.⁴

WHO IS MOST VULNERABLE?

Travellers with underlying chronic conditions or immunosuppression are more likely to present with a low-diversity, Escherichia/Shigella-dominated gut microbiota profile — a pattern associated with dysbiosis, a pro-inflammatory state, and potential antibiotic-resistance gene reservoirs. These individuals may be particularly deserving of targeted microbiome-supportive strategies. Notably, in one study, travellers reported substantially lower probiotic use in the preceding year compared with non-travellers (13.3% vs 44.4%; P = 0.006).⁴

THE PRE-TRAVEL MICROBIOME MATTERS

Not all travellers experience the same degree of disruption — responses vary considerably between individuals. Pre-travel microbiota composition, rather than baseline diversity alone, appears to influence resilience: travellers whose microbiomes remained stable were enriched pre-travel in probiotic-associated genera including Bifidobacterium, Lactobacillus, Faecalibacterium, and Roseburia. Several of these – including Agathobaculum, Faecalibacterium, and Roseburia – are short-chain fatty acid (SCFA) producers; SCFAs serve as a key energy source for colonocytes and support intestinal health.¹ This suggests that supporting gut microbiome health prior to departure – not just during travel – may be clinically relevant.

SUPPORTING THE MICROBIOME WITH PROBIOTICS

Given the evidence of travel-induced microbiome disruption even without diarrhoea, there is a reasonable basis for advising travellers to use probiotics before and during travel, and to continue them after return to support microbiome recovery. Certain probiotics have demonstrated effectiveness in reducing the risk of TD: a 2018 meta-analysis of 12 randomised trials (n = 3,736) found that Saccharomyces boulardii prophylaxis significantly reduced TD incidence (risk ratio 0.79; 95% CI 0.72–0.87; p < 0.001), while Lactobacillus rhamnosus GG showed a trend toward reduction (p = 0.008).⁵ The role and effects of probiotic use among travellers more broadly warrant further investigation.⁴

BEING PREPARED FOR TRAVELLER'S DIARRHOEA

TD affects approximately 10%–40% of travellers, with the highest risk in those visiting parts of Africa, South and Southeast Asia, Latin America, and Mexico. Risk is further elevated by longer duration of travel, younger age, underlying gastrointestinal conditions, use of acid-suppressing medications, and consumption of high-risk foods. Most episodes are self-limiting, with approximately 50% resolving within 48 hours and an average duration of 4–5 days; however, complications including post-infectious irritable bowel syndrome can occur.⁵

Beyond the acute episode, TD can drastically reduce microbial diversity, promote colonisation by pro-inflammatory bacteria, and facilitate acquisition of multidrug-resistant organisms that may persist after return.^2,6 These organisms can affect household contacts through onward transmission, and recent international travel should be considered in the clinical history of patients presenting with infections such as urinary tract infections or postoperative infections.² Travellers should be advised to carry a non-absorbable antibiotic for use if TD develops. These agents achieve high intraluminal intestinal concentrations with minimal systemic absorption, and are effective for uncomplicated (non-invasive) TD; they are not appropriate for dysentery or invasive disease, where alternative agents should be considered. Preventive measures – including careful food and water selection and hand hygiene – remain the foundation of TD prevention.⁵