How International Travel Spreads Antibiotic-Resistant Bacteria
A silent health threat is crossing borders, and you might be carrying it without even knowing.
Imagine returning from the vacation of a lifetime, only to discover you're carrying an unseen souvenir—antibiotic-resistant bacteria acquired during your travels. This isn't science fiction but the startling finding of a comprehensive Dutch study that revealed how international travel contributes to the silent spread of superbugs across the globe. For approximately 30% of travelers, these invisible stowaways represent a significant threat to personal and public health 1 6 .
To understand the significance of the Dutch study, we must first grasp what researchers were tracking. Extended-Spectrum Beta-Lactamase–producing Enterobacteriaceae (ESBL-E) are a family of bacteria, including familiar names like E. coli and Klebsiella pneumoniae, that have developed a powerful defense mechanism.
These bacteria produce enzymes called ESBLs that act like molecular scissors, cutting apart and neutralizing a wide range of commonly used antibiotics, particularly penicillins and cephalosporins 4 . When infections with these bacteria occur, they become notoriously difficult to treat, leading to longer illnesses, extended hospital stays, and higher medical costs 3 4 .
The World Health Organization now lists ESBL-E as high-priority pathogens requiring urgent attention 7 . What makes them particularly concerning is that the genes encoding these resistant enzymes are often located on mobile genetic elements like plasmids, which can easily transfer between different bacterial strains, rapidly spreading resistance 4 .
ESBL genes are often located on plasmids, which are mobile genetic elements that can transfer between different bacterial species, rapidly spreading resistance.
Prior to the Dutch study, research had begun to hint at a troubling connection between international travel and the acquisition of resistant bacteria. Studies from 2007-2010 had documented that 18-25% of returning travelers from countries like Australia, Canada, Sweden, and the United States carried ESBL-E 1 . The stage was set for a more comprehensive investigation to determine exactly how widespread this phenomenon was and which factors increased a traveler's risk.
In 2013, a team of Dutch researchers published a groundbreaking prospective cohort study that would provide some of the most compelling evidence yet for how travel spreads antibiotic resistance. Their investigation followed Dutch travelers to document precisely how many acquired ESBL-E during their journeys and what factors made this transmission more likely 1 6 .
The researchers designed their study with meticulous care, recruiting 370 adults who planned to travel outside Europe, North America, and Australia through travel clinics in Leiden, Netherlands. The study followed a clear, step-by-step process:
Participants completed a detailed questionnaire and provided a rectal swab sample before departure to establish their pre-travel bacterial carriage status.
Immediately upon return, participants provided another rectal swab and completed a follow-up questionnaire about their travel experiences.
Travelers who tested positive for ESBL-E after returning were asked to provide additional swabs six months later to determine how long the bacteria persisted.
If participants remained positive at six months, their household members were also tested to check for potential spread 1 .
In the laboratory, researchers used sophisticated detection methods, culturing samples on specialized ESBL screening agar and confirming ESBL production through combination disk diffusion tests. Molecular characterization of the resistant genes was performed using microarray technology, and genetic diversity of the E. coli isolates was analyzed through multilocus sequence typing 1 .
The findings, published in Emerging Infectious Diseases, were striking. Of the 370 travelers included in the analysis, 32 (8.6%) were already colonized with ESBL-E before their journeys began. More alarmingly, 113 travelers (30.5%) who had tested negative before departure acquired ESBL-E during their travels 1 6 .
Data compiled from the 2013 Dutch traveler study 1
Perhaps most concerning was the persistence of these bacteria. Of the 113 travelers who acquired ESBL-E abroad, 19 (16.8%) still carried the bacteria six months after returning home. This long-term carriage creates extended opportunities for these resistant bacteria to spread to others or cause future infections in the carriers themselves 1 .
Travelers who acquired ESBL-E during travel (n=113) still colonized at 6 months
Travelers positive before and after travel (n=20) still colonized at 6 months
Data from the 2013 Dutch traveler study 1
The genetic analysis revealed extensive diversity among the E. coli isolates, suggesting travelers were picking up many different strains rather than one common type. The predominant resistance genes detected were from the CTX-M enzyme group, which has become the most widespread type of ESBL globally 1 .
The Dutch study pinpointed travel to Asia as the strongest risk factor, but what explains this geographic disparity? Subsequent research has helped complete this picture.
A 2025 study from Chiang Mai, Thailand, found that 50% of chicken meat and 48% of pork from local markets was contaminated with ESBL-producing organisms 4 .
The genes responsible for ESBL production are often located on plasmids, mobile genetic elements that can readily transfer between different bacterial species 4 .
A 2025 meta-analysis confirmed that the Western Pacific Region continues to show the highest pooled prevalence of ESBL-producing E. coli 3 . Food appears to be a significant transmission route.
This means a resistant bacterium acquired from contaminated food or water can potentially share its resistance genes with other bacteria in a traveler's gut, creating new resistant strains that the person carries home.
Identifying ESBL-producing bacteria requires specialized laboratory techniques. The Dutch study and current clinical practice employ multiple complementary approaches.
Selective growth medium for presumptive ESBL-producing Enterobacteriaceae
Initial screening; creates colored colonies for easier identification 4
Phenotypic confirmation of ESBL production
Gold-standard confirmation; detects increased zone diameter with inhibitor 4
Molecular detection of specific ESBL and carbapenemase genes
Characterizing resistance mechanisms; identifies specific gene variants 1
Genetic characterization of bacterial strains
Tracking bacterial lineages and transmission patterns 1
Rapid molecular detection of CTX-M genes
Quick screening for most prevalent ESBL genes 5
Recent advances are making detection faster and more sophisticated. A 2025 Dutch study demonstrated that machine learning models can now predict ESBL production directly from routine antibiotic susceptibility testing results with high accuracy, potentially reducing detection time . Meanwhile, whole-genome sequencing provides the most comprehensive analysis, identifying resistance genes and tracking transmission pathways with precision 7 8 .
The 2013 Dutch traveler study fundamentally changed our understanding of how antibiotic resistance spreads globally. It demonstrated that nearly one-third of international travelers acquire ESBL-E during their journeys, with particularly high risk in Asian destinations, and that a significant number remain colonized for months after returning home 1 .
Subsequent research has confirmed that ESBL prevalence remains high in many regions, with a 2025 meta-analysis reporting an overall pooled prevalence of 25.4% for ESBL-producing E. coli 3 .
A Dutch hospital study showing a declining trend in ESBL-E carriage from 2013-2022, down to an average of 4.6% among hospitalized patients 7 .
The story of ESBL-E in travelers highlights the interconnected nature of antibiotic resistance. It respects no borders and can hitch rides with asymptomatic carriers. Contaminated food, environmental exposure, and person-to-person transmission all contribute to a complex web of spread that requires coordinated global surveillance, enhanced antibiotic stewardship, and continued research to address this pressing public health challenge 3 4 .
"Active surveillance for ESBL-E and contact isolation precautions may be recommended at admission to medical facilities for patients who traveled to Asia during the previous 6 months" 1 .