Researchers at Osaka University find short trips to a developing country can increase the appearance of a resistance gene to the antibiotic colistin.

The emergence of antibiotic resistance among dangerous pathogens is increasingly problematic worldwide. Many strains of infectious bacteria have become multidrug-resistant, and cannot be treated with common antibiotic therapies.

While the antibiotic colistin can often be used to treat infections by multidrug-resistant bacteria, colistin resistance is also on the rise and represents an emerging global health threat, further limiting available treatments.

Now, in a study published in Infection and Drug Resistance, researchers followed Japanese travelers to find out how short trips to a developing country can lead to the spread of colistin resistance.

"Extended-spectrum, beta-lactamase (ESBL) producing bacteria are resistant to most first-line antibiotics," first author Tatsuya Nakayama explains.

"Colistin is typically used as a last-resort treatment when there are no other therapy options available," Nakayama says.

"Unfortunately, we're seeing an increase in ESBL strains carrying the mcr-1 gene, which confers colistin resistance to bacteria. In our study, we were interested in tracking the genotype of bacteria carried by international travelers to understand how short-term travel can impact the prevalence of mcr-1."

The researchers followed 19 Japanese participants who traveled to Vietnam for less than 2 weeks. They collected fecal samples before and after each trip, and used a mix of biochemical and genetic assays to identify bacteria carried by each of the travelers.

The team found that short-term trips led to a significant increase in ESBL-producing bacteria: resistant strains were found in nearly 90% of travel events, and in the majority of cases travelers had shown no sign of ESBL bacteria prior to their journey.

More concerning, however, was the researchers' finding that the mcr-1 gene–which was absent among all travelers before leaving Japan–was carried back to Japan by three of the returning travelers during three separate travel events.

The mcr-1 gene is carried on plasmid, a mobile DNA molecule that can readily jump from one strain of bacteria to another, which means even an isolated case of colistin resistance can quickly spread through a human population.

The findings of the study thus suggest that even a relatively short international trip has the potential to serve as a starting point for the spread of colistin resistance.

"Our study supports the notion that even short-term travel can bring colistin-resistant strains back to the country of origin," senior author Yoshimasa Yamamoto concludes.