Antibiotic Resistance May Hitchhike Along Roads

Vans and cars loaded down with people, goods, and maybe chickens weave out of the delta town of Borbón in northwest Ecuador, zipping onto the region's new two-lane highway. Soon, many veer off onto narrower roadways, heading to villages on the Santiago and Cayapas rivers. But canned beans, people, and poultry aren't the only cargo, a new study suggests—antibiotic resistance could travel those same routes, too.

For centuries, getting to the more than 100 Afro-Ecuadorian villages dotting the dense forests in the Ecuadorian province of Esmeraldas was an adventure: Travelers had to go by ocean, then continue down miles of winding rivers. Then in 1996, the new highway joined Borbón to much of coastal Ecuador, says study co-author Joseph Eisenberg, an epidemiologist at the University of Michigan, Ann Arbor. Once-isolated communities became much more open to the flow of people and goods—not to mention foreign microbes, potentially including antibiotic-resistant bacteria on the run from hospitals in Quito and other city centers.

To test the theory, Eisenberg and colleagues conducted a 5-year census: They visited 21 communities, both short and long drives from Borbón and with nearly identical rates of antibiotic usage, collecting more than 2000 stool samples. The group then cultured the samples, looking for Escherichia coli bacteria resistant to common antibiotics, particularly ampicillin and sulfamethoxazole drugs. Genes that protect microbes against these two frequently prescribed cell killers typically sit together on plasmids, small circlets of DNA that, similar to children trading baseball cards, pass easily between bacterial species. The shorter the drive time to Borbón, the more antibiotic-resistant bacteria villagers had in their guts, Eisenberg and colleagues report online today in the Journal of the Royal Society Interface.

The team drew up a mathematical simulation based on its data to estimate how antibiotic usage might influence the flow of resistant bacteria. The more antibiotics villagers in this study took, Eisenberg and colleagues calculated, the more likely they were to transfer the resistant E. coli to their close neighbors. When people take antibiotics, the resistant bacteria become the dominant strains in their guts and the ones they transmit, Eisenberg says. So, in communities in which the antibiotics flow freely, villagers tend to swap resistant gut frequently, especially because sanitation tends to be poor—latrines are few and far between, and community members usually draw their drinking water directly from freshwater streams. But that intravillage swapping likely becomes rarer when ampicillin and similar drugs are more restricted, in which case the majority of new antibiotic-resistant bacteria presumably flood in from the outside.

The team's results fall in line with a number of studies that warn against excessive antibiotic use, says Stuart Levy, a physician scientist at Tufts University School of Medicine in Boston and a member of the advocacy group Alliance for the Prudent Use of Antibiotics. Even if some individuals are responsible about how much antibiotics they take, they're still vulnerable to catching antibiotic-resistant strains from their neighbors who take too many antibiotics, Levy says.

Such widespread studies are a major undertaking in rural settings like these, says Randall Singer, an epidemiologist at the University of Minnesota, Twin Cities. But he's not convinced that the authors have nailed the argument that roads were a major culprit in the spread of antibiotic resistance. Some resistance genes are very old, he says, often because they protect bacteria against toxins. So the defensive microbes the team spotted may have been in the villages all along.