A NEW study has shown that, contrary to popular belief, local domesticated animals are unlikely to be the major source of antibiotic-resistant salmonella in people, based on a detailed study of DNA from salmonella samples collected over a 22-year period.
According to an announcement from the Wellcome Trust Sanger Institute in the U.K., by studying the genetic variation in salmonella bacteria and their drug resistance genes, researchers found that distinguishable bacterial populations exist in human and animal populations living side by side.
By comparing the salmonella genomes in humans and animals, the researchers have provided important new insights into the likely sources and spread of antibiotic-resistant infections: First, the salmonella bacteria largely remained within their original host populations, and second, there were more varied combinations of drug resistance in the human-infecting bacteria, according to the announcement.
"For the first time, we've determined in detail and on a large scale how salmonella strains taken from humans and animals in the same setting and over the same time period relate to each other," first author Dr. Alison Mather of the Wellcome Trust Sanger Institute said. "Our genomic data reveal how the salmonella bacteria spread during the course of a long-term epidemic. We found that people have a more diverse source of infection and antibiotic resistance than just the local animals, pointing towards alternative sources."
The team sequenced DNA from 373 samples taken from humans and animals infected with Salmonella typhimurium DT104 over a 22-year period, mainly from Scotland but also from other countries.
This is the largest study of its type; whole-genome DNA sequencing delivers the highest level of resolution possible to examine how closely related the bacteria are, enabling the team to unravel the details of this epidemic, the announcement said.
The team discovered that, contrary to much current thinking, the populations of salmonella in humans and animals were distinguishable. They also found that the estimated number of times the bacteria had jumped from animals to humans (and vice versa) was remarkably low. In addition, there was greater diversity in antibiotic resistance genes in salmonellae isolated from humans.
Taken together, these findings suggest that the contribution of local animal populations to human infections with DT104 may previously have been overstated, the institute concluded.
The team speculated that international travel and imported foods may be major sources of antibiotic-resistant strains of salmonella. However, to understand fully the routes of infection and find ways to prevent it, further research into other bacteria and other environments will be needed.
"Discovering that the animal and human populations of salmonella were as distinguishable as they were was a great surprise to us," Stuart Reid, co-author from the Royal Veterinary College, said. "This finding in no way undermines the importance of prudent antimicrobial use in all species, but our study does demonstrate that greater effort needs to be focused on understanding the natural history of the pathogens and on identifying the major sources of resistance in our global ecosystems."
The research was published online Sept. 12 in Science Express.