Local animals unlikely source of drug-resistant salmonella

Genomic study shows that salmonella populations in people and animals living side by side are more different than expected.

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 humans, based on a detailed study of DNA from more than 370 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, the announcement said.

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," Dr. Alison Mather of the Wellcome Trust Sanger Institute and first author 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 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 that 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 S. Typhimurium DT104 may previously have been overstated, the institute concluded.

"This is a study that uses the latest genomic approaches and a unique collection of samples to address a significant public health problem," says Professor Nicholas Thomson, senior author from the Wellcome Trust Sanger Institute. "Our data provide a very simple message, challenging the established view that local animals are the predominant source of salmonella infections in Scotland. This finding will reinvigorate discussions on the sources of antibiotic-resistant salmonella infections in humans in other environments."

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.

The research was published as: Mather AE et al. 2013. Distinguishable Epidemics of Multidrug Resistant Salmonella Typhimurium DT104 In Different Hosts. Science Express, published online 12 September 2013.

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