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Salmonella Strain’s Path to Virulence Uncovered

Agricultural Research Service (ARS) scientists have uncovered genetic evidence about the evolutionary path that transformed Salmonella enteritidis from an innocuous bacterium into a virulent pathogen.

S. enteritidis, like many bacteria, reproduces very quickly–every 20 minutes in optimal conditions, according to veterinary medical officer Jean Guard-Bouldin in the ARS Egg Safety and Quality Research Unit in Athens, Ga.

“To reduce current levels of infection, we’re studying how S. enteritidis evolves and infects hens on the farm,” says Guard-Bouldin. “Using mutational changes in the Salmonella genome as a sort of ‘breadcrumb trail,’ we’ve tried to determine the first time this bacterium became capable of getting inside the egg from hen reproductive organs.”

Such a fast reproductive pace allows the organism to accumulate genetic variations. Only healthy competitors go on to reproduce, survive and develop the mechanisms needed to infect the egg. Using DNA analysis, Guard-Bouldin is looking at evolutionary evidence to determine how some S. enteritidis strains became pathogenic. Studying how S. enteritidis evolves and infects hens on the farm may someday help reduce levels of infection.

Guard-Bouldin and her colleagues found S. enteritidis strains to be so similar genetically that they appear identical, yet they may behave differently inside the hen. To distinguish between the apparently identical genomes, researchers must use a technique called “whole-genome mutational mapping” to analyze multiple strains.

Through these analyses, the researchers developed a timeline of when S. enteritidis first became capable of getting inside the egg from hen reproductive organs–approximately 36 years ago. It appears that a large-scale swap of DNA between strains, in association with the emergence of egg contamination, created a hybrid strain of S. enteritidis.

This hybrid strain had the ability to contaminate the internal contents of eggs. Later, it also split very quickly into two lineages, each carrying one virus. Both of the newly split lineages continued to evolve and eventually began to vary in their ability to contaminate eggs and to survive on the farm.

The data from this research is being entered into a publicly available database by the National Center for Biotechnology Information, part of the National Institutes of Health.

“This information about differences between genomes could help streamline the process of finding out how human disease organisms evolve to become more virulent,” says Guard-Bouldin. “The main focus for us now is to continue sequencing entire genomes and searching for more genetic changes that help us understand the Salmonella organism.

“Up until recently, genomic techniques for delving this deeply into the genetic code of multiple Salmonella strains weren’t available or cost effective.

“If we can understand how Salmonella evolved to become pathogenic, perhaps we can apply the same principles to other foodborne pathogens and begin to study foodborne illness the way influenza is being monitored-with equal emphasis on the importance of small, as well as large, genetic changes.”

According to the Centers for Disease Control and Prevention, about 40,000 cases of salmonellosis are reported in the United States every year. Most result in diarrhea, fever, and abdominal cramps lasting 4 to 7 days, but severe cases caused about 70 deaths in 2000. Adequate cooking eliminates the risk of infection from eggs.

SOURCE: USDA Agricultural Research Service
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