Salmonella Strain's Path to Virulence Uncovered
April 7, 2009
Agricultural Research Service (ARS)
scientists have uncovered genetic evidence about the evolutionary path that
transformed Salmonella enteritidis from an innocuous bacterium into a
S. enteritidis, like many bacteria, reproduces very quickly--every 20
minutes in optimal conditions, according to veterinary medical officer
Guard-Bouldin in the ARS
Safety and Quality Research Unit in Athens, Ga.
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.
more about this research in the April 2009 issue of Agricultural
ARS is the principal intramural scientific research agency of the
U.S. Department of