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Findings on Epidemic Legionnaire's-Causing Bacteria Could Lead to New Tools

NEW YORK (GenomeWeb News) – Comparative genomics is helping scientists understand and characterize the bacterial strain behind most cases of Legionnaire’s disease, French researchers reported today.
Researchers at the Institut Pasteur in Paris used DNA arrays to assess whether the genetic background of Legionella pneumophila — the causative agent for Legionnaire’s disease — influences the bacterium’s evolution and pathogenesis. In so doing, they also started to unravel the potent properties of an epidemic L. pneumophila strain found around the world. The work appeared in the online version of the journal Genome Research today.
The findings could potentially lead to the development of new tools for detecting Legionella in water systems or other sites susceptible to contamination, the researchers believe.
“DNA arrays are powerful tools to assess this inter- and intra-species variability at the genome level,” the authors wrote, noting that similar approaches in other bacterial species “allowed scientists to develop typing tools, find vaccine candidates, and to understand the evolution and clonal expansion of particular virulent clones.”
First identified more than 30 years ago, Legionnaire’s disease is a lung condition that often affects elderly individuals or those with a weakened immune system. It is characterized by severe pneumonia, fever, chills, and muscle aches. Although antibiotic treatments are available, Legionnaire’s disease can be serious or even life-threatening.
Many Legionella species exist — commonly occurring in water from drinking water reservoirs, showers, or air-conditioning units. But one is best known for causing disease: L. pneumophila. And within that species, the Sg1 serogroup causes roughly 84 percent of Legionnaire’s infections, though other, genetically diverse strains exist.
In an effort to understand this genetic diversity, the researchers compared 217 L. pneumophila strains along with an additional 32 Legionella strains that don’t cause pneumonia. They then assessed these strains using DNA arrays and comparative hybridization against probes from L. pneumophila’s genome as well as known or suspected virulence genes.
Although L. pneumophila strains are highly genetically diverse overall, the virulence genes were highly conserved in all the L. pneumophila strains. In contrast, these virulence genes were either absent or variable in other Legionella species. Several L. pneumophila genes were also remarkably similar to eukaryotic genes.
Taken together, the researchers believe these virulence and eukaryote-like genes may explain L. pneumophila’s ability to cause infections and evade the human immune system.
“We discovered core virulence- and eukaryotic-like genes are highly conserved, indicating strong selection pressures for their preservation,” senior author Carmen Buchrieser, a researcher at the Institut Pasteur, said in a statement.
Buchrieser and her colleagues also found genetic traits that were distinct in the epidemic, L. pneumophila Sg1 strain. For example, several Sg1 strains contained a group of lipopolysaccharide biosynthesis factors — genes that influence bacterial cell wall production — that are apparently swapped between bacteria from different genetic backgrounds.
This finding could be significant because humans’ immune systems often recognize and mount an immune response against bacterial cell wall components. If L. pneumophila strains do swap LPS genes, this horizontal gene transfer might contribute to the bug’s pathogenicity. “The LPS of Sg1 itself may confer to Sg1 strains the high prevalence in human disease,” Buchrieser said.
Ultimately, the researchers said, such improvements in their understanding of L. pneumophila’s genetic diversity may improve the detection and management of strains that are dangerous to human health.
“The findings of this comparative genomics approach will be invaluable for the development of novel tools to rapidly detect Legionella-associated risk factors in water distribution systems of hospitals and other potential sites for Legionella infection,” Buchrieser noted.

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