Worldwide-Distributed Clone of Bacteria Responsible for Legionnaire's Disease

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Researchers

Identify a Worldwide-Distributed Clone of Bacteria Responsible for

Legionnaire’s Disease

02/06/2008

A study published online today in

Genome Research describes new insights into Legionella pneumophila, the

bacteria responsible for most cases of Legionnaires’ disease. This report

investigates the genetic background of L. pneumophila, provides clues to the

evolution and emergence of this pathogen, and describes the identification of a

worldwide-distributed epidemic clone.

Legionnaire’s disease is

characterized by severe pneumonia, afflicting the elderly and individuals with

weakened immune responses in particular. While L. pneumophila, a genetically

diverse species and one of many Legionella species, is common in natural and

drinking water supplies, the majority of Legionnaire’s disease cases

worldwide (approximately 84 percent) are caused by a single serogroup, L.

pneumophila Sg1. Recent work has suggested that even though Sg1 is responsible

for most clinical cases, this serogroup accounts for only about 30 percent of

environmental Legionella.

As the prevalence of Sg1 in

Legionnaire’s disease cases does not appear to be a result of

environmental predominance, the frequent occurrence of Sg1 in disease is likely

due to higher virulence. In this study, researchers led by Dr. Carmen Buchrieser

of the Institut Pasteur conducted a comparative genomics analysis to gain

insight into the basis for the higher virulence of Sg1. To compare Sg1 and

other Legionella isolates, the researchers constructed DNA-arrays containing

genes known to be variable in L. pneumophila strains, including a set of known

and potential virulence genes.

“We screened the gene

content of 217 L. pneumophila strains and 32 other Legionella (non-pneumophila)

strains that were isolated from humans and the environment,” Buchrieser

says. “We discovered core virulence- and eukaryotic-like genes are highly

conserved, indicating strong selection pressures for their preservation.”

A cluster of lipopolysaccharide

(LPS) biosynthesis genes was found to be common in Sg1, even in different

genetic backgrounds. This suggests that the gene cluster could be transferred

horizontally between strains. “The LPS of Sg1 itself may confer to Sg1

strains the high prevalence in human disease,” explains Buchrieser.

Variation in LPS, a component of the cell wall normally recognized by the

innate immune system, could allow the bacteria to evade host immune responses.

Most significantly, this study

identified a specific clone of Sg1 that is present in both sporadic cases and

outbreaks worldwide. “The identification of this clone opens exciting

possibilities of research to find out which genes contribute to improved

interaction with the host, or to improved fitness in the environment, or to

both,” Buchrieser says.

Buchrieser cautions that there

may be other genetic factors involved in the emergence of an epidemic strain.

“Although the strains carrying this LPS cluster seem to be particularly

adapted for causing human disease, additional genetic factors present in the

genome may have allowed a particular clone of Sg1 to evolve within this highly

diverse species.”

In addition to gaining insight

into the genetic basis for L. pneumophila Sg1 virulence, Buchrieser suggests

this work may lead to new methods of detection. “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.”

Scientists from the Institut

Pasteur/Centre National de la Recherche Scientifique (CNRS) (Paris, France) and

the Centre National de Référence des Legionella/Institut National de la Santé

et de la Recherche Médicale (INSERM) (Lyon, France) contributed to this study.

This work was supported by grants

from the Institut Pasteur, the Centre National de la Recherche Scientifique

(CNRS), the Agence française de sécurité sanitaire de l'environnement et du

travail (Afsset), and the Israeli-French Research Networks Program in

Bioinformatics.

Reference: Cazalet C., Jarraud

S., Ghavi-Helm Y., Kunst F., Glaser P., Etienne J. and Buchrieser C.

Multi-genome analysis identifies a worldwide distributed epidemic Legionella

pneumophila clone that emerged within a highly diverse species. Genome Res.

doi:10.1101/gr.7229808.

Source: Genome Research/Cold

Spring Harbor Laboratory Press