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Resistant bacterium modifies its genome to cause infection


Scientists have identified a virulence region never seen before in the genome of Enterococcus faecalis—a leading cause of bacterial infection among hospital patients. This bacterium lives peacefully in the human gut, but it also thrives on wounds and burns. The researchers identified a group of genes that may contribute to the bacterium's transformation from a harmless gut homebody to a menacing invader.

Enterococcus faecalis in a blood culture.

A growing concern among doctors is the potential of E. faecalis to pass on antibiotic-resistant genes to other strains and pathogens. Hospital wards are by their nature a good place for the transfer of virulence factors among bacteria.

The discovery of the region, known as a pathogenicity island, could help in the fight against bacterial infections. The island may contain "useful markers for detecting unusually pathogenic strains," the researchers say. Armed with information about these harmful strains, hospitals could establish controls to prevent the spread of infection within their wards.

Michael S. Gilmore of the University of Oklahoma Health Sciences Center and colleagues compared the genomes of several E. faecalis strains, including the strain responsible for an infectious outbreak twenty years ago. They also studied the first strain known to be resistant to the drug vancomycin—an antibiotic of last resort. All infectious strains contained a 150,000 base-pair region not found in the non-infectious strains.

The researchers also report that subtle variations within the structures of the pathogenicity island "enable strains with the element to modulate virulence." These variations, they note, occur at a high frequency.

Enterococcus faecalis V583 chromosome

"The danger of the organism is that it can be genetically flexible in persons with compromised immune systems," says Gary M. Dunny of the University of Minnesota in Minneapolis. The bacterium's virulence, he adds, probably involves other genetic elements outside the pathogenicity island.

The pathogenicity island contains genes coding for proteins thought or known to be important for virulence or survival of the bacterium. The region also codes for the surface protein Esp, which promotes bacterial attachment to surfaces and biofilm growth. Newly identified gene clusters in the island may be targets for new anti-bacterial therapies, the researchers report in Nature.

Scientists at The Institute for Genomic Research (TIGR) in Rockville, Maryland, have sequenced E. faecalis and the data are available on TIGR's Website.

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Shankar, N. et al. Modulation of virulence within a pathogenicity island in vancomycin-resistant Enterococcus faecalis. Nature 417, 746-750 (June 13, 2002).

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