Scientists say they have identified the exact process by which the bacteria which causes cholera -- historically one of the most deadly scourges of humanity -- overwhelms and kills its victims.

In cholera, usually caused by drinking contaminated water, the small intestine is infected with the bacterium Vibrio cholera, leading to the disease's characteristic severe watery diarrhea that brings on severe and sometimes fatal dehydration.

Now Swiss researchers have discovered a "predatory" technique in which V. cholerae uses a tiny spear to stab and kill neighboring bacteria -- even other V. Cholerae -- and then steals DNA from it.

Known as "horizontal gene transfer," the mechanism turns the cholera bacterium even more deadly and virulent by allowing it to absorb the genetic traits of its bacterial prey, the researchers report in the journal Science.

That could explain why that although humans can build immunity to some strains of cholera, new strains sometimes appear.

The ability to take DNA from other bacteria -- including other strains of cholera -- might be one reason these new strains appear, the researchers say.

"That's what we think -- what we see is part of what makes the most virulent strains so virulent," says study co-author Melanie Blokesch, an assistant professor of microbiology at the Swiss Federal Institute in Lausanne.

The DNA transfer capability of V. Cholerae may have been involved in a cholera epidemic in the early 1990s that ravaged Southeast Asia, she says.

Blokesch, in her laboratory, tested a number of strains of cholera gathered from various parts of the globe, including strains thought to have caused a cholera pandemic in Indonesia in the 1960s that eventually spread to Asia, Latin America and Europe and is still affecting populations in those regions today.

"Using this mode of DNA acquisition, a single V. cholerae cell can absorb fragments containing more than 40 genes from another bacterium," says Blokesch. "That's an enormous amount of new genetic information."

The phenomenon of horizontal gene transfer is common in many bacteria and can affect virulence factors and antibiotic resistance, the researchers say.

Activation of V. Cholerae's spear-killing device likely renders it more deadly to humans when they ingest it, they suggest, as this molecular spear might also be capable of killing protective bacteria present in the human gut.

Further studies will focus on the interplay between the creation of the bacterium's spear and horizontal gene transfer, Blokesch says.

"By studying this interplay, we can begin to better understand evolutionary forces that shape human pathogens and maybe also transmission of the disease cholera," she says.