Magazine article Science News

Cholera Hides Sinister Stowaway

Magazine article Science News

Cholera Hides Sinister Stowaway

Article excerpt

Cholera has come nearly full circle, again. Beginning in Indonesia in 1961, the tireless traveler has almost completed its seventh circumnavigation of the globe since 1817, leaving freshly dug graves as evidence of its passage. In Latin America alone, cholera has claimed more than 10,000 lives since its landfall in Peru less than 6 years ago.

Now, scientists trying to determine what makes Vibrio cholerae so deadly and unstoppable have discovered that the virulent bacterium does not travel alone.

Deep inside it resides a viral stowaway packing all the genes needed to turn even harmless strains of cholera into killers.

The virus-known as a bacteriophage, or phage-leaps readily from one cholera strain to the next, a capability that had been postulated for phages but never demonstrated before in the laboratory or in nature.

The cholera-infecting phage attaches itself to slender receptors called pili, which, in the human intestine, bristle from the cholera bacterium like a bad haircut. The phage then slips into the bacterium and deposits into its chromosome a tidy package of genes that code for cholera toxin. Without these genes, the bacterium is harmless.

"We are a hospitable environment for the infection of Vibrio by phage," says Matthew K. Waldor of the Tupper Research Institute of New England Medical Center in Boston, who, with John J. Mekalanos of Harvard Medical School's Shipley Institute of Medicine, reported the finding in the June 28 Science.

Waldor and Mekalanos began by removing cholera toxin genes from the prevalent El Tor strain and substituting genes that confer resistance to antibiotics.

They showed that a phage carried the resistance genes as it moved from the infected strain to a strain that lacked the virulence genes. In this experiment, the gene that usually switches on toxin production conferred resistance instead. …

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