Delhi Belly Sent Packing

Scientists have decoded the genome of the common gut parasite Giardia lamblia, a frequent cause of gastric upset amongst backpackers and even children at nursery.

The genome sequence, published in this week's Science by Mitchell Sogin and his colleagues from the Marine Biological Laboratory at Woods Hole, US, should help scientists to develop better ways to prevent and treat the infection.  This is because giardia leads a doppleganger lifecycle: it spends part of its life swimming around inside the intestines as a microscopic "trophozoite", soaking up nutrients and multiplying.  Then, when it is shed from the body, it forms a tough cyst that can survive harsh chemical and environmental conditions.

But, when an individual ingests one of the cysts, usually by drinking contaminated water, the acidic environment of the stomach resuscitates the dormant cyst so that, within hours, it has turned into an intestinal trophozoite.

To be able to co-ordinate such a complex life-cycle the bug requires a significant genetic arsenal, which can also allow it to bypass the action of many of the drugs used to control it.

"Existing drugs can effectively treat people with Giardia infections, but as with many pathogens, the concern is that the parasite will develop resistance to these medications," says Anthony Fauci, from the National Institute of Allergy and Infectious Disease, which funded the study. "But the Giardia genome shows us that the parasite has a large complement of unusual proteins that are potential targets for new drugs or vaccines."