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Cholera bacteria are more infectious after trip through human body


Cholera bacteria become highly infectious while traveling through the human gut and remain in this state even after they are purged from the body, according to a new study. The bacterium's ability to transform itself into a highly infectious state may help explain the epidemic spread of the disease.

Cholera bacterium, Vibrio cholerae.

The discovery is relevant to the development of cholera vaccines, because most research uses bacteria samples from the laboratory rather than the hyper-infectious strains from humans. Cholera is spread through contaminated water and food, and outbreaks still occur in countries such as Bangladesh, where researchers collected samples for this study.

Andrew Camilli, of Tufts University School of Medicine in Boston, Massachusetts, led the research. His team is now trying to understand how the bacterium enters a 'transmittable' state. They believe genes are involved: Certain genes may switch on (or off) before the bacterium detaches from intestinal walls and leaves the body to find a new host.

To test this hypothesis, the researchers compared gene expression in bacteria from soil and freshwater to bacteria taken from three cholera patients. Using gene chips, they identified about 230 genes whose expression changes significantly after passing through the human host. The researchers are now investigating the functions of these genes.

Camilli and his team also infected laboratory mice with the different forms of Vibrio cholerae. The human-derived bacteria were a hundred times more likely to cause disease in mice than bacteria from soil or water. Furthermore, these bacteria remained highly infectious after living in pond water for five hours.

The human gastrointestinal tract is not only a suitable environment for the cholera bacterium to live and grow, it also helps prepare the pathogen to infect other humans, the researchers conclude in Nature.

Given the new information, the fact that some vaccine projects are using laboratory strains of the cholera bacterium represents a potential problem, says Camilli. As a possible solution, his laboratory is trying to identify proteins on the surface of the human-derived cholera bacteria that can be vaccine targets.

The three patients in the study were infected by the cholera strain that was sequenced in 2000 (the 01 Inaba El Tor strain N16961 from Bangladesh). The gene that is most highly induced in the patients has not been previously identified, perhaps because it is small and overlaps with another gene.

"The significance of this gene in the transmission of cholera—or for life in the environment—has not been investigated," says Camilli. "It is on our list of things to do."

See related GNN articles
»Potential cholera vaccine for travelers passes safety test
»Comparative study reveals genes related to cholera pandemic

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Merrell, D.S. et al. Host-induced epidemic spread of the cholera bacterium. Nature 417, 642-645 (June 6, 2002).

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