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Infant mortality: New clues from the sequenced Shigella genome
By Birgit Reinert

Chinese scientists have sequenced the genome of a bacterium that is a leading cause of infant mortality in developing countries. About one million people die of Shigella infections every year, most of them children. The bacterium Shigella flexneri causes sudden and severe diarrhea in humans, known as shigellosis.

Highly infectious microbe Shigella flexneri.

New treatments are needed for this highly infectious microbe because antibiotics are often inadequate and drug-resistant strains are on the rise. Currently, no vaccines exist and the World Health Organization considers the development of a vaccine a priority.

The publication of the genome sequence is an important step achieving this goal. The researchers identified regions of DNA linked to the virulence of the organism; these are promising targets for vaccines.

The sequenced S. flexneri strain was isolated from a patient with severe acute shigellosis in Beijing in 1984. The bacterium is commonly found in water polluted with human feces. It is transmitted in contaminated food or water and through contact between people. Upon infection, humans develop severe abdominal cramps, fever, and frequent passage of bloody stools.

The bacterium has about 4,700 genes. The S. flexneri genome consists of a chromosome and a smaller DNA structure called a virulence plasmid, which contains genes important in causing disease. The plasmid includes regions that are densely populated with genes called pathogenicity islands.

So far, the scientists identified 64 putative pathogenicity islands. "We are working to explore what percentage of the islands is involved in virulence," says Qi Jin of the State Key Laboratory for Molecular Virology and Genetic Engineering in Beijing, China, who led the study.

As part of the study, the researchers compared the sequenced Shigella pathogen to E. coli, which was sequenced last year. The comparison revealed that both microbes may be closely related and even constitute the same species. The findings are reported in Nucleic Acids Research.

Shigellosis is not only a significant cause of infant mortality in developing nations but maintains endemic levels of infection worldwide. According to the US Food & Drug Administration, an estimated 300,000 cases of shigellosis occur annually in the United States. Polluted water and unsanitary handling of food are the most common causes of disease.

More information on the S. flexneri genome is available from the Microbial Genome Center of Chinese Ministry of Public Health here.

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Jin, Q. et al. Genome sequence of Shigella flexneri 2a: insights into pathogenicity through comparison with genomes of Escherichia coli K12 and O157. Nucleic Acids Res 30, 4432-4441 (October 15, 2002).

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