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Back To Vidyya DNA Blueprint Of Deadly Cholera Bacterium Unveiled

Decoding An Ancient Infectious Foe

As reported in this week's issue of the journal Nature, scientists have determined the entire order of paired chemical building blocks that make up the DNA of the deadly cholera bacterium, an ancient infectious foe. The comma-shaped microbe, Vibrio cholerae, causes a severe diarrheal disease that has been endemic in southern Asia for at least 1,000 years. Since 1817, it has spread worldwide to cause seven pandemics. Between outbreaks, the organism thrives in brackish waters in both harmless and disease-causing forms.

"Determining the genomic sequence of medically important pathogens such as Vibrio cholera holds enormous promise for helping us fight some of the world's most intractable infectious diseases," comments Anthony S. Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases (NIAID), which funded the project. "Besides contributing to our understanding of how a microbe causes disease and survives in the environment, sequencing studies enable scientists to locate genes that may lead to potential new targets for vaccine candidates, drugs and diagnostic tools.

"Cholera has been extensively studied by many people, yet important new discoveries continue to be made," he adds. "Having the sequence available will facilitate these efforts immensely."

A talented team of cholera and genome sequencing experts contributed to the success of the project. They include John Mekalanos, Ph.D., of Harvard Medical School, a renowned microbiologist who probes how bacterial virulence factors cause disease; National Science Foundation Director Rita Colwell, Ph.D., of the University of Maryland, an expert in how V. cholerae persists in the environment; and genomics experts Claire M. Fraser, Ph.D., and John Heidelberg, Ph.D., of The Institute for Genomic Research (TIGR) in Rockville, Maryland .

The project began in late 1996, one of the earliest microbial genome sequencing efforts financed by NIAID, and its outcome has been long-awaited. Since that time, NIAID has rapidly expanded its portfolio of such grants, and they now number more than 30.

One of the most unusual findings of the cholera project is that instead of having one circular chromosome, like most bacteria, the organism has two, notes Dennis Lang, Ph.D., NIAID's bacterial and viral enteric diseases program officer. "When the team became aware of new research indicating that cholera strains may have two chromosomes, they rapidly assembled the final sequence into two separate elements."

The larger chromosome, comprising nearly 3 million base pairs, contains most of the organism's critical genes, including those coding for disease-causing toxins and proteins that carry out essential cell functions. The smaller chromosome is roughly one-third its size. "Both chromosomes are essential," says Dr. Lang. "You couldn't do away with either one of them and have the organism be viable."

Besides illuminating the bacterium's role in disease, the sequence information will enable scientists to investigate specific questions about how V. cholerae survives and persists in the environment, which it sometimes does by colonizing algae and other sea life. "Hundreds of different strains of the bacterium exist, and how they interact and evolve is largely a mystery," Dr. Lang notes.

Dr. Colwell and others have long believed that the bacterium enters a quiescent state where, much like bacterial spores, it is alive but fails to multiply unless triggered by specific environmental conditions. "The authors of the Nature paper found some genes in the cholera genome," Dr. Lang says, "that appear to be related to genes from sporulating bacteria. What role these cholera genes may play in that quiescent state, if it exists, remains to be seen, but this information will provide much fodder for that research."

The V. cholerae strain that was sequenced, a virulent El Tor strain used for many years in clinical studies, is also a strain that NIAID has produced in quantity and supplies to investigators worldwide for vaccine studies.

Cholera has a rapid onset and most often occurs in epidemics spread through contaminated water. Oral rehydration is an effective treatment, but left untreated, cholera causes severe diarrhea that has a high mortality rate, particularly in young children. According to information reported to the World Health Organization in 1999, nearly 8,500 people died and another 223,000 were sickened with cholera worldwide.

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Editor: Susan K. Boyer, RN
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