DNA Blueprint of Cholera.

As reported in the journal Nature (week of August 2, 2000), scientists have determined the entire order of paired chemical building blocks that constitute the DNA of the deadly cholera bacterium. Vibrio cholerae, a comma-shaped microbe, causes a severe diarrheal disease that has been endemic in southern Asia for at least 1,000 years. 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. Since 1817, the disease has spread worldwide to cause seven pandemics. 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. 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 cholerae holds enormous promise for helping us fight some of the world's most intractable infectious diseases," said 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 vaccines, drugs, and diagnostic tools."

A talented team of cholera and genome-sequencing experts contributed to the project. The team included John Mekalanos, Ph.D., of Harvard Medical School, a 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 in Rockville, Maryland.

The project began in late 1996, one of the earliest microbial genome-sequencing efforts financed by MAID, and its outcome has been long awaited. Since that time, NIAID has rapidly expanded its portfolio of such grants; 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, noted Dennis Lang, Ph.D., NIAID's bacterial and viral enteric diseases program officer. The larger chromosome, comprising nearly three million base pairs, contains most of the organism's critical genes, including those coding for the diseasecausing toxins and proteins that carry out essential cell functions. The smaller chromosome is roughly one-third the size. "Both chromosomes are essential," said Dr. Lang. 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 said. Dr. Colwell and others have long believed that the bacterium enters a q uiescent 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 said, "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.