Cholera needs guts to survive

Human stomach boosts cholera bacterium's infectivity.
6 June 2002

TOM CLARKE

Vibrio cholerae bacteria infect between 100,000 and 300,000 people each year. © SPL

Human digestive juices switch on genes in cholera bacteria that make the microbes hundreds of times more infectious, new research suggests. The finding identifies potential targets for cholera vaccines or diagnostic tests

Vibrio cholerae bacteria infect between 100,000 and 300,000 people each year, causing acute vomiting and diarrhoea. One in every 100 victims dies from dehydration.

Although dehydration is easily treated, cholera is a major problem in countries with poor sanitation. Current vaccines are only partially effective, and are banned by some countries because of harmful side effects.

Ten important genes are active in bacteria from the stools of cholera victims that are inactive in lab bacteria, Andrew Camilli of Tufts University School of Medicine in Boston, Massachusetts, and colleagues have found¹. The team collected the samples during a cholera outbreak in Dhaka, Bangladesh, in April 2001.

Some of the newly activated genes help V. cholerae feed, and others enable it to swim. All of these genes seem to contribute to making the bacteria between two hundred and seven hundred times more infectious - to mice at least. How is not yet clear.

The proteins that these genes produce could be ideal vaccine targets, says microbiologist James Kaper of the Center for Vaccine Development in Baltimore. Existing cholera vaccines may be flawed because they were developed using lab-reared bacteria. Tests based on the same proteins could also help to chart the spread the disease, says Kaper.

The Bangladeshi V. cholerae were highly infectious for at least five hours outside the body, maximizing their chances of re-infecting another human nearby. This suggests that V. cholerae "may have evolved to optimize their transmission", Camilli suspects. "Humans are a good growth environment for cholera and a perfect vehicle," he says.

Most of the time, V. cholerae bacteria live in stagnant water and reproduce very slowly. In humans, their numbers explode - V. cholerae causes diarrhoea that flushes all competing bacteria from the gut. Victims can have 100 million cholera bacteria in just one millilitre of stool.

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