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The bacterium that causes tuberculosis (TB) is so genetically distinct in different parts of the world that tackling the disease could require separate vaccines for each region, say researchers.

In a study published yesterday (13 February) in Proceedings of the National Academy of Sciences, the researchers say the bacterium has evolved into six distinct lineages.

Each of these was mostly found in a different human population around the world, prompting the researchers to classify them as the East-African-Indian, East-Asian, Euro-American, Indo-Oceanic, and two West-African lineages.

"This study shows that TB is not just TB: people developing vaccines should work with different strains for different parts of the world," says lead author Sebastien Gagneux of Stanford University Medical Centre, United States.

Tuberculosis kills about two million people a year. About two billion people, one-third of the world's population, carry the bacterium, but it is active only in about ten per cent of these people.

The current TB vaccine varies in effectiveness depending on where it is used. Scientists have suggested this is due to environmental factors or using different bacterial strains to create the vaccine, but Gagneux's study suggests that the TB bacterium's genetic variability could be to blame.

The researchers analysed genes from 875 samples of the bacterium that came from 80 countries. Their findings suggest that distinct bacterial strains have adapted over time to specific human populations.

Joanne Flynn of the University of Pittsburgh School of Medicine, United States says this is important as different strains might use slightly different mechanisms to cause disease "and this may be specific to the population it is most likely to encounter".

"A vaccine that is effective against one strain might not be effective against another strain," she told SciDev.Net.

Understanding the differences between strains could explain why some are more dangerous than others, and identify possible vaccine or drug targets, she added.

Link to full paper in Proceedings of the National Academy of Sciences

Reference: Proceedings of the National Academy of Sciences 103, 2869 (2006)

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