Bubonic plague, the bacterium blamed for the Black Death of medieval Europe and now a potential biological weapon, has had its entire genome sequenced. The genes seem to be "unusually fluid", readily re-arranging themselves and picking up new genes from other microbes.

That could mean that more virulent strains of plague might emerge. More ominously, it suggests that enhanced strains might be relatively easy to develop as weapons.

The bacillus that causes bubonic plague, Yersinia pestis, commonly infects rodents in Asia, Africa and the Americas. But it occasionally spreads to humans, with lethal effect.

It does so by infecting both insects and mammals. Fleas that feed on infected rodents swallow the bacteria, which infect and block their midguts. The starving fleas feed voraciously, but only regurgitate the blood they try to swallow - along with the bacteria. So plague spreads among rodents, often causing only mild disease.

If the infected flea bites a human, however, up to half the victims die, unless treated with antibiotics. If the bacteria invade the lungs of such patients, and they cough them out, nearby people may catch "pneumonic" plague. This is always fatal without treatment.

Pneumonic plague is the form feared as a potential biological weapon, as it can be released as an aerosol and can spread directly among humans, without the intervention of fleas. The Soviet Union developed such a plague weapon.

"Pathogenicity islands"

The gene sequence of Yersinia pestis, produced by Julian Parkhill of the Sanger Centre in Cambridge, UK and colleagues, shows how the bacillus learned to infect both insects and mammals.

It picked up genes directly from baculoviruses that infect insects, including one for a toxin that damages the midgut. It also acquired "pathogenicity islands", assemblages of genes from other bacteria that help cause human disease.

The sequence also reveals novel surface molecules, which might provide new ways to attack plague. But "this genome displays unusual fluidity," comment Stewart Cole and Carmen Buchreiser in Nature, where the genome is published. Numerous Yersinia genes have been copied backwards and have swapped positions within the genome, sometimes creating different variants in the same population.

These recombinations could mean differences in virulence in a single batch of plague, they note. That could also mean that the bacteria - or bioweapons developers - have the genes at their disposal for new and potentially nastier strains of disease.

Journal reference: Nature (vol 413, p 523)
 
SOURCE: NewScientist.com on the 3rd October 2001