TB is a major world
killer |
A virulent
form of tuberculosis was created in the laboratory by experts trying to
alter its genetic structure.
The mutant form of the bug multiplied more quickly, and was more lethal
than its natural counterpart.
Researchers from the University of California at Berkeley, US, had
actually been trying to disable genes and make the bacterium less deadly.
"This is one of the very few hyper-virulent organisms ever created,"
said scientist Dr Lisa Morici.
Tuberculosis is one of the world's biggest killers, and scientists are
probing its genetic structure in a bid to find weaknesses that might be
exploited by new treatments.
The Berkeley study, published in the Proceedings of the National
Academy of Sciences, concentrated on a particular collection of genes
thought to give TB some of its virulence - its ability to infect.
Growing threat
They disabled these genes, and expected to find a weakened form of TB
as a result. Instead, the organism grew in virulence.
It killed laboratory mice within seven months of exposure, while those
infected with normal TB survived the experiment.
Further investigations suggested that the genetic changes had the
unexpected effect of undermining the body's own immune response against
TB.
Professor Lee Riley, who led the study, said: "These findings came as a
complete surprise to us.
"We thought we had made a mistake, so we repeated the test several
times, and we always got the same result."
Bioterror played down
There have been fears that similar genetic modifications might lead to
a new form of TB that could be used in bioterrorism, but Dr Morici said
this was unlikely.
The bacterium is hard to deliver in an aerosol - the accepted method of
spreading it over large populations - and despite its extra virulence in
the lab, still grows relatively slowly and can be treated by antibiotics.
"There are several other organisms out there that are easier to
manipulate than TB," she said.
Dr Neil Stoker, from the Royal Veterinary College at the University of
London, UK, said that his research had also uncovered hyper-virulent
strains of TB.
The emergence of these strains should pose no risk to humans, he said.
"These are not going to become 'super-strains'," he said. "They are
already going to be out there, and they have not become dominant."
TB is such a "successful" pathogen, he said, for completely the
opposite reason.
"It is such a phenomenal pathogen because it does not cause disease in
everyone it infects.
"It has this extraordinary ability to transmit itself, and nine out of
10 people who have it will never fall ill."
