Zooming in on superbugs

An international team lead by researchers from Britain’s Wellcome Trust
Sanger Institute used very high-throughput gene sequencing machines to
compare individual MRSA bugs from patients and show precisely how they
were genetically related.

Methicillin-resistant staphylococcus
aureus (MRSA) causes infections such as blood poisoning and pneumonia
and can kill. It is one of a group of drug resistant bacteria, or
“superbugs” that are major problems in hospitals around the world.

Stephen
Bentley, who led the study published by the Science journal, said the
new technology had allowed scientists for the first time to find
precise differences in strains of the bug – a “fundamentally important”
step to tackling infection.

“It allows researchers and public
health officials to see how infections are spread, from person to
person, from hospital to hospital, from country to country,” he said.

Until
now, even the best methods for identifying genetic differences between
bacteria have been unable to pick up tiny differences – leaving
uncertainty about how infections spread.

The success of the new
method relies on comparing whole genetic codes, the scientists said.
The ability to track strains in this way will help researchers
understand how strains can spread so rapidly, and should lead to new
control strategies, not only for MRSA but also for other emerging
superbugs.

The researchers looked at 62 MRSA samples. One set of
42 was taken from hospitals in North and South America, Europe,
Australia and Asia from patients who became infected with MRSA between
1982 and 2003, and 20 were from a hospital in Thailand, from patients
who developed MRSA within seven months of each other.

“We wanted
to test whether our method could successfully zoom in and out to allow
us to track infection on a global scale – from continent-to-continent,
and also on the smallest scale – from person-to-person,” Simon Harris
of the Sanger Institute told reporters at a briefing.

The team
sequenced the whole genomes of all the samples and were able to spot
single-letter changes in the genetic code and identify differences
between even the most closely related bugs.

From the results
they created an “evolutionary tree” which showed that MRSA infections
are often clustered in locations, but can be spread across borders by
patients travelling between one place and another and visiting
different hospitals.

Drug-resistant bacteria kill about 25,000
people a year in Europe and about 19,000 in the US. The European Centre
for Disease Prevention and Control says superbug infections cost 900
million euros ($1.31 bln) a year in extra hospital time and 600 million
euros a year in lost productivity.

Sharon Peacock of Britain’s
Cambridge University who also worked on the study, told reporters the
work could “flag up hotspots for MRSA transmission … and these could
then be examined to improve infection control strategies.”

Dutch researchers said that all hospital patients should be screened for MRSA to try to halt its spread.