The tracker that could head off hospital bugs

Scientists have devised a way of tracking the spread of antibiotic-resistant bugs within a hospital by analysing the smallest changes to the genetic signature of the microbe as it spreads from person to person.

The development could lead to new ways of identifying and blocking the infection routes that allow potentially fatal superbugs to spread between hospital patients as well as individuals in the wider community, the researchers said.

A study found that it is technically possible to distinguish between two methicillin-resistant Staphylococcus aureus (MRSA) microbes infecting two different patients attending the same hospital by analysing the smallest genetic mutations in the DNA sequences of the bacteria.

The approach means that scientists should be able to construct a genetic “family tree” of the MRSA microbes found within a hospital which will reveal how the superbugs have come to spread from one individual to another, and possibly how they entered the hospital in the first place – by visitor or patient.

The scientists emphasised that the technique, which still has to be refined to make it cheap enough to use within the NHS, is not designed to apportion blame on individuals but to work out the best ways of identifying the weak links in the chain of measures designed to stop superbugs from spreading.

“It provides us with an opportunity to study MRSA in such find detail it will be possible to trace the routes of transmission in a localised setting,” said Sharon Peacock of the department of medicine at the University of Cambridge, one of the authors of the study published in the journal Science.

“We’re not trying to find blame in any individual and this is not a case of fingerpointing at any individual who might have spread a strain to another individual – that certainly wouldn’t be the point of this. The point really would be to strengthen hospital policy. It would allow us to find any breaks in this chain of infection control.”

She added: “We’re talking to the Health Protection Agency who are very excited about this technology.”

The technical development has come about because of the dramatically falling costs associated with sequencing the full genome of a microorganism. Ten years ago it took about three years and cost about half a million pounds to sequence the genome of a bacterium, now it takes between four and six weeks, and costs about £200, said Stephen Bentley of the Wellcome Trust Sanger Institute near Cambridge.

“Perhaps if you can get the cost of an individual test down to £20 or £30 that it may be viable to introduce that into a hospital setting as a routine test,” Dr Bentley said.

The study sequenced the genomes of 63 isolates of an MRSA strain, 43 of these were from a global collection and had infected people between 1982 and 2003, and the remaining 20 isolates came from a hospital in Thailand where the patients in question had become infected within seven months of one another.

The different genome sequences from the two groups of MRSA samples allowed the researchers to investigate the global spread of the bacterial strain from continent to continent, as well as the individual transmission from person to person within a single hospital, Dr Bentley said.

“Telling the difference between isolates within one species is fundamentally important in the development of public health strategies. It allows researchers and public health officials to see how infections are spread: from person to person, from hospital to hospital and from country to country,” he said.

The study concluded that the MRSA strain probably first evolved in Europe in the 1960s as a result of the widespread use of antibiotics. The outbreak in the Thai hospital was found to involve two genetically different strains that had been introduced into the hospital on separate occasions, said Simon Harris, co-leader of the study from the Sanger Institute.

“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,” Dr Harris said.