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Predicting survival from MRSA superbug infection by DNA sequencing

8 August 2017

Dr Ruth Massey, Microbiologist who joined the School of Cellular and Molecular Medicine (CMM) at the University of Bristol in July 2017, has published a new paper in Nature Microbiology entitled 'Clonal differences in Staphylococcus aureus bacteraemia-associated mortality.' This paper looks at predicting survival rates from MRSA superbug infection by DNA sequencing.

Dr Ruth Massey, who joined CMM in July 2017, from the University of Bath, led a team of researchers from Universities of Bath, Bristol, Exeter, Cambridge and London School of Hygiene and Tropical Medicine at the Milner Centre for Evolution. They were able to study blood samples from around 300 patients with septicaemia and look at the different strains, how they behaved and assess their lethality.  

The scientists looked at the details of the 300 patients they had information about and examined the genetic code of the infecting MRSA bacteria and paired the information. They looked at all the risk factors including age, other illnesses and if the patient was still alive after 30 days of the infection, and if deceased, did the MRSA contribute to their death.

Dr Ruth Massey said "Our study is important because it's the first time we've collected data in human patients rather than relying solely on animal models. We've combined information from real people with phenotypic and DNA sequence data from the bacteria causing the infections.

"For the first time, we've been able to predict which strains are most virulent, or likely to cause disease and the outcomes of infection.

"We've previously found that the most highly toxic MRSA strains were less likely to cause septicaemia, however, this research shows that when it does, it is more deadly than other strains. What is clear is that we still have a long way to go to understand how this pathogen causes disease."

MRSA is a common bacteria that normally lives harmlessly on the skin, however, if it were to enter the body through a cut, then it could cause septicaemia (blood poisoning), a potentially life threatening infection. This affects many thousands of patients throughout the UK each year, and with MRSA becoming resistant to most types of antibiotics, up to 20 per cent of those patients with invasive infections die. There are two main strains of MRSA in UK hospitals, CC22 and CC30.

In the study DNA sequencing was performed along with measuring toxicity (ability to kill human cells) of the MRSA strains and their abiltiy to form dangerous biofilms. A biofilm forms when a group of bacteria secrete proteins which stick together and coat surfaces in slime, these films make it easier for bacteria to evade patients immune system and block antibiotics. In CC22 strains it was found that both toxicity and biofilms played an important part in patient survival, but it was not the case for CC30 strains which indicates it would kill patients in a different way.

Dr Massey said "Unfortunately 20 per cent of patients with septicaemia die and cases are on the increase - this suggests that existing infection control and treatment options are insufficient to tackle this important health problem.

"We've identified that MRSA kills people in different ways depending on the strain and that the low toxicity CC30 strains are killing patients in an as yet unknown mechanism. It could be that they are better at evading the immune system. We need further research to find out how they do this, in order to develop new treatments for these patients.

"Our study further demonstrates the use of a combined genomics and data analytic approach to enhance our understanding of bacterial infection at the individual level, which will be an important step towards personalised medicine and infectious disease management."

Further information

Follow the link to read the full paper in Nature Microbiology.

Dr Ruth Massey provides more details of the background to her paper in her online article: Predicting mortality from S. aureus bacteraemia

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