dna sequencing stops mrsa spread

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DNA Sequencing Stops MRSA Spread

Professor Sharon Peacock, University of Cambridge

This week, the journal Lancet Infectious Diseases included a rather unusual detective story one

where rapid DNA sequencing was used in a hospital to track an outbreak of MRSA down to one

unsuspecting carrier. It bears all the hallmarks of good television forensic crime drama, but it

actually took place right here in Cambridge, and the research paper they released is said to be the

first example of using this technique to bring an infection to a close.

Chris - The author of that study is Professor Sharon Peacock from Cambridge University. Shes

with us. Hello, Sharon.

Sharon - Hello, good evening.

Chris - Why did you do this?

Sharon - We want to control MRSA 1.., the passing of it from one patient to

another, as much as possible because 2.any infections that might arise. And so, we

need to see when MRSA does move from one person to another and we need to see it fast.

The way we do that at the moment is we work out whether two patients who are MRSA positive

couldve got it from each other because they're in the same ward at the same time. But what we

can't do at the moment is to do any sort of3to see how related the two are.

If we had a tool where we could see that they're were either4., that would be

very helpful because we could work out if transmission had occurred from patient A to patient

B,but those tests aren't available at the moment and thats a problem for us.

Chris - So, we know that the two patients have got MRSA, but there's more than one form of

MRSA at a molecular level, so we dont know whether theyve come into the hospital

independently with their infection or something in the hospital is transmitting these infections

between these 4.

Sharon - Thats exactly right. You can't see the 5between patients. So, what this

new technology does is to actually sequence the whole genome with the bacterium.

Even just a few years ago, sequencing a bacterial genome wouldve cost hundreds of thousands of

pounds and taken several years to do a single one. There's new technology now available whichallows you to do sequencing of multiple genomes within a day actually, very, very fast and the

price has actually fallen very rapidly. That means that we can bring this technology to clinical

practice quite soon. In this research study, we really wanted to test out how useful that could

actually be in trying to understand better an MRSA outbreak on our Special Care Baby Unit at

Addenbrookes Hospital.

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Chris - Can you talk us through how you actually did the study? What were the steps and how did

you investigate what was going on?

Sharon - Well, we had access to this technology and we knew that there was a suspected

7of MRSA on our Special Care Baby Unit, which should be investigated by our

infection control team. Now, they were not sure whether an outbreak had occurred over anextended period of 6 months. There were cases coming into Special Care who were MRSA

positive, but we werent sure if they were related. And the reason for that is because throughout

that 6 months, there were quite large gaps when there were no people who were MRSA positive

at all. And so, one has to ask, how has that transmission gap been closed?

So, what we did in the first instance was, we took all of8..and we sequence them,

and were able to say very quickly that they were all so related at the genome level that this had to

be an outbreak. At the same time we extended our search to look for bacteria that are coming

from GPs actually in outpatient departments. We sequenced those and we found that actually,

the outbreak was 9..originally thought and that there were cases actually in the

community, people had developed infections. We really only linked that to the Special Care Baby

Unit through the sequencing.

Chris - So that meant that you were then able to say right there, there are outbreaks

occurring. How did you then go and find the individual that was causing those outbreaks?

Sharon - Well, having identified that there was definitely an outbreak, we put the Special Care

Baby Unit under very close 10to see if new carriers or people infected with

MRSA popped up. Actually, 2 months after the previous MRSA carrier, we found a new MRSA

carrier, an infant, and we were puzzled by this because there's obviously a very long gap when we

saw no MRSA at all. And it was at this point that we thought there must be a carrier amongst amember of staff.

And so, we gathered the staff together and got full agreement that they would be

11and we saw with 154 staff members, we found just 1 that was

a12... And so, of course we immediately sequenced that and it was a 13to

the outbreak strain. What we think is that this person was involved in the outbreak. What that

allowed us to do was to actually treat that individual so that the MRSA carriage is removed from

them and that effectively stopped the outbreak from continuing any further.

Chris - How do you know that they gave it to the kids and not the other way around?

Sharon - I think that it will be difficult to be absolutely unequivocal about that individual causing

spread throughout the entire outbreak, but what we did do with the staff carrier was we

sequenced lots of single bacteria from their carriage population and we got genetic matches to

before the 2-month gap and after the 2-month gap. We think that that is fairly strong evidence

actually that that person was at least involved in that transmission event.

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Chris - This is obviously extremely helpful in terms of guiding infection control strategies, but are

there any other uses for this sort of technology in guiding how we tackle bugs of all kinds in the

hospital?

Sharon - I think actually that therell be numerous applications for this. Well need to work out

exactly where, when, and how to use it, but for example, it could be really key in investigating thefood-borne outbreaks to work out if there is an outbreak and to help contain that. We could use it

for example to get rapid drug susceptibility testing for people with tuberculosis. So, there is a very

14for this technology and I think that we will be seeing this brought into use in

the next few years.

carrier / transmission pathways / swabbed / twice as big as / individuals/ we want to contain

/outbreak / direct match / wide range of applications/ related or unrelated /surveillance/ typing

on the bacteria / Transmission / those isolates

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