Hedgehogs harbouring superbugs

A battle between bacteria and fungi on the skin of Hedgehogs may be how MRSA developed
07 January 2022

Interview with 

Mark Holmes, University of Cambridge

HEDGEHOG

Hedgehog outdoors

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Chris Smith interviews Mark Holmes about his team’s discovery of the prickly origin story of the MRSA superbug...

Chris - This week, there's been a surprising finding that hedgehogs might be the origin of one form of the antibiotic resistant bacterial superbug MRSA (which stands for Methicillin-resistant Staphylococcus aureus). New research, which has been published in the journal ‘Nature’ this week, has found that a microbial war going on on the skins of our prickly hedgehog friends is probably where MRSA came from in the first place. Here is Cambridge University vet and immunologist Mark Holmes, who's one of the team who made the discovery. How did you link MRSA and hedgehogs in the first place? It's not an intuitive leap.

Mark - No, it isn't. About 10 years ago, we discovered this new strain of MRSA in dairy cows and at the time we assumed that antibiotic use in treating cows might have selected for this, but we looked more widely to see where this type of MRSA was and we found quite a bit of it in wildlife - in fact, scattered throughout Europe. Then we almost stumbled upon the fact that hedgehogs seem to have lots of this MRSA. We started off with a survey in Sweden and in Denmark and it turns out that around 50 or 60% of hedgehogs have this MRSA.

Chris - Someone just swabbing hedgehogs for the fun of it? How did this come to light? It doesn't seem an intuitive thing to do.

Mark - No. There's always the aspect, I think, of most scientists being a bit curious and a bit stamp collector-y. When we found it in farm animals our initial thought was: "Where are the farm animals getting MRSA?" The obvious place they'd get it from is people, but we still swab wildlife to see, because actually one of the other aspects of our research is what makes a bacteria particularly want to live on a particular host. Particularly something like Staphyloccocus aureus - we find it in many species of animals, but we don't necessarily find the same strains on the same species.

Chris - Why have they got it at all?

Mark - About 60 years ago, researchers in New Zealand had made an observation that there's a particular fungus, a skin disease, on hedgehogs. When you've got hedgehogs with this skin disease, you tended to be able to detect penicillin resistant bacteria. So when we made our observation and read those papers in the dark history of microbiology, we put two and two together and wondered if this is what was selecting for the MRSA on hedgehogs, and there followed a lot of work where we sampled lots of hedgehogs, we looked for fungus on hedgehogs, we sampled the MRSA, we sequenced those bacteria, and are able to show pretty well without much doubt that the hedgehog MRSAs firstly existed all that time ago. The ones we find in people are closely related to the ones we see in the hedgehogs, and they are similarly geographically distanced. Particularly, one of our sample sets is from Denmark and they're all on one of the islands. All the people and all the hedgehogs have one strain of the MRSA, and on another island in a different part of Denmark, again, the hedgehogs and the people share very similar strains of the MRSA, but two geographical strains are clearly distinguishable.

Chris - To make this clear then - some hedgehogs get fungal skin infections and that fungus is doing something that enriches for the presence of the Staphylococcus and specifically the Staphylococcus that happens to be resistant to, you say, from New Zealand penicillin, but that's going to include methicillin, isn't it? MRSA? So, it's the presence of the fungus carried by the hedgehog that then selects for resistant bacteria on those hedgehogs.

Mark - Yes - And it won't come as a surprise to many listeners who'll remember that Alexander Fleming discovered penicillin because he had a fungus growing on a plate of bacteria. So we'd known for a long time that fungi are a good source of antibiotics, things that kill the bacteria, and the fungus produces this antibiotic in order to outcompete for the food resources on the skin of the hedgehog.

Chris - And the genetic know-how for how their particular MRSA does what it does, that's then shared with strains that then cause superbug infections in hospital? That's basically how we have got those things: they already existed, but our use of antibiotics in hospital has enriched the genetic sharing of the know-how among strains that infect humans?

Mark - Yes. I mean, it's probably true of almost every type of antibiotic resistance: they're actually evolved in nature. It was selected a long time ago. So these resistant genes are scattered about, throughout nature and throughout the environment. However, what we do as people is we use vastly more antibiotic than ever a fungus secretes. Many of these antibiotics end up in the environment, and what's happening with human use of antibiotics is that we've upped the anti as far as selection pressure. The amount of antibiotics that bugs are exposed to is much more than it would be in nature. They come across it more frequently at higher concentrations because of the medical and veterinary use of antibiotics.

Chris - Mark, thank you very much indeed for sharing that discovery with us. That's Mark Holmes, he just published that in the journal 'Nature.'

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