Antibiotic Resistant Bacterium MRSA Evolved In Hedgehogs 200 Years Ago, Study Finds

One type of methicillin-resistant Staphylococcus aureus – MRSA, the antibiotic-resistant bacterial scourge of modern clinical medicine – may have evolved 200 years ago in wild European hedgehogs, according to a recent study.

As you’re reading this, your body is teeming with bacteria. There’s about a 30% chance that your normal, healthy microbiome includes a thriving population of a bacterial species called Staphylococcus aureus either on your skin or in your nose. Most of the time, these little round bacteria are just harmlessly along for the ride – but they can turn deadly.

Even a small cut or break in the skin can let S. aureus inside, where the infection can cause blisters, boils, cellulitis, or impetigo. It’s also a common cause of food poisoning. In the bloodstream, S. aureus can hitch a ride to vital organs; it can also infect bones, joints, and even the surfaces of medical implants. And it’s getting harder and harder for doctors to fight.

Nearly all S. aureus strains circulating today are resistant to penicillin, and about 60% are also resistant to methicillin – part of a class of antibiotics developed specifically to fight penicillin-resistant bacteria. Doctors started prescribing methicillin in 1959. MRSA reared its head the very next year.

Logically, all types of MRSA must have evolved in hospital patients as the next step in the ongoing arms race between people and bacteria – or so it’s seemed for 60 years. A recent study, published in the journal Nature, suggests that at least one strain of S. aureus evolved genes for methicillin resistance at least 200 years ago on the skin of wild hedgehogs.

“This Ain’t A Scene; It’s An Arms Race”

It started, as most science does, with rigorous data collection – which in this case means that biologists chased down a lot of wild hedgehogs in order to swab their skin with Q-tips and check for microbes. About 60% of the hedgehogs waddling around Denmark and Sweden carry mecC-MRSA, a specific strain of methicillin-resistant S. aureus. Biologist Evan Harrison and veterinary medicine researcher Mark Holmes, both of Cambridge University, found surprisingly high levels of MRSA in hedgehogs elsewhere in Europe, too, as well as in New Zealand.

S. aureus is a normal part of the microbiome for most hedgehogs, just as it is for about 30% of people. On hedgehogs, though, the bacteria live alongside a fungus called Trichophyton erinacei. To protect itself against the bacteria, the fungus produces methicillin. To maintain a fighting chance against the fungus, the bacteria apparently evolved to resist methicillin.

This kind of evolutionary arms race plays out constantly in nature, and that’s normally a good thing for us. Most of the chemical compounds we use as antibiotics are borrowed from fungi and bacteria trying to defend themselves against their microbial rivals. In this case, however, the T. erinacei bacteria living on wild hedgehogs managed to do the same thing the modern medical system has done: create enough selective pressure on bacteria to evolve antibiotic resistance.

But the hedgehogs’ microbiomes got there about 200 years ago, according to Harrison, Holmes, and their colleagues. They sequenced the DNA of the mecC-MRSA they found on hedgehog skin and compared it to the DNA of other strains. DNA accumulates tiny mutations over time, at a pretty consistent rate. Geneticists can count those mutations to estimate how long ago two organisms last shared a common ancestor – in other words, they can estimate how long a particular strain of MRSA has been around.

“We have traced the genes that give mecC-MRSA its antiobiotic resistance all the way back to their first appearance and found they were around in the 19^th century,” said Harrison.

What is mecC MRSA?

MRSA infections are hard to treat because the bacteria can resist not just methicillin, but a whole class of similar antibiotics called β-lactam antibiotics. That stubborn resistance is the work of a gene called mecA. In regular, non-resistant S. aureus, a different gene makes a protein that binds to β-lactam antibiotics, giving them a chance to latch onto the bacterium and destroy it. But in MRSA, mecA produces a protein that won’t bind to β-lactam antibiotics. An S. aureus bacterium with mecA is resistant to methicillin and its classmates because the antibiotics literally can’t get a grip on the germ.

About 1 in every 200 MRSA infections – in people, that is – have a different version of that gene, called mecC. It also produces a protein that won’t bind to β-lactam antibiotics, but its genetic code looks different than mecA. The two genes are homologs, meaning they share a common ancestor, but over time they’ve evolved slightly different versions of antibiotic resistance.

Holmes and his team first found mecC MRSA in people and dairy cows in 2011. By 2014, it had turned up in 14 different species in 13 European countries. At the time, Holmes and others assumed that mecC MRSA had evolved in dairy cows, because farmers routinely give their cattle huge doses of antibiotics to prevent infection. This practice is now being discouraged because it creates a hotbed for evolving antibiotic resistance.

But the new study suggests that mecC MRSA may have arisen in hedgehogs first, long before people, or cows, were taking antibiotics.

“We think MRSA evolved in a battle for survival on the skin of hedgehogs, and subsequently spread to livestock and humans through direct contact,” said Harrison.

Please Don’t Lick The Hedgehogs

“This study is a stark warning that when we use antibiotics, we have to use them with care,” said Holmes. “There’s a very big wildlife ‘reservoir’ where antibiotic resistant bacteria can survive – and from there it’s a short step for them to be picked up by livestock and then to infect humans.”

Because nearly all of the antibiotics we use today – and probably most of those we’ll use in the future – were found in nature, it’s likely that resistance to them exists somewhere in nature, too. “It isn’t just hedgehogs that harbor antibiotic resistant bacteria,” said Holmes. “All wildlife carries many different types of bacteria, as well as parasites, fungi, and viruses.” For that matter, so do all humans.

This isn’t a reason to be afraid of hedgehogs. Only about 0.5% of human MRSA infections today are the mecC type, which makes it extremely rare – despite having been waddling around out there for 200 years.

But out of an abundance of caution, you should avoid licking hedgehogs if possible; they are still very spiky.