Roses are red, violets are blue, frogs are icky, but may be good for you

By: Zuri Sullivan

Sorry, kids, but if you kiss a frog, it’s highly unlikely that he’ll turn into a prince. The most you’ll probably get out of it is some icky frog skin slime on your mouth. That may sound terrible, but a new study published in Antimicrobial Agents and Chemotherapy suggests that some components of icky frog skin can have some unexpected benefits for our health.

As you might imagine, frogs are pretty distantly related from humans, and consequently, their immune systems work quite differently from ours. One thing we have in common, however, is that we both produce molecules specialized for killing bacteria, called antimicrobial peptides (AMPs). Humans produce lots of AMPs in their guts— last week, I wrote a story about how the helpful bacteria in our guts manage to survive in the face of these harsh molecules, while dangerous bacteria don’t. Unlike humans, frogs and other amphibians produce AMPs in their skin. To a certain extent, you could say they wear their immune systems on their backs.

Scientists haven’t quite figured out all of the ways that AMPs actually kill bacteria, but if the bacteria are susceptible to them, they work incredibly well. So from the bacteria’s perspective, AMPs are pretty nasty. From biomedical scientists’ perspective, however, this means that AMPs from a variety of species may be able to be used to treat bacterial infections when our own AMPs are inadequate.

This was the focus of the study I mentioned above, led by principal investigator David Craik at the University of Queensland in Australia. His research group wanted to know whether frog AMPs could be used to treat Staphylococcus aureus infection. You may know of Staphylococcus aureus by its more colloquial nickname, Staph. Staph infections are a major problem in hospitals, and the widespread use of antibiotics has led to the emergence of antibiotic resistant staph, also known as MRSA (for methicillin resistant Staphylococcus aureus). Antibiotics are great, but when misused or overused, they can promote the emergence of antibiotic resistant bacteria, necessitating the development of new antibiotics. It’s a vicious cycle, but AMPs could be new weapons in our anti-bacterial arsenal.

Can you really take AMPs from frog skin and use them to treat a staph infection? In mice, the answer seems to be yes. Dr. Craik’s group isolated AMPs from frogs, and studied their chemical structure. If they’d had to run out and catch a frog every time they wanted to do an experiment, the study would have taken forever, so they synthesized molecules that were very similar to the frog AMPs. These synthetic AMPs were then used to treat Staph-infected mice. Surprisingly, they worked! And not only that, but when they tested the compounds to find out if they were toxic to the mice, they seemed to be safe.

This doesn’t mean you should go out and lick a frog next time you’re feeling sick. As Helen and I tried to illustrate before, findings in mice need a lot of work to be extrapolated to humans. We know that a drug’s effect on mice is not always the same as its effect on humans, so clinical trials, despite being absurdly costly and tedious, are really important to make sure our drugs have the effects we want them to. AMPs are also expensive and difficult to synthesize, and scientists will need to find easier ways of making them if they want to try to use them to treat human infections.

Don’t give up on frog AMPs yet, though. With time, scientists may be able to turn them into real treatments for human infections. And in the meantime, you can put them to good use by applying the old Russian method of dipping a frog in unpasteurized milk to keep it from spoiling.