Quirks & Quarks

Flesh eating bacteria stun the immune system into silence - with pain

This hints at why necrotizing fasciitis is so painful, and why the body doesn't fight it better
A US soldier has a foot-long scar along his shinbone to remember his encounter with flesh-eating bacteria. (U.S. Air Force photo by Senior Airman Cassandra Locke)
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Our pain is the infection's gain

One of the hallmarks of necrotizing fasciitis or "flesh eating disease" is how painful it is. That might not seem surprising for a disease that literally eats your body, but compared to many other infections, it is unusually painful, especially in its early stages before it's done major damage.

This pain may have a purpose - for the bacteria.  A team of researchers at Harvard Medical School have evidence that the bacteria might be inflicting pain to silence the body's normal immune system response.

Pain short-circuits the immune response

In research in mice, the researchers found that Streptococcus pyogenes, the bacteria that causes the disease, releases a toxin that specifically stimulates pain in the nerves around the infection. The body then interprets this as a wound, not an infection. That's important because the normal wound healing response damps the immune system in order to prevent too strong an immune response, which can interfere with the healing process. S. pyogenes seems to have evolved to short-circuit this normal process, and open a window to a runaway infection.

Dr. Isaac Chiu, who led the research team at Harvard, says there were two hints that led to this discovery.  The first was the unusually painful signature of an early necrotizing fasciitis infection. The second was the unusual inactivity of the immune system in response to the infection.

This raised flags for him, because his lab focuses on the relationship between the nervous system and the immune system. Researchers are realizing that the nervous system can modulate the activity of the immune system - and vice versa, and this study suggests one interesting example of this phenomenon.

Block the pain signal to block the bacteria

The study also suggests possible ways we might exploit this discovery to fight necrotizing fasciitis infections. In their experiments in mice, Dr. Chiu's team used botulism toxin - similar to the drug used for cosmetic wrinkle-removing treatments - to damp the nerves' ability to send pain signals. This also prevented the nerves from silencing the immune system, so the mice were able to successfully fight off the infection. They think other methods to intercept the signal the nervous system is sending to the immune system might also prove effective.

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