Back in 1862, soldiers fighting in the American Civil War noticed something odd. Some of the injured had wounds that glowed blue, and those who did, seemed to be more likely to survive. They called this weird light the Angel's Glow, and figured that it must be a sign of supernatural protection. And for more than a hundred years, nobody knew what caused it, until a pair of high school scientists figured it out. The soldiers with glowing wound were being protected by bacteria.

So, as you might have heard, the American Civil War was a pretty rough place to be. New developments in weaponry made the battles especially bloody and brutal, resulting in extremely high number of causalities on both sides of the conflict. and the Battle of Shiloh was a particularly awful place to be, when the bacteria cleared after two days of fighting, more than 23,000 people had been injured or killed, making it the bloodiest battle in American history up until that point. 

The medics on both sides weren't prepared to deal with so many injuries, so many wounded soldiers had to wait for days before they received medical attention. And as night fell on the first evening, some soldiers were surprised to discover that their wounds were glowing with a soft blue light. And, it turned out that those soldiers, whose wounds had glowed in the dark, were more likely to survive, and their injuries healed more cleanly than those who didn't -almost as if the blue light were protecting the soldiers who had it. 

In 2001, a 17-year-old named Bill Martin learned about the Angel's Glow and immediately thought of his mother's research on glowing bacteria for the US Department of Agriculture. Phyllis Martin was studying a type if bioluminescent bacteria known as Photorhabdus luminescens, which is found in soil and glows a pale blue. So Bill, along with his friend John Curtis, decided to see if the blue glow of the soldier's wound could be related to the blue glow of the bacteria. 

The knew that P. luminescens has a mutualistic relationship with a round worm, or nematode. But the worms have other important relationship in their lives, they're parasites of some insects. When these round worms infect an insect, they spit up any P. luminescens that they happen to have in their guts. Then, the bacteria release toxins that kill the insect and enzymes that break down its tissue. So both the worm and bacteria get a tasty dinner.

The duo took their project to the lab, where they examined different strains of the bacteria under different environmental conditions to figure out what suited them best. When they compared those results to the conditions described in historical records of the Battle of Shiloh, they found that the soil would have been great place for both the bacteria and the worms to thrive. And if the bacteria were living in soldier's wounds, that would explain the Angel's Glow, and why the soldiers who had it were more likely to survive. 

So here's what they figured happened after the Battle of Shiloh: There would probably have been insects on or near the soldiers' wounds because they were outdoors on a battlefield. When nematodes infected those insects, P. luminescens would have released the toxins that normally help kill the insects. But in this case, those compounds would also have helped kill off other, more dangerous bacteria, protecting the soldiers from infection. Then, the worms would have moved on in search of their next meal. And of course that whole time, the P. lumisecens would be glowing, giving off a faint, ghostly blue light.

So, it seemed like Martin and Curtis had solved the mystery. But there was one more wrinkle to iron out. Normal human body temperatures are too hot for P. luminescens. So how could the bacteria be living in the soldiers' wounds? After some thinking through, the pair came up with an answer. The battle occurred in early April in Tennessee, where it would have been pretty chilly at night. The wounded soldiers would have been sitting on the cold, damp ground waiting for medical care. Under those conditions, it wouldn't take long for hypothermia to set in, and the soldiers' lowered body temperature would have been just cool enough for P. luminescens to do their thing. Then, once the soldiers were found and brought indoors for care, their bodies would warm back up again, killing off the P. luminescens  before they could spread for enough to become dangerous.

These experiments and conclusions earned Bill Martin and John Curtis the top prize in the 2001 Intel International Science and Engineering Fair. By combining their knowledge of history and microbiology, and trying a few experiments of their own, they solved the 139-year-old mystery of the Angel's Glow. 

Thank you for watching this episode of SciShow, which was brought to you by our patrons on Patreon -who recently helped us reach our goal of making 7 videos per week on SciShow, thank you so much for supporting us. If you wanna help support this show, just go to Patreon.com/SciShow. And don't forget to go to YouTube.com/SciShow and subscribe.  

(Outro)