In one of the first great battles of the Civil War, Grant’s Union Army was pitted against Johnston’s and Buell’s Confederate forces at Shiloh, in Tennessee. A now bucolic place in Tennessee, Shiloh is near the southern Tennessee border with Mississippi and Alabama, on the Tennessee River. A vicious, costly two-day struggle, the battle was the largest fought in America and had the highest number of casualties so far in the war. 100,000 men fought, and the end of the battle found 24,000 men killed, missing or wounded.

It was spring of 1862, and the weather was cool and wet. It began raining at 10:00 pm the night after the first day of battle, and at midnight the rain became a storm with thunder and lightning. Hungry and tired men slept in the open without blankets, and rain and cold weather added to their misery. The battle continued on a second day, with thousands of wounded men waiting two nights or more for aid.

In the darkness after the battle ended, as they struggled to retrieve the wounded men, soldiers and medics noticed that certain gashes emitted a faint blue‑green shimmer. The doctors and medics found that soldiers with these mud‑streaked wounds developed fewer infections and recovered faster. It was, at the time, believed to be divine providence, so the phenomenon became called “Angel’s Glow”.

This Angel’s Glow story has been passed along for 160 years since the Civil War and has become standard folklore.

Then something interesting happened in 2002. Bill Martin, a high school student and Civil War enthusiast, visited the Shiloh battlefield. When he heard about Angel’s Glow, he was curious about the cause. Serendipitously, his mother Phyllis, worked as a microbiologist and happened to specialize in Photorhabdus luminescence, a soil bacterium that produced its own light.

Bill and his friend Jon Curtis investigated the story for a science fair project. They (with assistance from Bill’s mom) worked to create their theory for their project, “Civil War Wounds that Glowed.” They studied everything about the battle conditions, the soil in the area, the weather, and of course, photoluminescent bacteria in that area. Their research revealed that the glow was most likely caused by a bacteria called Photorhabdus luminescens. This bacterium produces an enzyme called luciferase, which fluoresces. They then cultured this bacterium, showing it could flourish in the battle’s cool, wet conditions, lighting muddy tissue while suppressing deadlier microbes. Their project took first place at the 2001 Intel International Science and Engineering Science Fair in San Jose, California. 

Let me explain. Wound infections were common during the Civil War because the dirt and mud and sweat of a battlefield created the perfect place for germs to thrive. Wounds are warm and moist, and damaged tissue is the perfect breeding ground for bacteria. Those of us who care for patients in the present day hate any bacteria in an open wound; it’s a worst-case scenario. But this appears to be a case where the right bacterium at the right time could actually save lives.

The life cycle of P. luminescens explains this. The bacteria hang out in the guts of various nematode worm species. The nematodes are tiny (some are microscopic) predators of the soil, hunting down insect larva which they devour with P. luminescens’ help. The nematodes burrow into the unfortunate larva’s bloodstream, where they release their bacterial payload. P. luminescens releases toxins that kill the insect larva, giving the nematode quick access to an insect buffet. These toxins also inhibit the growth of bacteria that decompose the insect corpse, letting the germ and the worm have plenty of time to feast and multiply in their prey’s carcass.

So, wounded soldiers would have attracted insects, who would have laid eggs, which would have hatched into larvae. These larvae would have attracted P. luminescens. Or the soil contaminating the soldiers’ wounds could have contained these predatory nematodes. Mistaking the soldiers’ flesh for caterpillars, the nematodes regurgitated their bacteria. The antibiotics the bacteria produced destroyed the other pathogens infecting the wounds. Having these bacteria in their wounds helped the wounded soldiers to fight off other, more harmful bacteria that could have caused an infection. And in the process, giving off their ghostly blue light.

One problem. P. luminescens doesn’t live at body temperature. Bill and Jon were able to show that, because of the cool rainy conditions at Shiloh, many soldiers would have suffered from exposure and hypothermia, and their wounds would have been at the right temperature for P. luminescens to thrive.

“These bacteria [that glow] don’t grow at human body temperature. This had to happen at a particular time when it was cold enough that the body temperature would be lowered by hypothermia, but not so cold that the soldiers would freeze to death,” Bill’s mom Phyllis said. Then, when the soldiers were taken in and warmed back up, their bodies would have naturally killed off the bacteria. For once, hypothermia was a good thing.

With that, the teenagers managed to present a plausible explanation for the Angel’s Glow, a phenomenon that was long thought to be little more than folklore.

There are other reports of glowing wounds on battlefields. During World War I, soldiers in muddy trenches sometimes developed “trench foot”, a non-infectious condition caused by prolonged exposure to cold, wet conditions. When open sores then developed, they became secondarily infected with non-pathogenic species of the bacteria Vibrio, which fluoresce.

Uh oh. Now I’ve said Vibrio, and you’re going to ask me about flesh-eating bacteria, which have garnered so much press recently. Vibrio vulnificus, a cousin of the fluorescent Vibrio, is often found in estuaries and coastal areas. It can cause serious infections in cuts gotten in marshes, for example, that have gotten the popular name “flesh-eating”. There are also many reports of life-threatening illness from eating raw oysters harboring Vibrio vulnificus. This Vibrio does not fluoresce. (As a Public Service Announcement, I’ll add here that the CDC no longer recommends eating raw oysters, due to the increased prevalence of Vibrio vulnificus in warmer coastal waters. Sigh.)

And now you’re going to ask me if those beautiful bioluminescent beaches in Puerto Rico and elsewhere are dangerous. The ocean itself doesn’t actually glow, but plenty of things that live in it do. There are hundreds of different kinds of organisms in the sea that can emit the chemical luciferase, and glow. They do this directly, or through bacteria they contain that create luciferase. They may be as tiny as microscopic dinoflagellates or as big as jellyfish. Many algae are bioluminescent. You may have read reports from World War II of being able to track battleships by their bioluminescent wake, created by stirring up these sea organisms. Some bioluminescence is safe to swim in, but some (like red algae blooms that fluoresce blue at night) are not. You have to ask local experts.

Have any of you ever seen a fluorescent wound? I have not. Please tell me if you have!