Story from Popular Mechanics on Apple News
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It took 139 years to understand what caused the mysterious “Angel’s Glow” at the Battle of Shiloh.
The American Civil War Battle of Shiloh produced a combined 23,746 casualties on April 6-7, 1862—a fact that’s chillingly ironic for a place whose name means “place of peace” in Hebrew. At the battle’s end, 16,420 soldiers lay wounded throughout the woodlands and ravines of the area. Many of the injured were unable to move. They shivered in the night, rain streaming down their faces as they waited for either help or death.
When physicians finally reached the scene of the battle’s aftermath, they were taken aback by eerie bluish lights glowing in the darkness. They soon realized that the odd radiance was coming directly from the soldiers’ gaping wounds. Even more strangely, the soldiers who had these eerily glowing wounds wound up having more of a fighting chance at survival than those with ordinary wounds did. As a result, the phenomenon came to be known as “Angel’s Glow,” and it was seen by some as divine intervention.
The true cause of Angel’s Glow remained a mystery for over a century and a half. Then, in 2001, seventeen-year-old high school students Bill Martin and Jon Curtis learned about Angel’s Glow in history class, and with the help of Jon’s mother (a microbiologist), the two decided to investigate the battle site. It turned out that what had once seemed paranormal was actually the bioluminescent bacteria Photorhabdus luminescens. Although bacteria that invade wounds are often pathogenic to humans, this particular species is an insect pathogen that lives symbiotically with soil nematodes, who feast on the insides of insects.
P. luminescens colonizes the intestines of nematodes in the family Heterorhabditidae, which infect insect hosts only as juveniles. The bacteria lie in wait for the microscopic worms to find prey, and once a nematode enters the bloodstream of an unsuspecting insect, it releases the bacteria. They, in turn, secrete toxins lethal to the insect and enzymes that decompose its carcass, allowing both bacteria and nematode to feed on the tissue. Additional secretions from P. luminescens prevent other bacterial species from colonizing the carcass and depleting a food source needed by both organisms to survive.
As it turned out, the bactericidal secretions of P. luminescens were likely what prevented wounded soldiers at Shiloh from succumbing to infections and gangrene, as so many others did during the Civil War. Why the bacteria glow is still a mystery, but what’s clear is that their bioluminescence is the product of the lux operon (“lux” from the Latin for “light”)—a cluster of genes that work together to produce the proteins responsible for light emission. P. luminescens also produces four different types of toxins, including one called “makes caterpillars floppy” (Mcf), which does exactly what its name suggests. These toxins have been harnessed to protect crops from insect pests.
What’s toxic to one species may be beneficial to another. Civil War-era doctors had no way of knowing that the glow they saw was the result of infected insects being attracted to the smell of blood and landing on open wounds, spreading the bacteria that were killing them, but that actually disinfected the wounds they landed on. The flies would have probably found this egregiously unfair if they had known what was happening.
Even after Bill and Jon realized that Angel’s Glow was actually coming from bioluminescent bacteria, a question remained: Why isn’t it frequently seen among humans? Luckily, their subsequent lab experiments held the answer. They showed that P. luminescens isn’t able to survive at average human body temperatures, but the weather conditions at Shiloh included sheets of rain that darkened into clammy nights. The pervasive chill that resulted made it possible for the disinfecting bacteria to thrive and secrete toxins that killed bacteria already infecting the wounds, while preventing other microbial species from gaining a foothold.
Not only did Bill and Jon’s study win them first place in the 2011 Intel International Science and Engineering Fair—they were also mentioned on Smithsonian Magazine’s website in a special report, Seven Obscure Facts You Didn’t Know About the Civil War. There was an angel among those wounded soldiers after all—just not the kind with wings.
