Photorhabdus luminescens sounds like a spell from the Harry Potter universe, maybe one that lights up a camera or ignites whatever mythical creature a “habdus” might be. In fact, the wizards and witches of Hogwarts aside, Photorhabdus luminescens was once the cause of what many likely considered magic. Just ask Confederate soldiers during the U.S. Civil War—especially the ones who inexplicably began to glow in the dark.
In April 1862, Union and Confederate forces met at Shiloh, Tennessee. The Battle of Shiloh was a clear Northern victory despite a heavy casualty count on both sides—each had roughly 1,700 soldiers dead and another 8,000 wounded. The Confederate medical crews were ill-prepared for those types of casualties, and many wounded Southerners were, therefore, left untreated for a few days. When night came, the wounds of some of the injured soldiers, still left unattended, began emanating a faint blue light. They created a soft glow in the otherwise-dark battlefield. When the wounded soldiers finally received treatment, many claimed that those who had the glowing injuries healed more thoroughly than those without the apparently supernatural halo.
It wasn’t a gift from the heavens, of course. It came from Photorhabdus luminescens, a type of bacteria. P. luminescens, as the cool kids call it, is a bioluminescent microbe that has a symbiotic relationship with roundworms, a parasitic nematode that infects insects. The roundworm invades an insect and, effectively, throws up a gut full of P. luminescens. The bacteria releases a toxin that kills the insect within forty-eight hours and an enzyme that breaks down the insect’s body. The nematode then eats the liquefied insect, returning much of the bacteria to its home inside the roundworm’s body.
The roundworms—and therefore the bacteria—were most likely present in the mud and dirt of the Shiloh battlefield. It’s further likely that the microbes made their way into the wounds of many of the injured Confederate soldiers and, because of other conditions, were able to thrive there. Even that required a bit of luck, which explains why only some of the soldiers began to glow.
While P. luminescens typically can’t survive in a human host because our body temperature is too warm for them, according to MentalFloss.com, prolonged exposure to the rainy and wet conditions of the battlefield caused many soldiers to suffer from hypothermia. This dropped the body temperature of those fighters, allowing P. luminescens to invade their wounds—and, being a bioluminescent creature with a blueish hue, to create the glow.
The good news for those soldiers is that P. luminescens isn’t all that infectious, and our bodies’ immune systems can typically handle the microbe. But before that happens, the P. luminescens do their human hosts a favor typically reserved for the roundworms. The toxins they produce that kill insects also happen to kill other bacteria in the area, keeping the P. luminescens and its host safe from infection. That’s almost certainly what happened in this case, which is why the glowing soldiers recovered more quickly than their standard-hued comrades-at-arms.
The gene of P. luminescens associated with the insect-killing toxin was discovered by a team of British researchers in 2002. They named the gene “mcf”—short for “make caterpillars floppy,” because that’s what the toxin does.