Answer: Bioluminescent Bacteria

The Battle of Shiloh was a particularly brutal battle in the U.S. Civil war that, in April of 1862, left over 16,000 Confederate and Union troops wounded and over 3,000 dead. The medics on either side of the battle were completely unprepared for such an astronomical number of wounded at one time.

Many of the soldiers simply had to sit and nurse their own wounds for days while waiting for medical attention. During those rainy days when dusk would fall, a curious phenomenon emerged. Some of the soldiers’ wounds would glow. All across the battlefield and the triage tents hundreds of soldiers had a faint blue glow radiating out of their wounds. Neither the soldiers nor the medics that treated them knew what to make of it, but one thing was clear, once the troops were moved to hospitals and given proper treatment, the soldiers who had experienced the strange glowing wounds on the battlefield had a much higher rate of recovery and survival. The soldiers nicknamed the phenomenon the “Angel’s Glow” and considered it some sort of divine intervention that alit, rather literally, upon some of the troops.

Today we have an answer to the mystery of the battlefield glow and how exactly the Angel’s Glow saved the lives of so many soldiers. The soil site of the Battle of Shiloh is rich with a particular parasitic nematode and in that particular nematode’s gut is a bacteria named Photorhabdus luminescens. The nematodes eat insects and P. luminescens has developed a very particular skill in helping its host thrive. As the nematodes consume their host, the bacteria also feeds and glows brightly via bioluminesce with the sole goal of making the body of the insect attractive to other insects so that a fresh host is nearby when the nematodes erupt from the husk of the insect they are currently feasting on. The glowing bacteria is essentially like a helper hailing a cab for the nematode, ensuring there is always a new ride waiting.

Now, that’s all very interesting but how exactly did it save the lives of all those soldiers? The glowing had nothing to do with it (and was just evidence the bacteria were present in the soldier’s wounds), the real life-saving magic was something that happened in parallel with the glow: P. luminescens also releases chemicals that kill off microorganisms in order to make life easier for its host (the nematode) to dominate the meta-host (the insect the nematode and bacteria hitchhikers are attacking). Sitting in the mud on those stormy nights in Tennessee, the soldiers were awash in the bacteria and the bacteria multiplied rapidly in the warm damp conditions of their wounds, all while excreting compounds that were killing off all the other harmful organisms nearby.

The Angel’s Glow might not have been truly angelic in origin, but it might as well have been; the soldier’s who were temporarily colonized with P. luminescens were getting an antibacterial and antimicrobial bath that even the best doctors’ of their day couldn’t replicate.