The Battle of Shiloh was a bloody engagement in the American Civil War. Fought on 6 April 1862, it lasted from dawn to dusk, leaving more than 3,000 soldiers dead and more than 16,000 wounded. Neither army had adequate resources to cope with such carnage and it was more than 48 hours before all the wounded were recovered from the battlefield.
The battle had taken place in a swampy area, and many of the wounded lay in mud and foul water while waiting for help. At night they noticed something odd: some of their open wounds had developed a faint greenish-blue glow.
When the men finally reached field hospitals for treatment, the medics discovered something else odd. Soldiers who reported glowing wounds appeared to have a much higher survival rate than those who did not. The wounds that had glowed seemed to have less infection, and so they healed faster and scarred less than non-glowing wounds. The phenomenon acquired the name “angels’ glow”, but no one could explain it.
Now fast-forward to the start of the 21st century. In 2000 a 17-year-old schoolboy, Bill Martin, visited the battlefield and heard about the glowing wounds. His mother happened to be a research microbiologist for the US Department of Agriculture and he knew she had studied luminescent soil bacteria. He asked whether such organisms could have caused the glow, and Mom suggested he should find out for himself as a science fair project.
Working with a friend, Jon Curtis, Bill learnt that Photorhabdus luminescens, the bacterium his mother had investigated, live in the guts of tiny nematode worms of the genus Heterorhabditis, which are common in the region of Tennessee around Shiloh. These worms prey on insect larvae in the soil and have been used effectively as a biological control agent against insect pests that destroy crops.
The nematodes and the bacteria have a symbiotic relationship. The worms hunt down insect larvae, burrow into them and release their bacteria, which emit a chemical cocktail that kills the insect host and suppresses or kills other micro-organisms already present. The symbionts feed, grow and multiply until the insect corpse is more or less hollowed out. The nematodes then re-ingest the bacteria, which by now have multiplied until there are enough to produce a distinct glow. Scientists believe that the luminescence attracts more insects, thus making it easier for the worms to transfer to a new host.
Bill and Jon developed a theory that Heterorhabditis nematodes were drawn to insects in the soldiers’ bloody wounds. The bacteria they released made the wounds glow, while at the same time killing micro-organisms that might have caused gangrene or other wound infections. This could explain the better survival rates and quicker recovery.
One problem with the boys’ theory was a finding in their laboratory studies that P luminescens cannot survive at human body temperature. However, historical records showed that the Shiloh area was cold and wet at the time of the battle. The boys reasoned that many of the open wounds would have been hypothermic, allowing the bacteria to survive — at least until the men were removed to the warmth of a hospital.
The teenagers’ study was impressive enough to earn them first place in team competition at the 2001 Intel International Science and Engineering Fair. I am not aware of further research to test the boys’ theory, but it seems to have become generally accepted as a plausible explanation for “angels’ glow”.