I first came across Photorhabdus luminescens while listening to a TWiM podcast and became quite intrigued by the lifestyle of this microorganism in the environment and how it has made its mark in human history, making it one of my favourite microbes.
P. luminescens is a mutualistic bacteria that lives in the intestine of ‘entomopathogenic’ nematodes. These nematodes parasitize insects and use them to reproduce and acquire nutrients. When the nematode infects a new host, P. luminescens is regurgitated out of the nematode (video courtesy of Dr. Todd Ciche from Michigan State University here) and into the blood of the insect. Here, P. luminescens unveils its pathogenic side, secreting toxins and hydrolytic enzymes that deteriorate the insect from the inside out and release nutrients that both the bacteria and nematodes metabolize. Once the lifecycle of the nematode is complete the nematode and bacteria reunite and leave the insect with a deadly fate, making this team a devilish duo. This switch from a mutualistic state in the nematode to a pathogenic state in the insect is a fascinating example of how microbes respond rapidly to different environments. A recent paper published in Science describes how a single promoter inversion controls these two lifestyles of P. luminescens (2).
A promoter switch inversion controls the pathogenic variant (P form) and mutualistic variant (M form) of P. luminescens as demonstrated by Somvanshi et al (Science 2012). The left panel depicts the M form (green) in the maternal nematode, while the P form (red) are visible outside the nematode. The right panel shows colonies of the P form (red) which outgrow the much smaller and slower growing M form (green). Images are courtesy of Dr. Ciche’s Microbial Symbiosis Laboratory at Michigan State University.
Now if you’re not already convinced that this two-faced microbe is worth remembering, perhaps a history lesson will lure you. During the American Civil War in 1862, the Battle of Shiloh claimed over 23,000 lives making it one of the bloodiest battles during the war. The bullets and bayonets left many soldiers with open wounds making them susceptible to lethal infections. Strangely, some of the soldiers’ wounds glowed at night and doctors and nurses noticed that the wounds that glowed healed faster (1). The eerie iridescence was termed “Angel’s Glow”, as the glow seemed to favour a soldier’s survival.
It wasn’t until 2001 that the mystery of “Angel’s Glow” was unraveled. In the midst of a bloody battleground open wounds were exposed to soil containing the nematodes that carry P. luminescens. The low temperatures at night resulted in many of the soldiers becoming hypothermic and a lower body temperature permitted growth of P. luminescens in the wounds. More unique features of P. luminescens are that it is bioluminescent and produces secondary metabolites, such as antibiotics, that kill other microbes. Thus, the supernatural glow was not a heavenly sign but rather a biomass of bacteria. Fortunately, P. luminescens is rather harmless to humans and its chemical warfare fended off other pathogens from infecting the tissue, likely saving lives during the Civil War.
A friend to humans and nematodes, a foe to insects, P. luminescens has a finesse for making the most of its host.