Photorhabdus luminescens

Photorhabdus luminescens brings bioluminescence to land through a remarkable three-way symbiosis. The bacterium lives mutualistically inside entomopathogenic nematodes (worms that kill insects). When nematodes invade an insect host, they release P. luminescens, which kills the insect and produces antibiotics that prevent other microbes from decomposing the carcass. The nematodes and bacteria then feed on the sterile insect remains. It's a hunting partnership where bacteria provide the killing toxins and nematodes provide transportation and access.

P. luminescens produces light through a quorum sensing-controlled lux operon homologous to V. fischeri's, but the function of bioluminescence in soil-dwelling, insect-killing bacteria remains mysterious. Unlike marine bioluminescence, which has clear functions in camouflage, communication, or predator attraction, glowing inside dead insects offers no obvious advantage. Some hypotheses suggest light attracts additional insects for nematode colonization, but this remains unproven. The persistence of luminescence without clear function illustrates how traits can be maintained through genetic linkage to essential functions.

The quorum sensing system, however, has clear function: coordinating the transition between mutualist and pathogen lifestyles. Inside nematode guts at low density, P. luminescens remains benign. When released into insect hemolymph and reaching high density, the bacterium produces toxins, antibiotics, and enzymes that kill and digest the host. This lifestyle switch—from cooperative partner to lethal pathogen—demonstrates how the same organism can play fundamentally different roles depending on context and population density.