As part of their innate immune system, insects use an enzyme called phenoloxidase to produce reactive molecules that kill bacteria and then encapsulate them in a dense coat of black pigment called melanin.
The researchers found that Photorhabdus produces a special phenoloxidase inhibitor to protect itself against this particular defence.
They identified the inhibitor as a small molecule called 1,3-dihydroxy-2-(isopropyl)-5-(2-phenylethenyl)benzene, known as ST for short.
This molecule is also an antibiotic and Photorhabdus produces it to kill off other microbes that might grow in the corpse of the dead insect.
To test their findings, the researchers produced a mutant Photorhabdus that is unable to make ST. Without ST, the bacteria were less virulent. The researchers then used a technique known RNA interference to prevent the insects from producing the phenoloxidase enzyme. These insects were more susceptible to regular Photorhabdus bacteria.
But when the two were combined, it was found that not being able to produce ST made no difference to Photorhabdus when colonising insects unable to produce phenoloxidase.
"This is conclusive evidence for a gene-for-gene interaction between the bacterium and the insect," said Richard ffrench-Constant (correct) of Exeter University.
"Photorhabdus is an important biocontrol organism that is used to control insect pests and reduces pesticide use, so the more we know about it, the more useful it can be.
"Insects are the major players in almost every ecosystem on the planet, so we need to know as much as we can about them."