Healthy moth larvae (first panel) can be preyed on by a species of bacteria called Photorhabdus luminescens. These organisms can attack insect immune cells (second panel; in red) using ‘weapons’ called Photorhabdus virulence cassettes; bacteria that produce these structures light up in green. The cassettes are complex microscopic machine formed of many proteins (third panel; in grey). They are enough to kill moth larvae (last panel; black insects). Image credit: Moths by Dr Nick Waterfield (CC BY 4.0); immune cells and bacteria by Dr Isabella Vlisidou (CC BY 4.0); Photorhabdus virulence cassettes by Dr Alexia Hapeshi (CC BY 4.0)
Photorhabdus are the only known group of non-marine bacteria that can produce their own light. These organisms prey on insects, which then glow in the dark once infected. The group also has an unusual weapon system formed of miniscule needle-like structures that can be sent out in the environment. These ‘Photorhabdus virulence cassettes’ are loaded with toxins that are injected inside host cells; the cassettes alone can kill a caterpillar within minutes. However, it is still unclear how exactly these structures work: are they like poison darts, with the toxin on the outside, or like hypodermic needles, with the toxin within?
Photorhabdus bacteria make lots of deadly substances, so to look at the needles on their own, Vlisidou et al. had them produced by another species of bacteria that does not carry these weapons. The experiments revealed that the cassettes packaged the toxic proteins inside, like a hypodermic needle. Alone in the environment, the toxin cannot penetrate host cells.
Creating the cassettes takes a lot of energy, and a closer look at Photorhabdus showed that, at any given time during an infection, only a small number of bacteria produce them. It is therefore possible that the bacteria share the high cost of making these deadly devices by using a division of labour approach.
With a better understanding of the cassettes, it could one day become possible to harness these molecular machines to control insect pests or parasites.
