Insects have been around for almost 400 million years. That’s plenty of time for evolution to fashion countless horrific deaths for them. Case in point: some insects die because a little worm vomits glowing bacteria inside their bodies.
The worm is Heterorhabditis bacteriophora, a microscopic creature used by gardeners the world over to control insect pests. Its accomplice-in-insecticide is a shiny bacterium called Photorhabdus luminescens, which only lives in the worm’s guts.
When the worm infiltrates an insect, it vomits out the bacteria. These reproduce madly and produce toxins that kill the insect, converting its fallen cells into nutrients that nourish the worm. The bacteria also make amino acids that the worm needs to reproduce, and antibiotics that kill other bacteria trying to colonise the insect. (In the US Civil war, soldiers were sometimes contaminated with P.luminescens, which gave their wounds a mysterious blue shine and protected them from blood poisoning – they called it the “angel’s glow”.)
This elegant partnership hinges upon a startling transformation. In the worm, P.luminescens lives a languid existence. It’s tiny and extremely hardy, it forms small colonies, and it grows so slowly that it might as well be dormant. This is the M-form. Once inside an insect, it transforms from Bruce Banner into the Hulk. Its cells get bigger, its colonies get larger, it glows more brightly, and it becomes violent and destructive. It unleashes a torrent of chemicals that kill insect cells and other bacteria, and others that spur the development of the worms. This is the P-form.
Until recently, scientists had almost always seen the P-form, and assumed that this was the bacteria’s default appearance. But Vishal Somvanshi from Michigan State University discovered that both states are important – one for destroying insects, and the other for partnering with worms. What’s more, he found that the bacteria change between the two states using a simple genetic switch – a small piece of DNA that can flip upside-down.
This flip-flopping piece of DNA is known, fittingly enough, as the madswitch. It’s a promoter – a piece of DNA that switches on nearby genes. In one direction, it does very little. In the other, it activates a set of genes that produce hair-like threads on the bacteria’s surface. These threads allow the microbes to latch onto the guts of their worm partners. If the madswitch is facing in the “OFF” direction, the bacteria stay in their insect-killing P-forms. If the switch flips to the “ON” direction, they convert into their worm-hugging M-forms. This simple volte face makes the difference between cooperation with one animal and conflict with another.
The madswitch’s flips aren’t carefully controlled. Instead, they happen randomly all the time, creating a mix of both the dormant M-form and the active P-form. The balance between these versions depends on the environment that the bacteria find themselves in. The P-form switches to the M-form almost 30 times more often than vice versa, but it also grows much faster. In lab cultures, or in an insect’s body, it overtakes its more docile counterpart. But in a worm, the M-form has the advantage and takes over.
In fiction, Dr Jekyll and Bruce Banner were only ever Mr Hyde and the Hulk some of the time. By contrast, P.luminescens has both sides of its character on display all the time. It simply lets its environment dictate which form will rise to the fore.
Reference: Somvanshi, Sloup, Crawford, Martin, Heidt, Kim, Clardy & Ciche. 2012. A Single Promoter Inversion Switches Photorhabdus Between Pathogenic and Mutualistic States. Science http://dx.doi.org/10.1126/science.1216641
Images: top photo by Penny Greb, middle image from Somvanshi et al.
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