Bugs with bugs: An accomplice to homicide


In my first couple of weeks as an undergrad, the truly awesome Dr. George McGavin (who you might know from Lost Land of the Jaguar / Volcano) gave me a tour of the Natural History museum. The most memorable part of this was his discussion of invertebrates. Having relayed the (possibly apocryphal) quotation from JBS Haldane that the Creator must have an inordinate fondness for beetles he then went on to say that anything we humans felt we had achieved (specifically in the bedroom) had already been done by invertebrates. “Providing gifts, tying each other up, group sex, sex with the dead… Yeah they got there first.”

It’s a dog eat dog world

Apparently invertebrates are also pretty good at coming up with awesome and decidedly creepy ways to kill one another. I could spend many hours telling you about parasitoid wasps, but todays biology lesson comes courtesy of Vishal S. Somvanshi et al from Michigan State who published a paper today in Science about a nematode (roundworm) called Heterorhabditis bacteriophora and its population of symbiotic bacteria Photorhabdus luminescens, which live quite happily in the gut of the worm. The bacteria and nematode are crucial to one another’s life cycles. The worm “infiltrates” the insect (I’m yet to find the explanation of exactly what this means… does the insect eat it? Does it burrow inside?!) and then vomit their friendly mutualistic bacteria into the insect. From there things get nasty

Dr Jekyll vs Mr Hyde

The bacteria have two distinct life stages. In the mutualistic (M-form) stage, they form small translucent colonies, without the haemolymph (blood-destroying) activity and they are not virulent to insects. They hang out quite happily in their host’s gut without causing any damage. The pathogenic (P-form) bacteria are yellow and opaque, produce an eery glow (bioluminescence), antibiotics, insecticidal toxins, and chelate (bind together) any iron in the nearby area. These are hardcore bacteria: they’re kicking asses and taking no prisoners.

Partial Figure 3 from Somvanshi et al (2012)

He had the opportunity: what about motive? 

So what actually makes the bacteria flip? What turns mild-mannered Bruce Wayne into hardcore crime-fighting batman? Well this is the topic of the paper. The genetic differences between the two forms of the bacteria are really quite dramatic: around 10% of the transcriptome (expressed genes) is completely different. But the individual switch is really quite simple.

Genes are stretches of code related to certain proteins. But the genes themselves do not tell the cell how and when to express them. That is the job of the promoter a nearby stretch of DNA that turns the genes on or off. The maternal adhesion genes (which allow the M-form of the bacterium to attach itself to its nematode host) are regulator by a promoter called the madswitch. By ‘locking’ the madswitch promoter ON or OFF the researchers were able to determine that this switch is sufficient to alter the entire transcriptome of the bacterium, and therefore whether it currently exists in the M or P form. When the promoter is ON the bacteria make the maternal adhesion proteins and bind to the worm. When it’s off they roam free in the insect’s gut, raping and pillaging. And this change doesn’t happen rarely or under strange conditions: it occurs fairly frequently in both directions, but the major selective advantage of each type in the ‘right’ situation means that it’s useful for both forms to continue to exist.

Pretty cool, huh?

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