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The bacteria that make insects eat their own brains

The views expressed are those of the author and are not necessarily those of Scientific American.


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As far as bacteria are concerned, other living creatures are just another niche to exploit, which means that pretty much every animal and plant has a set of bacterial pathogens that come along with it. These bacteria have made the animal in question their speciality, and are highly adapted to live inside their hosts. While these bacteria often make the host ill, or less fit, or sometimes dead, the longer they live with their host, overall, the less they damage it. After all, it’s no help to the bacteria if their home drops down dead right after they’ve moved in.

A great example of this appeared in PLoS Pathogens this month (reference 1), concerning the bacteria Wolbachia. These bacteria infect insects and other arthropods and are much beloved of journalists (well, compared to other insect bacteria at least) because one of their effects is to stop insects producing male offspring (so only female survive to pass on the bacterial genome), which gives journalists the opportunity to write silly headlines.

An electon micrograph of an insect cell, with three Wolbachia bacteria inside (the large circular blobs with white lines surrounding them). Image from reference 2.

As well as passing from females onto their offspring, Wolbachia can also be transmitted horizontally, that is between insects in the same generation. In its normal host the Wolbachia is not hugely damaging (apart from removing all males from the population) but when transmitted to a new species it causes various unpleasant nervous system complications, often leading to death. Clearly, the bacteria are more virulent when they encounter a new species. However when the bacterial infection was closely examined, it was found that infected individuals of both species contained the same number of bacteria. It wasn’t just that the new species couldn’t respond to the infection, it was in fact the way they responded that was doing the damage.

As it turns out, the reason Wolbachia are more dangerous in new species isn’t because the bacteria go wild in the unexplored territory, rather it’s because the new host doesn’t know how to deal with them. The insects that are used to dealing with the presence of the bacteria have developed ways to contain the infection, or just tolerate it. New species however, tend to panic, particularly as the bacteria tend to congregate in the gonads (sex organs) and the central nervous system, which even insects understand are bad places to have bacteria.

As the bacteria are found inside cells, the best way for an insect immune system to get rid of them, is by destroying the cells that house the bacteria. Which, as previously mentioned, are mainly the gonads and the central nervous system. When the Wolbachia get into a new species, the first response of the insect is to quickly and efficiently destroy any cells which have bacteria inside them. As a consequence the unfortunate insect basically destroys its own brain, leading to various unpleasant symptoms and death.

The carpenter ant, Camponotus pennsylvanicus. Many species of Camponotus are infected with Wolbachia. Image from reference 3

Even in insects, the immune system is vital to defend animals from bacterial, fungal, and viral attacks. However it’s fascinating to see the cases where the immune system (even ‘primitave’ immune systems that consist of nothing more than infected cells quickly being removed) can lead to issues by over-reacting to a threat. The best response to the Wolbachia is for the insects to learn to deal with it, rather than to attempt to launch counter-attacks which can be damaging for the animal as a whole.

Reference 1:Le Clec’h W, Braquart-Varnier C, Raimond M, Ferdy JB, Bouchon D, & Sicard M (2012). High virulence of wolbachia after host switching: when autophagy hurts. PLoS pathogens, 8 (8) PMID: 22876183

Reference 2: (2004) Genome Sequence of the Intracellular Bacterium Wolbachia. PLoS Biol 2(3): e76. doi:10.1371/journal.pbio.0020076

Reference 3: Wernegreen JJ (2004) Endosymbiosis: Lessons in Conflict Resolution. PLoS Biol 2(3): e68. doi:10.1371/journal.pbio.0020068

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. scientific earthling 8:05 pm 09/3/2012

    Now we need to find a similar bacteria to control the extreme overpopulation of Homo sapiens.

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  2. 2. S.E. Gould in reply to S.E. Gould 4:19 am 09/4/2012

    There are a large number of bacteria that are already more than capable of killing off humans, however we keep finding new ways of destroying them, through medication and antibiotics.

    Also infectious diseases aren’t really the most ethical way to deal with human overpopulation.

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  3. 3. fire1fl 5:49 pm 09/5/2012

    Overpopulation is the incurable disease, only temporarily forestalled by treating symptoms. Biology cares not about ethics.

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  4. 4. ferncanyon 6:01 pm 09/5/2012

    Having one’s own immune system dissolve one’s own brain is obviously an unpleasant outcome for the individual, but could it be an effective way to prevent the spreading of the bacteria through a colony? Social insects often sacrifice individuals to protect the colony. Perhaps that’s what is happening here. It might not be over-reacting after all–just a different strategy of dealing with the threat.

    It would be interesting to know whether the insects who contain the parasite still suffer ill effects from being infected and whether the other insects that sacrifice the individuals help keep the colony healthier.

    I wonder whether the same thing might happen in humans.

    Thanks for the thought-provoking article.

    Cheers,
    John

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