The fungus that makes ‘zombie ants’ could use biological clocks to control their minds

Ant killed by Ophiocordyceps fungus. Credit: Katja Schultz/Flickr

The world of parasites can sometimes be extremely gruesome.

Take, for example, the charming female jewel wasp, which uses a cockroach as a living
incubator for its larvae. The wasp stings the roach in the brain, and leads the much bigger host by its antennae into a burrow before laying an egg inside its abdomen. The cockroach, being completely under the jewel wasp’s spell, doesn’t protest. Once the egg hatches, the larva consumes the cockroach from the inside out. Lovely.

Or how about the delightful horsehair worm? The larvae of this species get eaten by small insects like mosquitoes, which are themselves gobbled up by bigger bugs like crickets. Once inside its host, the larva matures into a beautiful butterfly foot-long worm. It then hijacks the cricket’s behaviour, causing it to jump straight into the nearest water source and drown itself. The worm emerges underwater in a writhing mass from the cricket’s abdomen, before mating and starting the cycle anew. (You can watch this happen in the video below – but be warned, this is something you cannot unsee.)

 

 This kind of mind bending isn’t limited to animals. Toxoplasma gondii is a protist that famously makes rats attracted to cat urine (bad news for the rodent, good news for the parasite). But probably one of the most bizarre examples are fungi of the species complex Ophiocordyceps unilateralis, which infect and produce so-called ‘zombie ants’.

Ophiocordyceps spores infect carpenter ants while they are out at night searching for food. The fungus grows inside the ant and eventually causes it to leave the nest, seek out a piece of vegetation and climb it (all while convulsing horribly, of course). Once it’s ascended to a particular height, the ant clamps down with its powerful jaws and remains there until it dies, whereupon the fungus consumes it and uses the energy to produce a fruiting body. This structure bursts forth from the ant’s head like something out of a Ridley Scott film, and will rain down spores onto more unsuspecting ants below.

So what’s going on here? How does the fungus manage to pull off this amazing/terrifying piece of brainwashing? Charissa de Bekker is an Assistant Professor from the University of Central Florida who works on Ophiocordyceps, and gave a talk on her research at our Annual Conference last month.

“There are many different fungal species, and as far as we know each ant species gets infected by its own fungus,” says Charissa. “It’s very specific. There has been a long co-evolution between the parasite and the host, an arms race where the host is trying to get rid of the parasite, and the parasite is trying to overcome the immune system and strategies of the host.”

This battle has been going on for millions of years?, and seems like the fungus has come out on top. It has developed the ability to subtly and precisely manipulate the ant’s behaviour, albeit in an extremely macabre way. For example, different ant species will be forced to seek out different plants and will latch onto specific parts, like a leaf or a twig. It’s only after this point that the fungus will enact its grizzly execution.

Carpenter Ants. Credit: Katja Schultz/Flickr

“When the fungus is inside the ant, it’s normally in a yeast-like state, living as single cells. But as soon as the ant dies, the fungus starts to produce hyphae, which form a big branching network of cells,” explains Charissa. “And if we look at the genes at this point, comparing when biting has taken place to when the ant has died, we see that the fungus quickly changes which ones it’s using.”

Using this approach, Charissa’s group is trying to figure out the molecular basis of this mind control. By infecting ants in the lab, following their behavior and looking at the genes expressed at different time points, it’s possible to piece together what’s happening inside both organisms.

“The way we do this is collect the heads of the ants, mash them up, get the RNA out and sequence it,” she says. “Then we map that back to the ant and the fungus.”

The lab has found that, after infection, the fungus starts to secrete a number of compounds into the ant, including enzymes that are known to enhance locomotive activity, and alkaloids that may be activating or deactivating receptors in the brain. As you might expect, after the ant has died, the fungus moves from creating compounds that manipulate behaviour to enzymes that can digest the victim’s innards.

Charissa’s latest work is also looking at the role that internal biological clocks, known as circadian rhythms, play in the fungus–ant interactions, as lots of aspects of the system seem to be tied to a certain time of day. For example, the Hughes Lab at Penn State, where Charissa did her postdoc, found that ants tend to bite down on the vegetation at solar noon, and that death follows swiftly a couple of hours later. The fungus only releases spores at a specific time of night when the ants are out foraging – and foraging behaviour is itself controlled by the insect’s circadian rhythms.

“So there’s all these different elements that made me think that a biological clock could be very important for this interaction,” says Charissa. “The fungus might be breaking into the behavioural output of the biological clock of the host, and maybe hijacking and taking advantage of it.”

The team has also learnt that the fungus has its own molecular clock, and contains versions of proteins known to be used for cycling in other fungal species. This may be important for orchestrating changes in the host.

“We’re definitely at the beginning of the exciting stuff,” says Charissa. “Most of the parasites we know about that change behaviour aren’t model organisms, so they haven’t been studied for as long as fruit flies or E. coli. But with sequencing technology, we have doors open, and we can actually start to look into what might be going on. It’s a very exciting, novel field.”

Anand Jagatia

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Spotlight on Grants: Eliminating plasmids for antibiotic resistance

Each year, the Microbiology Society awards a number of Research Visit Grants that enable our members to work in another laboratory anywhere in the world.

PhD student Alessandro Lazdins fom the University of Birmingham writes about his trip to Sydney earlier this year, to work on plasmids in mice.

In addition to their chromosomes, many bacteria contain plasmids – small, circular DNA molecules that typically carry advantageous genes, most notably those involved in antibiotic resistance. Plasmids are particularly problematic because they can be easily passed between different bacterial populations by a process called conjugation. This can rapidly spread antibiotic resistance genes in all sorts of environments, from sewage water to the guts of humans and animals.

As a PhD student at the Institute of Microbiology and Infection at the University of Birmingham, I have been studying whether we can eliminate these plasmids from their hosts. This would be particularly useful in clinical settings, where many infections are due to bacteria that are normally found in the guts of patients. If we could remove resistance plasmids from a patient’s gut before an infection develops, then treating that infection would become a lot easier.

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What is a microbiome?

A microbiome is the community of micro-organisms living together in a particular habitat. Humans, animals and plants have their own unique microbiomes, but so do soils, oceans and even buildings. Watch our short animation to learn more.

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Podcast: Annual Conference 2017 Roundup

Earlier this month, the Microbiology Society hosted our Annual Conference 2017 in Edinburgh. While we were there, we managed to catch up with just a few of the researchers presenting posters and giving talks.

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New to Science: Microbes from dogs, fish and ice cores

Each month, the Microbiology Society publishes the International Journal of Systematic and Evolutionary Microbiology (IJSEM), which details newly discovered species of bacteria, fungi and protists. Here are a few of the new species that have been discovered and the places they’ve been found. 

Spring is here. Well done everybody!

There’s a triptych of new Chryseobacterium species in our latest issue. C. cucumeris has been isolated from the root of a cucumber by a team from South Korea, while another group from the country has discovered C. nepalense in oil-contaminated soil. The final species, C. endophyticum, was isolated from a maize leaf by microbiologists from Taiwan. Continue reading

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Annual Conference 2017 – a view from Twitter

The Microbiology Society’s Annual Conference 2017, which took place 3–6 April in Edinburgh, was our biggest event to date. After becoming fully booked weeks before it began, the four days saw over 1,800 delegates come through the doors and 600 posters on show – we managed to exceed our personal best that we set last year in Liverpool in 2016. Have a look at our Twitter summary of the best of this year’s Annual Conference. Continue reading

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Investigating Lyme disease on the South Downs

The South Downs National Park is an area that stretches for 140 km along the South Coast of England, and is home to a huge diversity of plants and animals.

Some of the smaller inhabitants of the park are ticks: biting arachnids that feed on the blood of a wide range of mammals and birds, including deer, sheep and rodents. A number of these ticks are colonised by bacteria of the Borrelia genus, which are known to be the causative agent of Lyme disease, a potentially serious infection that can affect many parts of the body.

Last year, researchers from the University of Brighton suggested that the national park includes two of the ten areas in the UK where Lyme disease transmission is most common.

What the team weren’t able to show was why the park has high levels of transmission. Today, at the Microbiology Society’s Annual Conference, Dr Ian Cooper from the University of Brighton is presenting a poster explaining how the team are looking to answer that question. Continue reading

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