Microbiome Manipulates Host
A TEDtalk by microbiologist Bonnie Bassler describes the symbiotic relationship between the Australian Bobtail Squid and a tiny incandescent bacteria that lights up at night. As her team of scientists dove deeper into their relationship, two freaky notions were uncovered. One, we too are made up of trillions of microbes at a 10 to 1 ratio with our own human cells. And Two, these microbes communicate with eachother using a chemical language that actually enables them to coordinate defenses and mount attacks on the host based on a community vote.
If that doesn’t make you a little uneasy, a recent issue of the prestigious Journal of Experimental Biology is entirely dedicated to similar examples of invisible zombies in nature. They are far from rare. Bacteria, viruses, fungi, protozoans, wasps, tapeworms and a vast number of other parasites can control the brains of their hosts and get them to do their bidding. But only recently have scientists started to work out the sophisticated biochemistry these parasites use.
In the rain forests of Costa Rica lives Anelosimus octavius, a species of spider that sometimes displays a strange and ghoulish habit.
From time to time these spiders abandon their own webs and build radically different ones, a home not for the spider but for a parasitic wasp that has been living inside it. Then the spider dies — a zombie architect, its brain hijacked by its parasitic invader — and out of its body crawls the wasp’s larva, which has been growing inside it all this time.
“The knowledge that parasites can manipulate their hosts is old. The new part is how they do it,” said Shelley Adamo of Dalhousie University in Nova Scotia, a co-editor of the new issue. “The last 5 to 10 years have really been exciting.”
To manipulate the spiders, the wasp must have genes that produce proteins that alter spider behavior, and in some species, scientists are now pinpointing this type of gene. Such is the case with the baculovirus, a virus sprinkled liberally on leaves in forests and gardens. (The cabbage in a serving of coleslaw carries 100 million baculoviruses.)
Human diners need not worry, because the virus is harmful only to caterpillars of insect species, like gypsy moths. When a caterpillar bites a baculovirus-laden leaf, the parasite invades its cells and begins to replicate, sending the command “climb high.” The hosts end up high in trees, which has earned this infection the name ‘treetop disease.’ The bodies of the caterpillars then dissolve, releasing a rain of viruses on unsuspecting hosts below.
David P. Hughes of Penn State University and his colleagues have found that a single gene, known as egt, is responsible for driving the caterpillars up trees. The gene encodes an enzyme. When the enzyme is released inside the caterpillar, it destroys a hormone that signals a caterpillar to stop feeding and molt.
Dr. Hughes suspects that the virus goads the caterpillar into a feeding frenzy. Normally, gypsy moth caterpillars come out at night to feed and then return to crevices near the bottom of trees to hide from predators. The zombie caterpillars, on the other hand, cannot stop searching for food.
“The infected individuals are out there, just eating and eating,” Dr. Hughes said. “They’re stuck in a loop.”
Why this matters
Other parasites manipulate their hosts by altering the neurotransmitters in their brains. This kind of psychopharmacology could be a far greater factor in human behavior than we’ve thought possible. Now that more and more medical fields are linking back to the gut microbiome and it’s trillions of tiny residents, (gastroenterology, neurology, rheumatology, cardiovascular disease and more) it seems possible that we’re never really alone. (queue moaning zombies)
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