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Hungry Bacterium: I Eats Plastic

March 10, 2016

Plastic_eatersIf you heard it on NPR it must be true. A newly discovered bacterium, Ideonella sakaiensis, can munch its way through a notoriously indestructible form of plastic, called polyethylene terephthalate, or the cute acronym PET. For recyclers, it’s the #1 plastic. From strawberry containers to Ishampoo bottles, it’s all around us. What makes it so conveniently immortal? It’s the concentration of aromatic rings; that is, usually six-carbon rings with alternating double bonds like benzene. This kind of molecule used to be called “xenobiotic,” meaning so alien to life that no living microbe could ever break it down.

But as microbiologists know, plastic is just another arrangement of carbons and hydrogens, and an occasional oxygen, so eventually with enough evolution, DNA will make a slightly twisted enzyme that can munch the polymer down to its monomer parts. Hence the Ideonella bacterial enzyme, PETase (the “ase” part means “I eats it”).

PET_enzymes

PETase hydrolyzes the ester bond that links each terephthalate link to its neighbor. You can think of ester bonds as the weak link in the chain, like the connection between two Lego bricks: If it’s going to break, there’s where it will happen. All kinds of biomolecules are connected by esters, including sugar chains (carbohydrates) and the phosphodiester bonds of DNA. What makes PET different is that the enzyme has to fit itself to the aromatic portion, to recognize where to stuff the ester into its active site. Afterward, still other enzymes such as MHETase have to break down the aromatic portion. Surprisingly, the microbial domain is full of aromatic degrading enzymes because wood and leaves are full of such compounds, called lignin.

So how did the Japanese discoverers find this bacterium? They collected hundreds of samples–from a PET bottle recycling site. Of course, in that environment, a soil bacterium that eats PET could find a competitive edge. Soil is one of the most competitive environments out there, full of predatory and cannibalistic microbes. To get ahead, either you cooperate with them (we’ve had many posts on that) or you out-eat them, eating something they can’t. The researchers had to (1) isolate the bacterium as colonies in culture–a real trick, as 99.9% of bacteria won’t; (2) prove that it actually breaks down PET and assembles the carbon into its own cell parts. A lot of work to find, but any contaminated waste site is a potential source of microbial recyclers.

8 Comments
  1. March 12, 2016 10:48 am

    I heard, from a fellow student back in the late ’60s, about a biologist who had been trying mass selection hoping to find a bacterium that would eat DDT (he did, but the breakdown products were just as toxic). The grad student who worked with him told me they’d abandoned the idea after discovering their best candidates weren’t selective — they also ate telephone wire insulation quite happily.

    Yes, they thoroughly sterilized everything, didn’t just pour it all down the drain — or we might all be using glass fiber instead of copper wire for telecom nowadays. Or semaphore flags …

    • March 13, 2016 1:26 pm

      Sounds fascinating!
      Actually, according to the Cornelius Van Niel principle, there’s a microbe already out there to eat anything. So, the ability to eat DDT will evolve again; no doubt it already has.
      We already have things that eat hard-to-digest things such as telephone insulation. But they grow slowly. They can take their time because nobody else eats the stuff. Also, they have to put energy in before they get energy out of complex plastics.

  2. brunoclarke2016 permalink
    March 13, 2016 1:18 pm

    from Lynn Margulis and Dorion Sagan, What is Life? (1995): “Although no bacteria yet degrade the refractory carbon-hydrogen compounds of most plastics, eventually some will evolve and, not limited by food supply, they will spread like wildfire from landfill to landfill through the biosphere.”

    • March 13, 2016 1:27 pm

      Yes, no doubt they exist and are spreading. But they grow slowly–and there’s little incentive to grow faster. Like the tortoise and the hare, the tortoise wins in the long run.

  3. March 14, 2016 11:00 am

    > little incentive to grow faster

    Oh, I imagine there are managers at big chem/bio companies currently funding exactly that sort of selection, imagining speeding up the process and capturing the waste plastics disposal market. Heck, they probably imagine they can keep selecting for faster and more efficient beasts for a few years then figure out what magical enzyme they’re using, make or extract that enzyme, and not let the beasts loose in the environment.

    Yeah, once they’re spilled into the real world out of the lab they’re once again not especially fit to survive. Except, well, in a few rare places where there’ s nothing much else to eat _except_ plastics so they aren’t competitively disadvantaged.

    Like, oh, sewage systems. Wire bundles under the streets. Great Pacific garbage patch. Medical devices. Yeah, medical devices. Lots of nutrients, 98.6 degrees F. What’s not for a plastic-eating bacterium to love about these environments, once it’s been enabled?

    “No matter how cynical you become, it’s never enough to keep up.”
    — Mary Jean “Lily” Tomlin

    • March 14, 2016 12:13 pm

      The Great Pacific would be a good place to eat up the plastic. Fish have already learned to make a home in plastic bottles.
      Still, it’s glacially slow. Think of the Gulf of Mexico, where oil seeps continually from natural leaks. So there are always some petroleum-eating bacterial. When the BP spill happened, it took them months to degrade the easy parts. The tougher molecules will take years or decades.
      I don’t think we’ll see a Hollywood blockbuster about plastic-eating bacteria, although the ending of Andromeda Strain was pretty good.

    • Paula Lieberman permalink
      March 28, 2016 11:30 pm

      There was the zoolipt in one of the Ann Maxwell Fire Dancer series… (the one that wasn’t Fire Dancer or Dancer’s Illusion.)

  4. March 18, 2016 2:33 pm

    > glacially slow

    Um …. a joke about changing rate of change there?

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