Hungry Bacterium: I Eats Plastic
If 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”).
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.