Bacteria eat plastic?

A new bacterial strain has been found to degrade polyethylene terephthalate, or PET, which is a plastic commonly found in water bottles and polyester clothing. The concept of breaking down long plastic chains via microbes, or biodegradation, is not a new one, but PET was previously thought to be non-biodegradable. The bacterium in question, I. sakainesis, was found to have two key enzymes that could degrade PET into two relatively harmless monomers: terephthalic acid and ethylene glycol.

biopolymers1
Examples of a few biopolymers. Source: Schnepp Group.

How does this work? We first start with a discussion on some basic polymer science:

Polymers are a chain of repeating monomer units. In the image above, we saw that cellulose, for example, is a polymer with glucose as the monomer. These chains can be very long, even as long as 15 million monomer units! But how can one small monomer become a large polymer? Most plastics undergo chain-growth polymerization, which occurs in three simple steps: initiation, propagation, and termination.

Initiation

Initiation is the phase where, as you probably guessed, the polymerization is initiated. The monomer is a stable chemical compound. In the case of PET, the monomer is ethylene terephthalate (ET), a compound that is formed by… ethylene glycol and terephthalate. ET needs to be activated in order to start the polymerization, which is the initiator step. Usually this activation is done via the addition of heat, light, or chemical ions to remove an electron from a chemical bond from the monomer, creating a radical. This step is shown below.

 

238px-PET.svg
Example activation of ethylene terephthalate (ET) monomer to form ET*, a radical. Delta represents heat, light, or chemical change. 

Propagation

Now that the radical has been formed, many more radicals should also be formed before starting the propagation step. Two radicals find each other in solution to grow the polymeric chain. As the chain gets longer and longer, with a sufficient monomer concentration one can imagine that it gets easier to grow a longer chain. This is depicted below.

PET_propagation
Propagation of PET growth. An activated monomer adds to the growing chain to form a longer radical. This growing radical is important for the final termination step.

Termination

Several long polymers have formed in solution, and it now becomes easier for these long chains to find each other in solution to complete the final phase: termination.  Two radicals combine to form a stable compound. This is described in the below diagram.

PET_termination
Termination of PET growth. 

Now we more or less know the basics of polymer growth. Where does the bacteria come into play? The polymer undergoes a depolymerization reaction that works by effectively reversing the above mentioned polymerization sequence. The two enzymes that the bacteria contains would probably act as acid catalysts to oxidize the PET to produce the base terephthalate monomer, with a ethylene glycol remnant from the monomer linkage. Depolymerization reactions are promoted at higher temperatures, and are entropically driven. This behavior was reproduced by the species in question when it broke down a PET film at 86°F over six weeks. The below diagram depicts the depolymerization reaction.

PET_depolymerization
PET depolymerization reaction. The base terephthalate is regenerated, along with ethylene glycol. 

So by reusing the mechanism to form the polymer, the bacteria is able to manipulate the chemical structure to reform the original monomer. Of course, breaking down a small film over six weeks is certainly nonideal. Perhaps with some higher temperature studies, the reaction rate could increase. The key again is the two enzymes that the bacteria contains for the depolymerization reaction.

[The writer must confess and say that he didn’t read the actual scientific article due to not having access to ScienceMag away from MIT. More on the enzyme behavior will be added after his return. In the meantime, he hopes you enjoyed this article so that he doesn’t need to continue talking in third person.]

Links

Original article

Autoacceleration: long chain formation

Glass transition temperature (Tg) of polymers

Depolymerization

Nylon-eating bacteria

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