They make plastic edible

With genes from two bacteria and a rat, E. coli produces the main ingredient of vanilla

Aleena Naeem

The food we eat is made up of carbon, hydrogen, oxygen, nitrogen, sulfur, a few other minerals… Plastic? Well, same parts, though arranged very differently.

Now, in a recently published article in Green Chemistry, researchers at the University of Edinburgh have shown how they used some rather cooperative bacteria to break down one common plastic, PET. The result is vanillin, which along with some other compounds gives vanilla its signature flavor.

Food from plastic seems odd. But then, a lot of animals — including us — have been champing on the remnants of bags and other plastic delicacies awhile now.

Plastics are made from fossil fuels, which are non-renewable, and they are choking up a lot of the planet – not to mention turtles, whales, fish and, yes, children.

Depending on the variety of plastic (how stable it is), breakdown can take a long time — though the media can throw numbers around to say the same thing.

PET treatment

One plastic with a pretty long life is PET, or polyethylene terephthalate. It is used as packaging material, plastic bottles, disposable cups, and plates. About 50 million tons of PET are generated annually, only a small amount of which (29.1% in 2018) is recycled. Researchers have been coming up with some original ways to deal with plastics, usually by breaking it down — using bacteria, fungi, or more.

But Joanna Sadler and her team came up with a novel spin: make the plastic edible. They relied on earlier work done by a team of Japanese researchers who got another bacteria, Ideonella sakaiensis, to break down PET to its subunits, terephthalic acid and ethylene glycol.

Sadler, and Stephen Wallace, of the School of Biological Sciences, University of Edinburgh, U.K., used a genetically engineered bacterium, Escherichia coli (MG1655 RARE), to convert PET into vanillin. Besides food, vanillin, which is in high demand, is used in cosmetics, cleaning products, herbicides, and antifoaming agents.

During recycling, PET is usually converted into its two subunits that are then reused. In some cases, bacteria convert terephthalic acid into polyhydroxyalkanoates. These are plastics that biodegrade more easily than PET.

But Sadler and Wallace just transformed these building blocks into vanillin.

A ‘simple project’

“The beauty of using whole cells is that you grow cells that’s ready to put in your reaction. It’s a very simple project,” Sadler told Truly Curious.

E. coli does not naturally convert PET into vanillin, of course. So it had to be armed with new enzymes. The researchers came up with a plan that involved four enzymes that would systematically convert PET into vanillin. The bacterium has an extra ring of DNA, called a plasmid. Genes that produce the required enzymes were inserted into two plasmids. The genes for two enzymes (they are a mouthful: terephthalate 1,2-dioxygenase, and dihydroxy-3,5-cyclohexadiene-1,4-dicarboxylic acid dehydrogenase) came from another bacterial species, Comamonas. On the second plasmid was one gene for carboxylic acid reductase, which comes from yet another bacterium, though it behaves like a fungus at times, Nocardia iowensis, and another (catechol O-methyltransferase) from a rat.

E. coli may have the right genes, but its cell membrane is firmly closed to PET. So the researchers added butanol, a form of alcohol with four carbon atoms, rather than the regulation two found at every local bar. The butanol poked holes in the E. coli membrane, thus clearing the way for PET to get in and be converted to vanillin.

You can guess the new problem: how to get the vanillin out. Never fear, there’s always another alcohol for the job — in this case, oleyl alcohol.

Fine-tuning operations

The researchers saw vanillin production go up if the E. coli was fed better nutrients, including breakdown products of milk for amino acids, trace elements, and benzyl alcohol. They found vanillin production at 22º Celsius (72º Fahrenheit) to be five times what they got at 30º Celsius (86º Fahrenheit).

“You can’t take anything for granted,” Sadler said. “You do have to test all sort of different parameters to really understand the process” to convert street-picked plastic bottles into vanillin.

Vanillin produced this way is similar to naturally occurring kind. But is there anything that makes it unsafe to consume?

“If the product is 100% pure then, there should be no problem,” Sadler said. “It’s better to use feedstock [raw material], which is currently an environmental pollutant and clogging up the oceans, than it is to use first-generation fossil fuels, which we are running out of.”

Wallace told Truly Curious that the research completely changes the perception of plastic waste as a problematic end product, converting it into usable feedstock in industrial biotechnology.

Future plans

Sadler and Wallace now aim to improve the process in a year, and try to increase the scale of production in the next. They are also eyeing other plastics, such as polyvinyl chloride (PVC), that they could convert into useful products.

“It is possible to up-scale these materials, but I think we first need to tackle the problem of degrading them,” Sadler said.

“We could actually use plastic as a substrate and replace some of these fossil fuels,” Sadler said. “And if we could use biology to do this process, we could replace some of those harsher chemical processes that are releasing a lot of venomous gases.”

Aleena Naeem is an M.Phil student in chemistry at the Government College University in Lahore, Pakistan. Her research interests lie in the extraction of bioactive compounds, their analysis and, kinetic modeling.