Bacteria becoming plastic bags

Given the right conditions, some bacteria will basically turn into ‘fat’ bags, as process technologist Katja Johnson discovered. The polymer that the microbes produce forms a resource for bioplastics.

The world urgently needs better plastics. The bags and packaging we use today are made from a finite source (fossil crude), and what’s worse: they have eternal life. In many developing countries shreds of plastic bags are strewn all over the place, and when the wind blows the plastic ends up stuck onto barbed wire. Eventually most plastic ends up at sea, where it joins the Great Pacific Garbage Patch – a giant gyre of floating plastic that threatens wildlife: seabirds, turtles and mammals alike.
Biodegradable plastics, preferably made from renewable materials, are starting to hit the market. Some are starch-based; cellulose based (cellophane for example), made from polylactic acid; or made from bacterial ‘fat’, called polyhydroxyalkanoate (PHA for short). Bacteria naturally produce PHA polymers from fatty acids as their energy stock, just as humans produce subcutaneous fat.

There are basically two different ways of making bacteria produce PHA. The first and industrially
applied method involves a monoculture of specialised bacteria producing PHA’s in a sterilised reactor from a very specific feed. That’s how Mirel, a company based in Massachusetts (US), produces its bioplastic from sugar.
The other way is being pioneered in the Delft laboratories in the life
science & technology building (Applied Sciences), where researcher Katja Johnson uses a mixed culture of bacteria, originating from a wastewater treatment plant in Rotterdam, in non-sterilised surroundings to produce plastics. So far, Johnson’s alternative process has only been operated in a small bioreactor for over a year. In the lab, she feeds the culture with a mixture of nutrients and a carbon-source; however, in practice, the culture should be fed with waste streams from the food industry or paper industry, or with agricultural waste.

The amazing thing, says her co-supervisor, dr. Robbert Kleerbezem, is the yield that Johnson reached with her alternative approach: a mere 90 percent of the biomass produced consists of PHA. The other 10 percent probably only consists of a stretched cell membrane and some squeezed proteins.
Kleerebezem: “In the past, pure culture fermented better than mixed cultures. Pure cultures would reach up to 80-90 percent, and mixed cultures would typically produce 60-70 percent. Which seemed fair, because the process costs would be lower as well. But it no longer holds true.”
Johnson: “It’s a major step forward. We can get the same yield without sterilisation of the bioreactor, without genetic engineering and with waste water as a substrate (feed, ed.).”

So how did she do it? By applying some good old Darwinian pressure, you could say. An essential step is the feast-famine cycle. After having fed the culture copiously, Johnson suddenly stopped the feed. Only those microbes that had produced ample quantities of PHA could survive on it. Thus, it is evolution on the lab bench. “I made the cycle length long for a strong selection on PHA production,” Johnson explains. “I made them grow fast, so that they grow big as well. And I raised the temperature to speed the process up.” Kleerebeezem looks amused: “And there was a luck factor as well.” Johnson nods in agreement.

At congresses on the production of bioplastics, Johnson is the odd one out. “I’m the exotic with the mixed cultures,” she says. Colleagues from industry are interested, but after the congress they tend to forget and go back to what they know.

A new STW programme should change that. The research programme called ‘Waste to Resource’ aims to elaborate the principle of using microbial communities to make products from industrial residues. There is interest from some major industries, and TU Eindhoven will also join to develop suitable applications for the bioplastics produced.
“Until now, we’ve been relatively successful”, says Kleerebezem. “Now we’re working to really get it going.” 

Katja Johnson will defend her thesis PHA production in aerobic mixed microbial cultures on Monday, 8 February 2010. PhD supervisor: professor Mark van Loosdrecht.

TU Delft’s new website continues to malfunction. The faculty home pages and student and staff portals have been off-line a few times and content pages are slow to load. Apparently there are various reasons for these problems. The TU homepage and staff and student portals recently changed to a new system, resulting in some teething problems. Additionally, the new house style templates place a heavy burden on the server, which causes the website to respond slowly and show the ‘503 error’ sign.

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