Bugs making plastic


Students learn to assess current developments in the use of biopolymers and describe a process currently used industrially to produce biopolymers

From the technical language in the title, you have probably already worked out that this page was written by a polymer scientist not a biologist. The thing that may surprise you is that we actually know what we are talking about when it comes to bacteria... read on!

Why don't we just get the bacteria to make the plastic?

This is already done commercially on a small scale. The polymers used are "condensation polymers", like cellulose, but have properties more like polyethene.

Polyhydroxybutyrate (poly-3-hydroxybutanoate) and Polyhydroxyvalerate (poly-3-hydroxypentanoate) (PHV).

These polyesters are produced by a number of different bacteria (Alcaligenes spp., Pseudomonas spp.) as a food storage material, just like we desk jockeys lay down layers of juicy fat.

PHB and PHB/PHV copolymers are produced in reasonable quantities under natural conditions (up to 30% of the dry weight of the bacteria), which can be increased to 90% polymer by weight in dry bacteria if fermentation conditions are carefully controlled to limit oxygen and nitrogen. The yield is dependent on feed material, and coincidentally researchers have shown ethanol to be the best food!

Genetic engineering has been done to transfer the polymer-making capacity to Escherischia coli (a very well studied and controlled bacterial system) and to higher plants. It is theoretically possible to modify starch forming plants (such as potatoes) to grow PHB and PHV instead, replacing petrochemical plants with potato fields and giving rise to the spectre of a world in the economic grip of OPEC (Organisation of Potato Exporting Countries).

The first commercial product made from these polymers was a bottle for a biodegradable shampoo, produced in Germany in 1990. In 1994, 300 000 kg/year was being produced, compared with approximately 85 000 000 000 kg/year for polyethene. Production capacity today is about 600 000 kg/year.

Specialty polymers

Since these natural polymers are biodegradable, they are ideal for materials that are hard to separate for recycling, such as nappies, kitchen film and sanitary pads. In parts of Europe, where recycling or biodegradability of polymers is mandated by law, this is an important consideration.

As well, these natural polyesters are "biocompatible" and can be used to make surgical thread and for other medical applications; if left behind in the body, they will easily be degraded with no harmful effects.

The properties of PHB-PHV blends are such that they could replace polypropene for many applications.

However, oil is still too cheap : in 1996 these polymers were still about 10 times as expensive (1996 figures: A$13/kg compared to aroud A$1/kg for polypropylene - 1 AUD (Australian dollar) is about 75 cents US - 2005). A recent study of all the energy costs of processing and transporting polymers made either in plants, or by bacteria, found that the amount required was very much greater than needed for producing plastics from petrochemicals - so much so that as long as our energy is derived from non-renewable resources, they will greatly increase carbon dioxide emissions.

Once an alternative energy supply is in place (solar, biomass, fusion, etc.) these biopolymers are likely to come into their own...