Plastics play a major role in our everyday lives. From water bottles to prosthetics, plastics can be seen everywhere. However, these oil-based polymers take many years to degrade, which poses an environmental problem in some areas. Fortunately scientists are discovering ways in which to produce more environmental-friendly plastics. Many of these methods involves the use of the eubacterium Alcaligenes eutrophus.
Alcaligenes eutrophus, which is now called Ralstonia metallidurans, is a gram- negative, non-spore forming bacillus. This bacterium thrives at an optimal growth temperature of 30 C in environments that contain millimolar concentrations of toxic heavy metals. A. eutrophus is often found in soils and sediments containing high contents of heavy metals in various geographical locations. This bacterium also is an obligate aerobe, a falcutative chemolithoautotroph, and can oxidize hydrogen to water for energy.
What makes A. eutrophus so unique is its ability to synthesize a biodegradable plastic called polyhydroxbutyrate (PHB). This polymer is very similar to polyhydroxalkanoates (PHA), a biodegradable polymer already used in industries. A. eutrophus synthesizes PHB as a way to store lipids in conditions in which excess carbon is present, but limited in nutrients such as nitrogen or phosphate. Up to 80% of the dry weight of A. eutrophus can be composed of PHB inclusions.
A. eutrophus synthesizes PHB from acetyl-Coa in a pathway involving three enzymes. The first enzyme, 3-ketothiolase, is used to promote the condensation of two acetyl-CoAs into acetoacetyl-CoA. Next, the second enzyme, acetoacetyl-CoA reductase reduces acetoacetyl-CoA to R(-)-3-hydroxybutynl-CoA. Finally, the third enzyme, PHA synthase polymerizes the R(-)-3-hydroxybutyrl-CoA to form PHB.
Scientists have just now realized the importance of A. eutrophus’s ability to produce plastic and are trying to harness this power. Many researchers are now growing mass cultures of A. eutrophus in conditions that promote PHB production and extracting the plastic. Others are using genetic engineering to insert the DNA sequences of the three enzymes discussed above into the DNA of the plant Arabidopsis thaliana, thus growing plants able to synthesize plastics. One group of Michigan Sate University researchers was successful in using A. thaliana plants to produce PHB granules. Incredibly, 14% of the plants’ dry weights were accumulations of PHB.
Plastics produced by such methods described above are already being used in the medical field, which serve as harnessing apparatuses for broken bones that deteriorate as the bone heals. Also, this new plastic is also used in the production of some shampoo bottles. One drawback with this new plastic is that it is very costly. Harvesting the plastic directly from A. eutrophus would cost $4.00/lb and transferring genes into A. thaanlia and then extracting the plastic would cost $1.50/lb, both methods, however, cost more than oil-based plastic which cost $0.50/lb. Another drawback is that only about 600 tons of the A. eutrophus plastic is produced a year, while 100,000 tons a year of normal plastic is considered a small amount for one factory.
Much more research is needed about A. eutrophus’s awesome ability to synthesize PHB before we are able to incorporate this biodegradable plastic into our society. However, I do not feel that it will be long until labels saying “Hecho en Mexico” are replaced with ones saying “Hecho en Alcaligenes eutrophus