Microbial production of plastics

1. MICROBIAL
PRODUCTIONOF
PLASTICS

2. INTRODUCTION
◦ The “traditional” plastics derived from coal, natural gas, and oil are one of the
most environmentally harmful substances produced by humans, but they are also
an important and useful part of our society. From mobile phones to packaging, it
is difficult to imagine a world without them.
◦ Plastics are extremely versatile; they can be transparent, cost-effective, lightweight,
strong, and durable, and they also possess vital properties that make them useful
in medical, agricultural, domestic, and industrial applications.

3. ◦ While these properties are desirable, the highly stable structure of plastics also
makes the process of natural degradation difficult; therefore, these materials are
overused, and they persist in soil, marine, and freshwater ecosystems when they
are irresponsibly dumped.
◦ Even attempting to destroy plastics via incineration can cause major air pollution
by releasing toxins, like dioxins, furans, mercury, and polychlorinated biphenyls.

4. ◦ The exponential growth of the human population has led to the accumulation of
huge amounts of non-degradable waste materials across our planet.
◦ Living conditions in the biosphere are therefore changing dramatically, in such a
way that the presence of non-biodegradable residues is affecting the potential
survival of many species.
◦ For this reason, many countries have promoted special programs directed towards
the discovery of new commonly used materials that can be readily eliminated
from the biosphere, and have designed novel strategies aimed at facilitating the
transformation of contaminants.

5. What are bioplastics?
◦ Biomaterials are natural products that are synthesized and catabolized by different
organisms and that have found broad biotechnological applications.
◦ Bioplastics are a special type of biomaterial. They are polyesters, produced by a
range of microbes, cultured under different nutrient and environmental
conditions.
◦ These polymers, which are usually lipid in nature, are accumulated as storage
materials (in the form of mobile, amorphous, liquid granules), allowing microbial
survival under stress conditions.

6. ◦ The number and size of the granules, the monomer composition,
macromolecular structure and physico-chemical properties vary, depending
on the producer organism.
◦ Currently, the main limitations for the bulk production of bioplastics are its
high production and recovery costs.
◦ However, genetic and metabolic engineering has allowed their biosynthesis
in several recombinant organisms (other bacteria, yeasts or transgenic
plants), by improving the yields of production and reducing overall costs.

7. Discovery of bioplastics
◦ Lemoigne first described a bioplastic — poly(3-hydroxybutyrate) (PHB) in
Bacillus megaterium.
◦ This initial observation was almost forgotten until the mid-1970s when, because
of the petroleum crisis, a scientific movement aimed at discovering alternative
sources of fossil fuel reserves was undertaken. However, the structure,
biosynthetic pathways and applications of many bioplastics have now been
established.
◦ Microbes belonging to more than 90 genera — including aerobes, anaerobes,
photosynthetic bacteria, archaebacteria and lower eukaryotes — are able to
accumulate and catabolise these polyesters.

8. ◦ The most widely produced microbial bioplastics are PHB, PHA and their
derivatives. However, other polyesters can also be produced by microorganisms.
◦ But, most of them either require similar biosynthetic enzymes or lack current
industrial applications.

9. POLYHYDROXYALKANOATES(PHA)
◦ Polyhydroxyalkanoates or PHAs are polyesters produced in nature by
numerous microorganisms, including through bacterial fermentation
of sugars or lipids. When produced by bacteria they serve as both a source of
energy and as a carbon store.
◦ These plastics are biodegradable and are used in the production of bioplastics.

10. Biosynthesis of PHAs
◦ To induce PHA production in a laboratory setting, a culture of a micro-organism such as
Cupriavidus necator can be placed in a suitable medium and fed appropriate nutrients so
that it multiplies rapidly.
◦ Once the population has reached a substantial level, the nutrient composition can be
changed to force the micro-organism to synthesize PHA.
◦ The yield of PHA obtained from the intracellular granule inclusions can be as high as
80% of the organism's dry weight.
◦ The biosynthesis of PHA is usually caused by certain deficiency conditions (e.g.
lack of macro elements such as phosphorus, nitrogen, trace elements, or lack of
oxygen) and the excess supply of carbon sources.

11. POLYHYDROXYBUTYRATE(PHB)
◦ Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA),
a polymer belonging to the polyesters class that are of interest as bio-derived
and biodegradable plastics.
◦ The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common
type of polyhydroxyalkanoate, but other polymers of this class are produced by a
variety of organisms.

12. Biosynthesis of PHB
◦ PHB is produced by microorganisms (such as Cupriavidus
necator, Methylobacterium rhodesianum or Bacillus megaterium) apparently in
response to conditions of physiological stress, mainly conditions in which
nutrients are limited.
◦ The polymer is primarily a product of carbon assimilation
(from glucose or starch) and is employed by microorganisms as a form of energy
storage molecule to be metabolized when other common energy sources are not
available.

14. Biotechnological applications
◦ Many different applications have been described for bioplastics since the first
industrial production of Biopol1 in 1982. Initially, they were used for the
fabrication of bottles, fibres, latex and several products of agricultural,
commercial or packaging interest.
◦ Currently, these polyesters have been employed for medical applications such as
sutures, implants, urological stents, neural- and cardiovascular-tissue
engineering, fracture fixation, treatment of narcolepsy and alcohol
addiction, drug-delivery vehicles, etc.

15. CONCLUSION
◦ To date, more than 160 different polyesters with plastic properties
have been described and this number is growing exponentially by
means of genetic and metabolic engineering techniques.
◦ The collection of novel polyesters using recombinant microbes
suggests that the biosynthetic limitations observed in the original
strain are not imposed by a strict substrate specificity of the anabolic
enzymes but, instead, are due to other physiological reasons.

16. ◦ Thus, it could be expected that many other bioplastics with different structures,
properties and applications could be obtained if the appropriate organism were
selected and genetically manipulated.
◦ In conclusion, because of their special characteristics and broad biotechnological
applications, bioplastics are compounds with an extremely promising future.