Plastic to Bioplastic: The Bacterial Journey
Shoham Ghosh, Sanjana Ghosh, Dr. Arup Kumar Mitra
St. Xavier’s College (Autonomous), Kolkata, West Bengal
Since its invention 162 years ago, plastics have been one of the most important and recognisable human inventions of post-Industrial society, a material capable of being moulded into all forms and made to be hard-wearing, rot-proof and virtually indestructible; yet those very properties now come back to haunt us. There is virtually no place on Earth untouched by plastics. It has been estimated that practically every gram of this material we ever created still exists on the planet in some form or another. Microplastics, which result from the natural erosion of larger plastic pieces, have now found their way into every habitat imaginable, including our own bodies. They are a growing concern in everyday life and the potential health risks they pose is debated and perhaps even grossly underrated. However, despite their flaws, plastics are still very useful materials. The trick is to make them sustainable and this is where the bioplastics come in and we turned to bacteria to achieve this metamorphosis.
Bacteria are one of the great heroes of the microbial world and many of them, when overfed with carbon sources such as glucose, store the food in the form of PHAs/ PHBs (Polyhydroxyalkanaoates and Polyhydroxybutyrates respectively) in intracellular vesicles for later use. These can be extracted from the cells and made into biodegradable plastics. Since the synthetic plastics polluting our environment are primarily made of carbon, in theory a bacterium can break them down and store the extra carbon in PHA/ PHB form, if they have the necessary genes to do so. While it may seem that the prospects of finding such organisms is remote, the source could, ironically, be the plastics themselves. Every year, almost 8 million tonnes of plastic ends up in our oceans. Here, in the limiting and harsh environment, microbes can attach themselves to these plastics and evolve unique mechanisms to exploit it. Research involving this was done by us, whereby a piece of plastic, a PET bottle, was obtained from the tidal line at a beach, and saline tolerant bacterial strains were isolated and screened for any potential plastic breakdown capabilities based on their production of four important classes of enzymes.
Bacteria primarily employ enzymes of the cutinase, esterase, lipase and laccase classes to break down plastics in the environment. Cutinases and laccases are predominantly used in the break-down of polyethylene terephthalate (PET) plastics into smaller subunits. These can be internalized into the cells where they are further broken down and enter, primarily, into the tricarboxylic acid (TCA) cycle. Esterases and lipases, on the other hand, are mainly used to break-down polyethylene (PE) plastics into smaller oligomers, which are internalized, converted into ketones by other lipases and enter the TCA cycle. From the screenings, five bacteria were identified to be producing all four enzymes at appreciable levels. Additionally, tests were conducted to identify whether they could also produce PHAs/ PHBs from glucose. The bacteria were observed to produce between 5 to 9 µg/ mL of bioplastics individually in the span of three days from a glucose source. With these results at hand, a test was conducted to observe if a consortium of these five bacteria could break down PET and use it to make the PHA/ PHB granules. The results yielded PET degradation rates of around 3% and bioplastic levels of more than 100 µg/ mL in the span of just 9 days. Further studies are required to be performed such as the range of plastics the consortium can break down, the chemical nature of the bioplastic produced, the identity of the bacteria and the genes involved in this remarkable alchemy.
The uniqueness of the consortium and the ease of growing it can make these bacteria useful in industrial settings where waste plastics are fed in one end and bioplastics are obtained at the other end. Since these are marine bacteria isolated from a tightly sealed bottle, they have high levels of saline tolerance, giving them the ability to be applied to many types of environments, including heavily degraded soils laced with fertilizers and the oceanic habitats where the damaging effects of plastics have been brought into the spotlight in recent years, especially the risks of microplastics bio-accumulating in the flesh of fishes and other sea creatures harvested for human consumption. The bacteria can potentially be used as an effective way to eradicate microplastics from agricultural lands, either directly or through the genes responsible for the entire mechanism being identified and used to genetically transform other, more familiar bacteria currently used, such as Bacillus subtilis and Escherichia coli. to create strains with this capability, perhaps even improving their degradation and bioplastic production capabilities. This has been a stepping stone to strike a chord with nature’s resilience.
Biotechnological innovation and emotional resonance have motivated us to embark on this journey of transformation of plastics to bioplastics. Technological advancements and scientific research have enabled us to find this novel approach and emotional resonance of achieving a cleaner and greener planet motivate us to keep pushing further. It also serves as a powerful reminder of the untapped potential of microbes isolated from different habitats, whereby if such smallest agents in this world can drive a change, so too can humanity.

Shoham Ghosh

University/College name : St. Xavier's College (Autonomous)