Bacteria can break down almost all chemical compounds. However, plastic poses a particular challenge for microorganisms and their enzymes, as these plastics or "synthetic polymers" are stable chains of repeating molecules: polymer means "many-part" in Greek. Polymers that we use every day and that also end up in nature are polyurethane (PUR), polyethylene (PE), polyamide (PA), polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC), epoxy-based polymers (EP), polypropylene (PP) and, in some cases, synthetic rubber (SR).

Only a few microorganisms can degrade some of these petroleum-based plastics under laboratory conditions. Microbial degradation is possible if, for example, an ester bond forms the backbone of the polymer; PET and ester-based PUR can be degraded accordingly. Around 65 enzymes are known to break down the ester bonds in PET and eleven enzymes for PUR. The best characterised enzymes come from microorganisms of the actinobacteria genera Thermobifida or Thermomonospora.

Another well-known example is the Gram-negative betaproteobacterium Ideonella sakaiensis 201-F6, which can even utilise PET as its main source of energy and carbon. Other organisms that act on PET are Pseudomonas aestusnigri, Vibrio gazogenes or Kaistella jeonii. Comamonas acidovorans. PUR is degraded by some pseudomonads, some of the PET degraders can also decompose PUR.

Presumably, synthetic rubber is also at least partially degradable if similar bond types are present as in natural rubber, which is the case with Rhizobacter gummiphilus. In addition, some enzymes can degrade short pieces (oligomers) of PA (nylon) or individual monomers of polystyrene (PS).

So far, however, polymer degradation has only been demonstrated under laboratory conditions with partially optimised enzymes. It is unclear whether these microorganisms and enzymes play a role in the degradation of PET, PUR or rubber in nature. It is certain, however, that the processes are significantly slower and very long periods of time must be assumed. Nature has only known these polymers for less than 80 years - too short a time for effective enzymes and degradation processes to evolve. For all other types of plastic, no microorganisms or enzymes are yet known that can break down synthetic polymers, at least under laboratory conditions.

Unlike the classic types of plastic mentioned above, modern biopolymers such as polylactate (PLA), polyhydroxyalkanoates (PHAs), polybutylene adipates (PBAT) and polybutylene succinate (PBS) are in principle biodegradable. In nature or in compost heaps, microbial and enzymatic degradation takes place, but the process is very slow and takes several months; rapid composting methods are therefore unsuitable. Unfortunately, their share of the total amount of plastics used globally is currently still very low (less than 5%) and they cannot replace many of the material properties of traditional polymers.

Research in the field of plastics should therefore not only focus on finding enzymes for the degradation of synthetic polymers, but also on producing better biopolymers that can be used more widely. Avoiding plastic is the best way.

The bacterium Commamonas sp. DHH01 on a PET fibre that it can degrade. Scanning electron microscope image (Quelle: DOI: https://doi.org/10.1128/AEM.01095-19).

Read more:

https://www.biospektrum.de/magazinartikel/marine-mikroorganismen-fuer-den-plastikabbau?dl=1

https://www.biospektrum.de/magazinartikel/ein-nachhaltiges-produktionssystem-fuer-plastik?dl=1

https://ami-journals.onlinelibrary.wiley.com/doi/10.1111/1751-7915.14135

The PAZy database provides a good overview of microbial enzymes and microorganisms involved in plastic degradation.

© Text und Figure, Wolfgang Streit/ VAAM, wolfgang.streit[at]uni-hamburg.de, use according to CC 4.0