Microbes, worms and plastic


Breaking down the microbiology world one bite at a time

Microbes, worms and plastic

Did you visit the grocery store today? Considering that you also paid for the packaging, how much plastic did you buy?

Did you recently buy a product that you did not need but just had a strong desire to possess? How much plastic was in the material from which this product was made?

Did you know that plastics are organic polymers, and yet they cannot be degraded easily because of their increased durability?

Lastly, do you know what styrofoam is? Remember, those take-out lunch boxes and those white super-light blocks that come with the packaging of electronic equipment. Styrofoam is made of polystyrene which is an extremely durable plastic.

Plastic pollution is a sensitive topic of discussion amongst the public, environmental scientists, media and consumer goods companies. Meanwhile, scientists found that plastic can be degraded by microbes residing in the gut of worms.

Why insects?

Insects can eat plastics. They have the ability to break plastic into smaller pieces by using their legs. When these pieces reach the insect’s gut, they are further degraded by the resident microbes. Several research studies show that a worm’s gut bacteria can eat and survive on Polyethylene and Polystyrene. A major problem with these studies is that researchers focussed on bacteria that can be isolated from a worm’s gut and cultured under lab conditions. A recent study sheds light on even those bacteria that were considered hard to culture in a lab. This study was focused on superworms, which are larvae of beetles.

Scientists kept 3 groups of superworms: the first group was fed polystyrene, the second group was fed wheat bran which is the common food for superworms, and the third group was not fed at all. You might ask ‘What is the point of having a group that was not fed anything?’ This group was used to identify if the worms gained enough energy from polystyrene for their survival. And what did they find? The wheat bran group gained more weight than the other two groups over the course of 30 days. The polystyrene group gained some weight in comparison to the starvation group. This means that polystyrene may not be the best source of food for the worms, even though scientists observed that worms chew their way by making narrow holes through the polystyrene blocks (See the figure below).

(a) Snapshots of worms eating styrofoam sheets; (e) Change in weight of worms (wheat bran-fed, polystyrene-fed and starvation groups) over time, (f) Change in weight of worms at the end of 15 day feeding period

How do microbes degrade polystyrene?

‘Polystyrene’ means a polymer of styrene (a big molecule made of several styrene molecules). Polystyrene can be degraded to small styrene molecules. Fun fact – styrene is an organic compound found in plants and fungi. Warning – Just because styrene is naturally found in plants/fungi, it is still not safe to consume (Not everything tagged organic is harmless!!!).

Plants and bacteria have an intricate relationship, where microbes utilize a lot of metabolites provided by plants, one of them being styrene. The same relationship holds for fungi and bacteria. This means that these smaller naturally occurring styrene molecules can be used as a food source by bacteria. But there is a catch! While it is known that polystyrene can be broken down to styrene by many microbe-produced enzymes, very little is known about how these enzymes facilitate this pathway. Styrene can be broken down to even smaller molecules by several pathways, one of them being vinyl side chain oxygenation. Styrene monooxidase is a crucial enzyme in this pathway and this enzyme can be produced by many bacteria. But, what are these bacteria?

Scientists found evidence that the gut microbiota of worms fed polystyrene harbored bacteria that specialize in degrading polystyrene, such as  Brevundimonas, Microbacterium, Pseudomonas, Sphingobacterium, and Streptococcus spp. These bacteria were present in greater amounts in the gut of polystyrene-fed worms than the wheat bran-fed worms and starving worms. Pseudomonas are particularly interesting bacteria because they are known to derive energy by feeding on polystyrene.

Remember that polystyrene degradation is a complex process and not all genes involved in this process are known. But in this study, scientists found genes coding for styrene monooxidase in the metagenome sequences of the polystyrene-fed worms group. On further investigation, it became clear that these genes have sequences similar to the ones found in styrene transport pathways in Pseudomonas putida.

What effect does polystyrene have on the health of these worms?

Efficient degradation of polystyrene by the collective efforts of worms and bacteria is a promising step in dealing with plastic waste. Yet, feeding on polystyrene is definitely not good for the worms. After worms ate styrofoam, they gained marginal weight compared to the starving worms. However, weight gain/loss does not necessarily mean poor health status. There are other ways to qualitatively measure the health status of worms after their exposure to plastic. Scientists observed the reproduction cycle of these worms post styrofoam feeding. Worms transition to pupae and pupae mature into adult insects. They found that the time taken for the pupae to reach maturity increased substantially as compared to the wheat bran fed worms.

What does this experiment imply for reducing plastic pollution?

For the short term planning of reducing plastic waste, using worms and their gut bacteria for plastic waste degradation may be an efficient process. However, in the long run, this idea may or may not sustain, depending on the effect of plastic exposure on the progeny of worms. Therefore, at the moment, this idea is useful for dealing with the existing plastic waste, but reducing the usage of plastic is still the best solution to ameliorate the impact of plastic pollution in the near future.

Link to the original post: Sun, J., Prabhu, A., Aroney, S. T. N., Rinke, C. Insights into plastic biodegradation: community composition and functional capabilities of the superworm (Zophobas morio) microbiome in styrofoam feeding trials. Microbial Genomics. 8 (6).

Featured image: created by the author