Difference between revisions of "Team:BGU ISRAEL/Results"

The bacterium Rhodococcus ruber R. ruber, known to utilize polyethylene (PE) as a carbon source, was grown in various liquid growth media with and without amino-acids and containing PE or PET pellets for a period of 30 days.

For control we used a SM liquid growth medium with glucose, glucose and amino-acids and a SM medium with no carbon source (for detailed instructions on preparation of the media see Protocols ).

As expected the results show that growth on glucose media was substantially faster and yielded high O.D₆₀₀ measurements throughout the experiment. We have observed similar results when the bacterium was grown on media containing amino acids (AA), suggesting the bacteria can utilize the AA as a carbon source. The media containing plastic as the sole carbon source (PET, PE), reached higher O.D₆₀₀ values from that of a medium with no carbon source (SM medium) but lower O.D₆₀₀ than on glucose media or plastic enriched with AA media. All samples have shown a steep rise in the absorbance on the first few hours of the experiment. We can assume that these results are due to the trace of rich medium that was left from the starter that was used prior to the begining of the experiment. This might also explain the increase in O.D₆₀₀ values in the SM medium. Lack of values of certain samples is due to contaminations or broken equipment.

After 30 days we ceased measuring O.D₆₀₀ values, but continued monitoring our samples for bacterial growth. Samples were left to incubate unopened for 3 months to test for the formation of biofilms.

As seen in the picture (fig. 2), after 3 months R. ruber developed a biofilm surrounding the pellet (in this case – PE). The orange color is a unique property of R. ruber.
We have witnessed similar results with PET pellets, suggesting R. ruber can also utilize PET as a sole carbon source.

We took samples from both PET and PE media (with pellets) and examined them with a Phase-contrast microscope and scanning electron microscope (SEM):

First, we could see that R. ruber was present in its planktonic form in the media.

After further examining the images we noticed the formation of vacuoles close to aggregates of bacteria. We assumed the vacuoles are made up of Extracellular polymeric substance (EPS) that was secreted by the bacteria into the media. We speculated that in this case the EPS is used for the absorption of the PET micro particles and allows the accumulation of secreted enzymes. The enzymes are used to catalyze the initial cleavage of the polymer and allow absorption of nutrients (Laspidou et al 2002). These images suggest a mechanism to improve degradation by increasing the extracellular degrading enzyme concentrations.

More samples from the same experiment were analyzed using SEM:

As seen from the images above, there is a substantial difference between the surfaces textures. The control's (fig. 5) surface is rough while the treated one is smoother (fig. 6). It can be explained by the ability of bacteria, i.e. its enzymes, to degrade exposed and less dense polymers with more ease, smoothening the outer layer.

We performed SEM on the PET samples in order to examine if the same effect on the polymer's surface is observed:

Contrary to our expectations, the untreated PET surface is smooth while the treated sample’s surface seemed to have gone through degradation (fig. 8).

Furthermore, we have come to notice the appearance of crystalline rods near bacteria aggregation sites. That can be explained by the preferential degradation of the amorphous material exposing the better defined (higher crystallinity) material that goes through a slower degradation (Shah et al 2008).

Our results suggest that R. ruber is able to degrade PET, having presented substantial changes before and after incubation with PET. It should be mentioned that it is not yet established that R. ruber is able to utilize PET as a carbon source. In order to verify this possibility, more tests are needed.
After identifying R. ruber's mechanisms of plastic degradation such as secretion of polymer degrading enzymes and forming vacuoles from EPS to increase the extracellular enzyme concentration, we decided to concentrate our efforts on improving a PET degrading enzyme.

It is important to note, that to the best of our knowledge, this is the first time that PET degradation by R. ruber is reported.

References

1. Laspidou, C. S., & Rittmann, B. E. (2002). A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass. Water Research, 36(11), 2711-2720.
2. Shah, A. A., Hasan, F., Hameed, A., & Ahmed, S. (2008). Biological degradation of plastics: a comprehensive review. Biotechnology advances, 26(3), 246-265.

To further characterize LC-Cutinase's PET degradation activity we used Scanning-electron microscopy of PET pellets incubated with E. coli expressing LC-Cutinase.

Control:

One control was PET pellets incubated for 2 days in LB liquid media with no bacteria.
As seen from the images, the surface of the PET pellets is relatively smooth, there are some high crystallinity rod shaped structures but they are fairly covered.

As a second control we used E. coli transformed with the pACYC vector with no insert (the backbone alone), to assess the effects of bacteria themselves on the PET.

PET incubated with LC-Cutinase expressing bacteria:

As seen above, after incubation with the LC-Cutinase expressing bacteria, the surface of the PET is rougher than the control. We also notice that the surface of the PET looks more porous and the rod shaped, high crystallinity PET, is more exposed than in the control.
Moreover, when comparing the bacteria expressing the LC-Cutinase gene and the bacteria transformed with an empty vector backbone, we see a clear difference in the texture of the PET. We assume that this is the result of the PET degradation activity of the LC-Cutinase enzyme.

Please note, that when comparing the appearance of degraded PET between LC-Cutinase degraded PET and R. ruber degraded PET, they looked the same, however, the difference was that it took a month to R. ruber to degrade PET to the observed picture (Fig. 8-10) while it only took two days to LC-Cutinase to reach the same. Although, this is not a quantitative measure, it is an indication of the enzyme’s high activity.

In order to test P. putida's ability to utilize TPA and PCA as a carbon source we measured O.D₆₁₀ values for a period of ~100 hours with P. putida in liquid M9 media with either TPA or PCA as a carbon source. To prevent contamination, we used Ampicillin and Chloramphenicol (at 100μg/mL and 34.1μg/mL respectively) as selection markers.

From these results we conclude that P. putida can utilize PCA as a sole carbon source, as previously reported. In contrast, our results show no growth with TPA, which lead us to the conclusion that P. putida cannot utilize it as a sole carbon source.

Reviewing these results leads us to the conclusion that the TPA degradation pathway must be cloned into the P. putida bacterium for efficient PET degradation.

In order to test the PET degradation ability of the LC-Cutinase enzyme variants, all variants were grown on M9 minimal medium plates with shredded PET pellets as a sole carbon source to test their ability to degrade PET. (for detailed instructions on the preparation of the media see Protocols).

As seen in the images above, all LC-Cutinase variants were able to grow on the plates containing shredded PET, suggesting that the PET is degraded and E. coli is utilizing at least one of its products (we assume - ethylene glycol).
We cannot conclude which variant degrades the PET with the highest efficiency, as the PET in the plates is not evenly spread and the amount of PET degraded cannot be measured in this assay.

However, we have clearly shown that our bacteria can utilize PET as a carbon source using the LC-Cutinase protein, which is a proof-of-concept for one part of our project. Looking at the entire design- our biofuel cell will consist of E. coli expressing LC-Cutinase that degrades the PET and consumes ethylene glycol (it includes the previous results in the E. coli ethylene glycol utilization results). We have managed to show that out E. coli will be able to break down PET in conditions similar to those in our designed biofuel cell and to survive without any other carbon source.

Moreover, our new LC-Cutinase genes consist of a PelB leader sequence that has been codon optimized to maximize expression levels in E. coli. The PelB leader sequence is a previously registered part (Part:BBa_J32015).

Our symbiotic approach relies on mutual growth of E. coli and P. putida. For this approach to succeed E. coli must successfully grow inside a dialysis bag.

In this experiment we grew E. coli strain BL-21 transformed with the pACYC vector containing the LC-Cutinase W.T. gene and induced with IPTG in a liquid M9 medium with PET.

As seen from the results, our E. coli was able to grow with only PET as a sole carbon source, suggesting it utilized PET's degradation product, ethylene glycol, as a carbon source.

Moreover, we see that E. coli remained viable throughout the experiment, verifying our hypothesis that it can grow inside a dialysis bag, and suggesting that our symbiotic design using a dialysis bag is plausible.

We consider that result as an important step and a proof-of-concept of our final goal.