Results

Synthesis Results

Characterization of pET29B(+)-phaC1J4

The objective of our project was to genetically engineer E. coli to synthesize PHB. We designed a pET29b(+)-phaC1J4 construct to allow our engineered E. coli to employ the beta-oxidation pathway to break down volatile fatty acids (VFAs) and use it to synthesize PHB. We have successfully ligated and transformed this part into E.coli. The figure below shows the digestion confirmation of the vector pET29B(+)-phaC1J4. We have also sequence verified this part.

To confirm the production of protein, we induced gene expression in E. coli BL21(DE3) transformed with pET29B(+)-phaC1J4 with IPTG and performed SDS-PAGE. Our results are shown in the picture below. The SDS-PAGE results indicated that our protein PhaJ4 was being expressed. It was difficult to distinguish PhaC1 amongst the other protein bands. However, because PhaJ4 is downstream of PhaC1, it is likely that PhaC1 was being expressed as well, but further SDS-PAGE runs must be performed for confirmation. Furthermore, our PHB synthesis experiment with E. coli containing pET29B(+)-phaC1J4 produced PHB as shown in Figure 3. The successful synthesis of PHB indicated the functionality of our exogenous proteins.

To confirm the ability of E. coli transformed with our pET29B(+)-phaC1J4 construct to produce PHB, we grew O/N cultures for 24 hours and induced them with 0.1 mM of IPTG while supplementing them with chemical media mimicking fermented synthetic poop supernatant (FSPS). Cells were then left to culture for 16 hours. To collect the resulting PHB, we performed sodium hypochlorite extraction on the cultures, which can be found on the Experiments page.

To confirm that the identity of the white powder made by the transformed E. coli was actually our desired product of PHB, we ran our results alongside a sample of pure industrial-grade PHB provided to use by PolyFerm by dissolving PHB in sulphuric acid, which can be detected via HPLC as crotonic acid. The analysis confirmed the identity of our PHB and its similarity to industrial-grade material because the presence of crotonic acid was indicated by the peaks in the spectrum below.

Detailed results of these experiments can be found on this part's Registry page.

Characterization of pET29b(+)-phaCBA

As part of our project, we also used a pathway that relied on acetyl CoA (produced from metabolism of glucose and some VFAs) to produce PHB. We designed pET29b(+)-phaCBA construct so that the E. coli breaks down acetyl Co-A and converts the products into PHB. We have successfully ligated and transformed this part into E.coli. The figure below shows the digestion confirmation of the vector pET29b(+)-phaCBA. We have also sequence verified this part.

To confirm the identity of the white PHB made by the transformed E. coli, we ran our results alongside a sample of pure industrial-grade PHB provided to use by PolyFerm Canada by dissolving PHB in sulfuric acid, which leads to the production of crotonic acid peaks. The analysis confirmed the identity of our PHB because the presence of crotonic acid was indicated by the peaks. Furthermore, our PHB was comparable to industrial-grade PHB because the peaks for the samples matched.

Detailed results can be found on this part's Registry page.

Secretion Results

Characterization of pSB1C3-Phasin-HlyA Tag

In order to secrete PHB, we engineered Phasin-HlyA Tag first into E. coli DH5α for plasmid propagation then into E. coli BL21(DE3) for protein expression. Our Phasin-HlyA Tag construct was designed for use in psB1C3. Figure 8 shows the successful digestion of Phasin-HlyA Tag with EcoRI-HF and SpeI then ligation into pSB1C3.

To confirm the functionality of Phasin-HlyA Tag, a "super" plasmid that contains both PHB-producing and PHB-secreting genes was made. This plasmid was created by ligating Phasin-HlyA Tag into pSB1C3-PhaCAB (PhaCAB is a PHB-producing part from Imperial College, 2013). This plasmid was transformed into E. coli BL21(DE3). The successful ligation of the "super" plasmid was confirmed with NotI-HF digestion, as shown in Figure 9.

For a secretion assay, triplicates of the "super" plasmid in BL21(DE3) were inoculated into LB media + chloramphenicol + 3 % glucose and triplicates of pSB1C3-PhaCAB in BL21(DE3) were used as a control. The bacteria were induced with IPTG then incubated at 37°C for either 24 hours or 48 hours. After that time, the samples were separated into intracellular and secreted PHB fractions. CaCl₂ was added to promote PHB in the secreted fraction to agglomerate so that it could be separated from cells via differential centrifugation. The amount of PHB collected in each fraction was measured and those results are presented below (Table 1).

The addition of CaCl₂ to the secreted fractions results in a pellet being obtained that contains both PHB and CaCl₂. To account for this, the mass of secreted PHB was corrected for the mass of CaCl₂ added (0.05549 g). The corrected results are presented in Table 2.

The assay showed that cells with PHB-secreting genes secreted more PHB than control cells that did not contains those genes, but this difference only became apparent after 48 hours of incubation at 37°C. At 24 hours, there was no meaningful difference between the amounts of PHB secreted by both E. coli variants (0.12851 g for the control and 0.12417 for the positive PHB-secreting samples). At 48 h, control cells secreted only 0.08051 g, but the PHB-secreting cells secreted 0.17251 g. This is a 114% increase in the amount of PHB secreted by cells with Phasin-HlyA Tag compared to the control. This indicates that our part, Phasin-HlyA Tag, improves secretion of PHB after an incubation time of at least 24 hours.

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These results can also be found on this part's Registry page.

Process Development Results

Methods for VFA quantification and characterization

As mentioned in our journal, determination of the total VFA concentration in the solution was an important step in the process – knowing how to quantify total VFAs in the solution helped to prove that the fermentation of human feces with naturally occurring bacteria increases the VFA concentration, as well as it helped to prove VFA presence in both - fermented and unfermented synthetic feces.

Titration is commonly employed by the wastewater treatment plants to give a rapid estimate of the VFA concentration in the solutions. We were able to successfully perform “Simple titration” experiments. The results (table 3) indicate that the method tends to give a slight overestimate of the total concentration – yet it can be used for quick estimations, as well as for determination of VFA concentration increase/decrease.

HPLC is another method commonly employed in laboratory setting for the VFA concentration determination. The advantage of the method is the fact that it provides the concentration of different volatile fatty acids in the solution.

VFA fermentation results

The first VFA fermentation experiment showed higher VFA production at 37°C than at 22°C (Figure 11). The control condition was E. coli BL21(DE3) transformed with pET29b(+) vector without any inserts (non-PHB producing), Imperial condition was PHB-producing E. coli transformed with the Imperial College 2013 part , and Tokyo condition was PHB-producing E. coli transformed with the Tokyo 2012 part . The reported results are an average of 3 titrations performed for each sample.

The second VFA fermentation experiment confirmed higher VFA production at 37°C than at 22°C (Figure 12). In addition to a 3-day fermentation (denoted as D3), a 5-day fermentation was also introduced during this experiment (denoted as D5). Higher VFA concentrations were observed after 5 days of fermentation. Similarly to the first experiment, the reported results are an average of 3 titrations performed for each sample.

In subsequent experiments, the supernatant from fermented synthetic feces was collected after the VFA fermentation experiments, sterilized, and cultured with PHB-producing bacteria for 2 – 3 days at 37°C. PHB-producing bacteria cultured in the supernatant from synthetic feces fermented at 37°C had little to no growth, resulting in little to no PHB produced. Although the supernatant from synthetic feces fermented at 37°C had higher VFA concentration (Figure 11, Figure 12), we believe the lack of growth at PHB production stage could be due to lower pH. Consequently, 22°C was selected as preferred VFA fermentation temperature due to optimal pH for bacterial growth, although lower VFA concentration.

Liquid-solid separation results

The very fist experiments for the solid-liquid separation were "Gravity driven sedimentation" and "Gravity driven filtration" experiments The results are summarized in the table 4.

It as clear the gravity alone would not do the required job, hence the"Staged Filtration" experiment was conducted using 25g of synthetic feces (recipe 2). The original sample contained 15g of water, yet only 10% of it was recovered, meaning that a more advanced and power intensive technology has to be considered for this stage of the process.

Finally, we decided to investigate the efficiency of centrifugal based extraction methods using the "Centrifugation for solid-liquid separation" experiment. When a 50g undiluted sample of synthetic feces (recipe 2) was tested, the mass of water recovered was 19.6g, while the mass of initial water present in the sample was 30g, meaning that there was 65% recovery of water. This result indicated that centrifugal based solid-liquid separation technology would be the best fit for our application.

PHB Extraction Results

Chemical Coagulation Results

We demonstrated in the lab that adding calcium chloride and then centrifuging increased the amount of PHB removed from a suspension in water. We carried out centrifugation at various RPMs and then collected absorbance data for the samples. The lower the absorbance reading, the higher the amount of PHB that was removed.

Details about the experiment can be found here. The absorbance readings and the standard deviations of the various conditions tested are shown in the figure below.

Electrocoagulation

We tested electrocoagulation of microscale PHB particles in suspension in water, nanoscale PHB particles in suspension in water, synthetic feces supernatant and a 1:1 mixture of microscale PHB suspension and synthetic feces supernatant.

From our preliminary experiments with just PHB particles in suspension in water we were able to demonstrate that PHB particles settle out via electrocoagulation. After we stopped running electricity through our cells, we were able to observe a layer of PHB at the bottom of the cell. However, after a few hours a layer of brown powder settled on top of the white layer of PHB. We hypothesized that this was probably iron(III) hydroxide, formed from the excess iron ions released by the anode that did not bind to the PHB.

From our experiments with synthetic feces supernatant we found that a layer of brown sludge settled at the bottom each time. Even with the 1:1 mixture there was no discernable layer of PHB within the sludge. We tried washing the sludge with dilute acid to remove the metal salts that might have been present in the sludge. However, we were still unable to separate PHB from the sludge.

The electrocoagulation experiments led us to conclude that while it is possible to settle out PHB using electrocoagulation, this method was not suitable for our media which contained a number of salts that interfered with the coagulation process and caused the formation of sludge.

High Performance Liquid Chromatography Analysis of PHB

During the first HPLC run with our PHB samples, we didn’t observe the crotonic acid peaks, which is the product of PHB digestion that can be detected using HPLC. This was likely due to the 3 hour digestion time, which was likely too long resulting in degradation of crotonic acid.

For the second HPLC run, we tested two different digestion times (20 minutes and 30 minutes) and observed peaks for crotonic acid for samples obtained from our phaCBA part and confirmed that PHB was present in the our samples. We also ran PHB produced using Imperial College 2013 part and observed crotonic acid peaks as well.

Although we were successful at detecting PHB using HPLC, quantification of PHB using HPLC would require further development. In particular, an optimal digestion time needs to be selected so that all of the PHB in the sample digests to crotonic acid, while crotonic acid does not start to degrade. We also calculated dilution factors for the samples by assuming an 80% conversion from PHB to crotonic acid which was reported in (Karr, Waters, Emerich, 1983). However, conversation factors in our HPLC runs ranged from 40% to 60% for the standard PHB samples, resulting in inaccurate dilution factor predictions. Accurate dilutions are important to ensure the sample concentration is within the HPLC detection range. The bleach PHB extraction method can also be optimized to minimize the amount of PHB lost during extraction.

For our third HPLC run, we digested the samples from the Beta Oxidation part for 15 and 30 minutes and observed clear peaks for crotonic acid. We also based our dilution factors on 60% conversion. More details about the HPLC results can be found on the beta oxidation part page and the gycolysis part page