From 2013.igem.org

Results

Constructs

PSB1A3_RCNA+ YFP:

Figure 1: Transformation of XL1-Blue bacteria with the ligation PSB1A3_RCNA+ YFP. Many bacteria grew on plates! We took some colonies and had their plasmids minipreped.

Figure 2: Digestion of PSB1A3_RCNA+ YFP miniprep. Digestions were made using EcoRI and PstI enzymes. A fragment of the expected size for RCNA+YFP (~ 1300 pb) can be seen in lane 4. 1Kb: molecular ladder. 1: non digested plasmid. 2: plasmid digested with EcoRI. 3: plasmid digested with PstI. 4: plasmid digested with EcoRI and PstI.

Figure 3: PCR of PSB1A3_RCNA+ YFP minipreps. PCRs were made using VF2 and VR primers. A fragment of the expected size for RCNA+YFP (~ 1300 pb) can be seen in lanes 1 and 3. 1Kb: molecular ladder. 1: miniprep 1. 2: miniprep 2. 3: miniprep 3. 4: miniprep 4. C-: negative control.

PSB1C3_TorCAD:

Figure 4: PCR of PSB1A3_TorCAD minipreps. PCRs were made using VF2 and VR primers. Fragments of the expected size for TorCAD (~ 120 pb) can be seen in all lanes. 1Kb: molecular ladder. 1: miniprep 1. 2: miniprep 2. 3: miniprep 3. 4: miniprep 4.

PSB1C3_TorCAD+RFP:

Figure 5: PCR of PSB1A3_TorCAD+RFP colonies. PCRs were made using VF2 and VR primers. Fragments of the expected size for TorCAD+RFP (~ 1000 pb) can be seen in some lanes. 1Kb: molecular ladder. 1 to 10: 10 different colonies used as templates for PCR.

Fluorometric Assays

The results shown in here were performed as described in Protocols. We used Varioskan Flash Multimode Reader (Thermo Scientific™) to do the reads of YFP constructs and Synergy 2 (Biotek) to do the reads of mCherry.

PSB1A3_RCNA+ YFP:

We have constructed the composite RCNA+YFP to detect IMA (ischemia modified albumin) in the serum of patients with cardiac risk. These patients present more IMA than normal individuals.

To prove that our construct works, we made some tests with transformed E. coli XL1-Blue. We added different concentrations of cobaltous chloride to bacterial cultures and measured their fluorescence (excitation: 514nm; emission: 527 nm) and absorbance (600 nm) for a certain period. The results for fluorescence(Figures 6) show a peak of fluorescence which can be seen 1 hour after 25 µM treatment, and another peak after 4 hours of 75µM treatment. However,normalizing the data according absorbance measures we can see only a peak after 4 hours of 75µM treatment.

Figure 6: Fluorometric reads of cultures of E. coli XL1-Blue carrying the plasmid PSB1A3_RCNA+ YFP, along the time, after treatment with 0, 25, 50, 75, 100, 125 or 150 µM of cobaltous chloride.

Figure 7: Fluorometric and absorbance reads of cultures of E. coli XL1-Blue carrying the plasmid PSB1A3_RCNA+ YFP, after treatment with different concentrations of cobaltous chloride. Bacteria were treated with 0, 25, 50, 75, 100, 125 or 150 µM of cobaltous chloride." After that, fluorescence and absorbance were read hourly, until 4 hours, and there were read 8 and 24 hours after treatment. A peak of fluorescence can be seen 3 hours after treatment. Intermediate concentrations of cobalt were more efficient in generating fluorescence.

As IMA binds less to cobalt than normal albumin, we expected that a serum containing more IMA would have more free cobalt than a “normal” serum. This excess of cobalt would be able to activate more RCNA promoter, which in turn would lead to expression of YFP, so we could be able to distinguish between a patient with cardiac risk from a normal patient by comparing the fluorescence generate by each serum.

First, we tested whether BSA (bovine serum albumin)would produce the expected result (more BSA, less free cobalt, less fluorescence). As can be seen on Figure 9, the result obtained meets the expected result.

Figure 8: Fluorometric assay to assess the BSA cobalt binding. In this experiment we measured the fluorescence produced by the RCNA-YFP modified E.coli incubated with 100 μM of cobaltous chloride and different Bovine Serum Albumin (BSA) concentrations. Each curve shows different concentrations of Bovine Serum Albumin (BSA) and its respective fluorescence along the time. As we can see, the fluorescence produced by bacteria increases according to the cobalt available in the media.

We have also added mice serum to bacteria containing the composite. These sera were from ischemic or non ischemic mice.

We can see that for two of the ischemic serum samples the fluorescences produced were higher during the entire experiment, and we can differentiate it from the normal serum samples. However, for one ischemic serum sample, its fluorescence pattern can be differentiated from the normal samples just at the zero time. We hypothesized that it happened maybe because its degree of ischemia is smaller than the others (Figure 10).

Figure 9: Fluorometric assay using serum from isquemic and non-isquemic (normal) mices. In this experiment, 100 μM of cobaltous chloride was incubated with serum from 3 different mices with induced isquemia or not using RCNA-YFP modified E. coli. The three curves above (Isq1, Isq2, Isq3) are the sera of ischemic mice and the three below (Nor1, Nor2, Nor3), the non ischemic mice sera. .

We can conclude that the three curves with most intense fluorescence are due to the ineffective cobalt chelation by the mice serum albumin. In the control sample we can see the opposite effect, showing our E. coli sensor working as expected.

PSB1C3_TorCAD+RFP:

We have constructed the composite TorCAD + RFP to detect TMAO (Trimethylamine N-oxide) in the serum of patients with cardiac risk. These patients present more TMAO than normal individuals.

To prove that our construct works, we made some tests with transformed E. coli XL1-Blue. We added different concentrations of TMAO to bacterial cultures and measured their fluorescence and absorbance for a certain period. The results (Figures 11 to 12) show a peak after 3 hours, during exponential phase.

Figure 10: Fluorimetric reads of cultures of E. coli XL1-Blue carrying the plasmid PSB1C3_TorCAD + RFP, after treatment with different concentrations of TMAO. Bacteria were treated with 0 μM, 1 μM, 10 μM, 100 μM, 1 mM, 10 mM and 100 mM. After that, fluorescence was read hourly, until 15 hours. The beginning of the fluorescence increase can be seen about seven hours after the treatment.

Figure 11: Fluorimetric and absorbance reads of cultures of E. coli XL1-Blue carrying the plasmid PSB1C3_TorCAD + RFP, after treatment with different concentrations of TMAO. The fluorescence reads shown in figure 11 divided by the absorbance, resulting in the graphic above.

Discussion and Conclusions

The peak of fluorescence after 3 hours (Figures 6 to 8) is probably related to the phase of growth in which bacteria are (exponential phase). At this phase, bacteria are more metabolically active, once they are dividing in a great rate, so they need to produce large amounts of proteins.

Concerning cobalt concentrations, it is likely that lower concentrations activate the promoter less than intermediate concentrations, whereas bigger concentrations might be saturating the promoter, or even causing negative feedback.

In the tests using BSA or mice sera, the results meet our model, in which more normal albumin(or BSA) leads to less free cobalt, resulting in lower fluorescence.

Thus, our results show that the composite RCNA+YFP generates fluorescence in the presence of cobalt. Furthermore, it can be used to distinguish between ischemic and non ischemic individuals. Further characterization, including usage of samples containing human IMA (ischemia modified albumin) and normal albumin, is needed, in order to improve our composite’s documentation.

Regarding TMAO, we found that the fluorescence increases after 7 hours of 100 µM TMAO treatment, showing that our TorCAD+RFP worked as expected. Further characterization using human sera is also needed for better composite’s documentation.

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