Team:Oxford/Experiments

Experiments

Introduction

Our enzymatic approach to the treatment of urinary tract infections (UTIs) is centred on the design of a "pathogen killing" engineered microbe which can in principle be incorporated into the urinary microbiome of UTI patients to achieve not only a therapeutic effect, but also potentially a prophylactic effect. The engineered microbe needs to contain three key features:

  • Constant secretion of biofilm-degrading enzymes - degrading the biofilms of the pathogenic bacteria reduces their resistance towards antibiotics
  • Production and intracellular accumulation of enzymes that can kill both the pathogenic bacteria and our engineered microbial host upon release into the extracellular medium
  • A quorum sensing mechanism that triggers the release of the antibacterial enzymes in the presence of pathogenic bacteria

Due to constraints in time and resources, we focused our experimental efforts towards the development of proof-of-concepts for only the first two features.

Through our experimental work with secretion assays, biofilm assays, and cell-killing assays we were able to obtain preliminary in vitro data suggesting that the BioBrick parts which we designed to allow our microbial host to produce the relevant biofilm-degrading enzymes and bacteria-killing enzymes are indeed able to function as expected, exerting antibiofilm and bactericidal activity against bacterial strains closely related by species and/or genus to the pathogens involved in catheter-associated urinary tract infections.

Bacterial Strains and Growth Cultures

E. coli DH5α was used for all cloning purposes. The E. coli strains MG1655 and RP437 ∆FliC, as well as the multi-effector knockout BSL-1 strain of Y. enterocolitica, IML421asd, were used as expression hosts. Cultures for cloning were grown in antibiotic-supplemented Lysogeny Broth (LB) at 37°C. The E. coli expression host cultures were grown in antibiotic-supplemented Lysogeny Broth (LB) at 37°C, while cultures of Y. enterocolitica IML421asd were grown in Brain Heart Infusion (BHI) media supplemented with diaminopimelic acid (DAP) and the appropriate antibiotic at 30°C.

Bacterial cultures were grown overnight (16-20 hours) to stationary phase before being subcultured for characterization experiments.

Plasmid Construction & Gene Cloning

Each gene sequence for our parts was directly synthesized pre-fused with 1) a sequence coding for a hexahistidine tag downstream of it, and 2) the BioBrick prefix and suffix sequences containing the EcoRI/XbaI and SpeI/PstI restriction sites attached upstream and (further) downstream of it respectively through IDT. The sequences were amplified using PCR and inserted into the pSB1C3 BioBrick standard vector backbone before subsequently being cloned into E. coli DH5α for plasmid storage and submission to the Registry.

For gene expression studies, our parts, contained in the pSB1C3 backbone, were extracted via Miniprep. The NcoI restriction site was introduced upstream of each of our gene sequences (except BBa_K1659501 and BBa_K1659601) via PCR to facilitate their insertion into the arabinose-inducible pBAD/HisB commercial expression vector, and the insert-containing expression vectors were subsequently cloned separately into the standard laboratory E. coli K-12 strain MG1655 as well as a chemotaxis knockout strain E. coli RP437 ∆FliC.

In the later stages of experimentation, BBa_K1659003[pBAD] was also cloned into Y. enterocolitica IML421asd for further characterization of gene expression.

Biofilm-degrading Enzymes

According to the design requirements, our genetic constructs coding for biofilm-degrading enzymes need to achieve the following:

  • Trigger the production of biofilm-degrading enzymes and facilitate the secretion of said enzymes from the engineered bacteria
  • Ensure that the enzymes, after secretion into the extracellular medium, are still correctly folded such that they retain their enzymatic biofilm-degrading function

The parts which we characterized for this section of the design are BBa_K1659211 (DsbA-DspBx) and BBa_K1659301 (DsbA-DNase).

Secretion Assaying

Click image for full protocol, including the recipes for the buffers used to purify each protein.

Stationary cultures of MG1655 DsbA-DspBx[pBAD] and MG1655 DsbA-DNase[pBAD] were subcultured into fresh media at a 1:20 ratio and enzyme secretion was induced for 4 hours using L-arabinose. Subsequent purification of protein from the cell-free supernatant and visualization using SDS-PAGE confirms that proteins of the expected size are present in the supernatant and hence most likely successfully secreted by the respective engineered bacterial strains.

Left figure: Lane D = DsbA-DspB, ~45 kDa; Right figure: Lane C = DsbA-DNase, ~22 kDa

Biofilm Degradation

To study the effects of the secreted enzymes on E. coli biofilms, we conducted two types of experiments:

  • Biofilm viability assays which investigated whether the secretion of the biofilm-degrading enzymes can prevent the therapeutic engineered bacteria, which are E. coli strains themselves, from forming biofilms.
  • Biofilm inhibition assays which investigated whether the enzyme-secreting cells can inhibit control cells which do not secrete the enzymes from forming biofilms.

    • Biofilm viability assay

    Secretion of functional biofilm-degrading enzymes should prevent the engineered bacteria from forming their own biofilms or at least reduce the amount of biofilms which they form. To investigate the validity of this hypothesis, 1:200 subcultures of enzyme-secreting cells were grown for 72 hours with different concentrations of arabinose inducer and the results were compared against control cells grown with the same set of arabinose concentrations.

    The results show significantly reduced OD590 values, which quantify the amount of stained biofilm remaining, in cultures where expression of DsbA-DNase (BBa_K1659301) or DsbA-DspB (BBa_K1659211) are induced to high levels (0.2% arabinose). The correlation suggests that the production and secretion of these enzymes can lead to reduced biofilm formation, and we proceed to investigate whether the enzyme-secreting cells can also inhibit control cells present in the same culture from forming biofilms as well.

    Co-culture Biofilm Inhibition Assay

    Screening of growth times versus density of biofilms grown for the culturing of E. coli biofilms showed that beyond 72 hours, biofilm density plateaus to more or less the same value independent of what the initial subculture concentration dilution was (1:100, 1:200, 1:500, 1:1000 were tested). In view of that, the ability of the enzyme-secreting cells to inhibit control cells from forming biofilms can be tested using a co-culture of the two cell types:

    • If the gene expression of BBa_K1659301 and BBa_K1659211 merely prevents the engineered bacteria from forming their own biofilms via some alternative intracellular mechanism instead of triggering the secretion of functional biofilm-degrading enzymes, the control cells in co-culture would still be able to form biofilms and over a time course of >72 hours achieve plateau biofilm density.
    • If the gene expression actually triggers the secretion of functional biofilm-degrading enzymes, the control cells in co-culture would form a lower biofilm density by the end of the time course.

    To test the hypothesis, co-cultures containing 4 parts engineered bacterial cells and 1 part control cells were set up using the same growth and analysis protocol as the biofilm viability assays above, for a growth time course of 120 hours:

    While DsbA-DNase failed to produce consistent enough results for conclusions to be drawn, DsbA-DspB was able to bring about a ~50% reduction in biofilm density in co-culture when compared against the controls.

Section Remarks

Overall, this section serves to provide some preliminary proof-of-function for the antibiofilm parts DsbA-DNase (BBa_K1659301) and DsbA-DspB (BBa_K1659211). The data is far from enough to be fully conclusive - it is yet to be established as to whether the secreted enzymes are able to degrade bacterial biofilms which were already pre-formed, as well as whether at experimental gene expression levels are the secreted enzymes able to degrade biofilms to an appreciable enough rate. Further experimental studies need to be conducted before a stronger causal link with biofilm degradation can be established and subsequently quantified.

Antibacterial Enzymes

As outlined in the design aim, the therapeutic engineered bacteria need to be able to produce and accumulate antibacterial enzymes intracellularly, and upon detecting the presence of a high concentration of pathogenic bacteria release the accumulated enzymes in a high-concentration pulse to simultaneously achieve both pathogen-killing as well as self-killing for the purposes of bacterial population control.

The artilysin Art175, our main antibacterial enzyme of study, is capable of killing Gram-negative bacteria by penetrating through its outer membrane and degrading its cell wall. Our experimental aims with regards to Art-175 are twofold:

  • The engineered bacteria must be able to produce and accumulate Art175 within itself without killing itself.
  • Once Art175 is exported to the extracellular medium, it must be able to kill both the expression host as well as the target pathogenic bacteria.

For the final implementable design, the proposed mechanism of export for Art175 is the permeabilization of the engineered bacteria's inner membrane using T4 Holin upon detection of quorum-sensing signals sent out by pathogenic bacteria. However, to achieve greater compartmentalization in terms of function-testing we opted to instead to investigate whether Art175 can fulfill the two requirements above through a substitute methodology - instead of using a separate export mechanism, we designed BioBrick parts separately coding for the production of Art175 with and without a secretion tag to mimic its behaviour in the intracellular and extracellular environments respectively.

Host Cell Lysis Assay

Of the three secretion-tagged Art175 parts (BBa_K1659001, 9002, and 9003), BBa_K1659003, the part containing the YebF tag has been shown to produce the most consistent results and hence will be the subject of comparison against the non-tagged Art175 part, BBa_K1659000.

Induction of gene expression for BBa_K1659000 (Art175) and BBa_K1659003 (YebF-Art175) separately in 1:100 subcultures of stationary phase engineered bacteria while tracking the changes in cell density by means of OD600 measurements in a 96-well plate allows us to determine whether Art175 behaves differently in terms of host cell-killing from the inside or outside of the cell:

As seen in the results, when compared against two control types (cells that are only cloned with a blank plasmid backbone (blue line) and cells containing the YebF coding sequence but does not have the gene expression induced (orange line)), production of non-secreted Art175 (graph on the left) does not lead to host cell lysis even at high levels of gene expression, whereas gene expression for the secretion-tagged Art175 (graph on the right) leads to significant amounts of host cell lysis some time during the mid-log phase which is presumably when the enzyme starts being produced and secreted in large amounts.

Target Cell Lysis Assay

Having obtained results showing that YebF-Art175 is capable of killing the engineered E. coli expression host cells, we needed an alternative expression host organism that is able to better survive the production and secretion of Art175 such that a high enough cell count within the culture can be maintained. This is to ensure that a high enough concentration of enzymes will be secreted into the extracellular medium, allowing us to test the antibacterial potency of its subsequent cell-free isolate against target pathogen cells.