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P_blaZ Promoter controlling gene expression of blaZ in S. aureus

The P_blaZ promoter is a part used in the Beta-Lactam Biosensor project of [http://2017.igem.org/Team:TU_Dresden iGEM Team TU Dresden 2017 (EncaBcillus - It's a trap!)].

This part is a composite of the bla operon found in Staphylococcus aureus and constitutes the promoter regulating gene expression of the gene blaZ, coding for a beta-lactamase. If the microorganism is exposed to beta-lactam antibiotics, a receptor, named blaR1 [1], senses the compound and a signal is transduced into the cytoplasm. Subsequently, the BlaI repressor protein [2] is degraded and frees the P_blaZ promoter. Following, the blaZ gene is transcribed and confers resistance to the antibiotic.

This part features the RFC10 prefix and suffix:

Sites of restriction enzymes generating compatible overhangs are indicated by sharing one color. (EcoRI and PstI are marked in blue, NotI in green, XbaI and SpeI in red)

Sequence and Features

Beta-Lactam Biosensor

Worldwide, multidrug-resistant bacteria are on the rise and provoke the intensive search for novel effective compounds. To simplify the search for new antibiotics and to track the antibiotic pollution in water samples, whole-cell biosensors constitute a helpful investigative tool. In this part of EncaBcillus, we developed a functional and independent heterologous Beta-lactam biosensor in Bacillus subtilis. These specialised cells are capable of sensing a compound of the beta-lactam family and will respond by the production of an easily measurable luminescence signal. We analysed the detection range and sensitivity of the biosensor in response to six different Beta-lactam antibiotics from various subclasses. The evaluated biosensor was then encapsulated into Peptidosomes to proof the concept of our project EncaBcillus. The encapsulation of engineered bacteria allows an simplified handling and increased biosafety, potentially raising the chances for their application in e.g. sewage treatment plants.

Design of the Biosensor

To achieve our goal of encapsulating bacteria into Peptidosomes that can sense antibiotics of the beta-lactam family, we first needed to develop a reliable biosensor strain. In Staphylococcus aureus the bla-operon encodes a one-component system, which is responsible for sensing and mediating resistance against beta-lactam antibiotics. The idea was to transfer the regulatory elements of this operon to Bacillus subtilis and replace the native output – being the beta-lactamase BlaZ – by an easy detectable signal. Thus, making Bacillus subtilis a Beta-lactam sensing biosensor. (see Figure 1).

For the creation of our biosensor in B. subtilis, the bla-operon from S. aureus was split into three genetic constructs: (A) The Receptor gene blaR1 under control of a strong constitutive promotor (P_veg), (B) the Repressor gene blaI under control moderate strong constitutive promoter (P_lepA) and the target promoter P_blaZ in front of the lux-operon (luxABCDE). In addition, an inducible version of the blaR1 construct was made by inserting the P_xylA promoter upstream of the blaR1 gene. [1]

The following results demonstrate the promoter activity of P_blaZ in the context of our biosensor. We could observe, that this promoter is very sensitive and strong. These findings are supported by the data below:

Evaluation of the promoter activity

During this project, we generated several strains to investigate the functionality of the heterologous constructs constituting the biosensor in Bacillus subtilis [file strains?]. This result section though will focus on the evaluation of the strains shown in Table 5 as these represent the most interesting ones.

In our first experiment, we performed plate reader assays in a 96 well plate format and measured growth (OD₆₀₀) and luminescence output for 18 hours every 5 minutes. Induction with the Beta-lactam antibiotics occurred after 1 hour. All strains have been tested in triplicates under the same conditions. Strains with the genotype penP::kan^R have been induced with lower concentrations compared to the wild type strain W168 (see Table 1 above).

After induction, we anticipate a strong increase in luminescence signal for the biosensor strains. The control strain W168 (wild type) and control 1, will presumably not show any luminescence output, while the positive control 2 is expected to show a steady luminescence signal regardless of the presence of any antibiotic compound.

As shown in Figure 2, the wildtype W168 (black with white dots) shows no increase in RLU/OD₆₀₀ values when induced with the different Beta-lactam antibiotics and controls. Control 1 (black tight stripes) behaves similarly to the wild type strain. The slight decrease of control 2 (light grey) in the bar chart where induction with ampicillin and carbenicillin happened, is mostly explained by the high growth inhibition caused by the chosen concentrations for W168 (with functional Beta-lactamase PenP). Most of the times, the constitutive expression of the lux operon resulted in an RLU/OD₆₀₀ of over 1.3 million for control 2 (see Figure 2).

Biosensor 1 gives an overall good signal for all Beta-lactam antibiotics tested, but also shows a higher basal activity in absence of the Beta-lactam compounds of 40.000- 90.000 RLU/OD₆₀₀ (see Figure 5, bar chart with bacitracin and dH₂O). Further, we could observe a difference in signal intensity dependent on the Beta-lactam antibiotic tested. Therefore, biosensor 1 gives the highest signal in presence of penicillin G, cefoxitin and cefoperazone with up to 2.7 million RLU/OD₆₀₀. Ampicillin and penicillin G again show a weaker increase in signal produced by biosensor 1, which could be due to the same reason as for control 2 (see Figure 2).

For biosensor 2, the detection range and sensitivity is comparable to biosensor 1, This strain strongly senses cefoxitin, ampicillin and cefoperazone reaching up to 2.4 million RLU/OD₆₀₀. Even the basal activity of the P_blaZ promoter in biosensor 2, as shown in the bar charts with bacitracin and dH₂O, conforms with the one from biosensor 1.