SensorA

The mechanism of Sensor A engineering bacteria

This gene circuit applies bistable mechanism, involving two types of promoters: recA and lac, which dividedly regulate the expression of the gene sequence of repressor lacl and lexA, whose product suppress promoter lac and recA. The reporter gene of lacl and lexA thereafter bring out RFP(red fluorescent protein) and GFP(green fluorescent protein), both of which could be output signals that represent the current state of the system. As upstream and downstream gene promote and mutually repress, the whole system has two stable output state:

State A: As promoter precA works, repressor lacl is expressed, then the product lexArepresses promoter lac, and repressor lexA is not expressed, thus cannot exert depressant action to promoter precA. When promoter recA works while promoter plac is shut down, the whole system remains perfectly stable. Afterwards, the downstream reporter gene of promoter precA, GFP, is turned on, then product can be detected.

State B: promoter plac works, repressor lexA is expressed, then the product lexA represses promoter precA, and repressor lexA is not expressed, thus cannot exert depressant action to promoter plac. When promoter plac works while promoter precA is shut down, the whole system remains perfectly stable. Afterwards, the downstream reporter gene of promoter plac, RFP, is turned on, then product can be detected.

Two external input signals: IPTG and radiation-produced SSB can separately counteract the inhibition of expression product of repressor lacl towards promoter plac and repressor lexA towards precA. Their involvement could make the whole system switch between the two states.

SensorB

Sensor B is a low-dosage-radiation-sensitive biological sensor. As back plan, its mechanism and effect is relatively uncomplicated and direct, which involves radiation response and grouping amplification.

Sensor B applies a completely different responsive mechanism, in order to supplement the one of cell radiation. Sensor B composes of three parts, altogether 4321bp long. In sequence, the whole circuit is made up with SoxS P-LuxI-LuxPL-LuxR-LuxPR-LuxI-LuxPR-GFP, and GFP works as the final reporter gene.

The first part is assembled by SoxS P-LuxI,while SoxS is a oxidation-mark-sensitive promoter. When cell gets radiated, it will produce a oxidation mark, which induces SoxS P to promote, thus making luxl produce AHL synthetase, catalyzing and synthesizing autoinducer information molecule AHL, and interacting with next part.

The second part composes of LuxPL-LuxR-LuxPR-LuxI. LuxPL makes LuxR express. When cell got radiated, protein LuxR integrates with autoinduer AHL and form complex, which induces LuxPR to start expression of Luxl, and then comes to produce more autoinducer AHL and complex, therefore amplifying the effect of gene expreesion.

The third part applies LuxPR-GFP structure, working as reporter gene in this sensor. Through the interaction and amplification in the first two parts, the complex will exert effect on promoter LuxPR and induce the expression of GFP. According to the fluorescence intensity, the radiation intensity could be detected directly and accurately after collecting and analyzing data.

Amplifier

In sensor, the amount of promoter expression stimulated by external condition can be detected by the expression amount of fluorescent protein, but most of all invisible. So, in order to increase it several times of growth, we need amplification effect. In common practice, quorum sensing, which is a grouping bacteria behavior regulation mechanism, is always applicable. It shows that bacteria respond to colony density and environmental change by sensing autoinducer. When the number of bacterial colonies reaches a certain quorum, it starts a series of gene expression to regulate group reaction. At the very beginning, we originally applied lux system in Vibrio Fischeri, for its mechanism is relatively simpler but innovatively harder. Then, we switched to las system and rhl system in pseudomonas aeruginosa.

In the plasmid of amplification system, two activating transcription factors are in constant expression and remain in a certain concentration. With a certain level of autoinducer PAO1, pao1 integrates with lasR, the complex of which are bond to the target site of las promoter, then activating it and control the expression of the downstream reporter gene GFP in accord with the concentration of the amount of combination. In addition, this complex can also work on the promoter rhl as transcriptional activator of rhl system, and afterwards activating it.

The efficiency of complexes activating two promoters are relatively different. Theoretically, it is more efficient for the complex to take care of its own promoter than to work on the thl promoter as a transcription activator.

Actually, las and rhl system are not orthogonal, while las system disturbs the rhl system. PAO1 can not only bind to the lasR protein, also thlR protein, even in a very low level. Furthermore, the complex PAO1-rhlR activates promoter to express RFP.

Micro-fluidic chip

Material

PDMS glue; glass; semipermeable membrane (specifics are as follows)

Sensor bacteria produces AHL molecules when receiving radiation, and transferred AHL to amplifier bacteria, which gives out fluorescent light, whose intensity can be detected by measuring the light intensity.

Based on the simplified mechanism, the basic function of micro-fluidic chip contains:

1/separate culture of sensor and amplified bacteria

2/deliver the metabolite from sensor to amplifier, and mix it with amplifier bacteria as much as possible

3/fluorescent light given out by bacteria B can be detected.

4/detect multiple samples at a time

Design of chips

Single channel pattern

And the chip model to meet the needs of multiple data at one time is as follows.

Through making model, pouring, punching and bonding, a perfect chip is made up.

Function

Sensor bacteria above receives radiation, and metabolizes AHL molecules, which pass the semipermeable membrane and transmit the amplifier to the detection hole.

The transmission method remains unknown for we cannot make sure of the transmembrane way of AHL.

Device

This device composes of three parts

1、 Internal circuit: “brain” and “heart” of the whole device, single chip microcomputer(SCM) as processing core. Control and sampled signals are received and processed in this unit, then making it possible to control device, switch signals, process algorithm and display on the screen.

2、 Optical structure: this is designed to detect these samples which are excited by laser and make out fluorescent light, then detector will receive it and make high-precision measurement.

3、 Appearance design: out product is not only beautifully designed, and also functionally sufficient.

Characteristics

Biological sample detection is a long tough process, which costs a great amount of money and labor. However, our device possesses great character of high-throughput and high-precision, also simplifies conductive process, visualizes detection result, and even substantially lowers the cost. Even though this device serves for project engineering, which can be applied only in our program, its electric cores and optical structure could be easily transferred to other relevant detection. This is greatly significant in further device refinement.