Our Idea

Figure 1. Explore possible approach to overcome the challenge
(Picture by Tsinghua iGEM team 2013)

To meet the foregoing challenges, a portable pathogen detector which could be kept within reach is required. The detector is designed as test paper. When pathogen appears, its existence is reported by the color change of the detector. But what can be used as the components to sense the pathogen as well as output the color change?

Figure 2. Budding yeast is wildly utilized in manufacturing
(Picture by Tsinghua iGEM team 2013)

In our project, genetically modified S. cerevisiae (budding yeast) is utilized as the main component on the test paper. The yeast detects the signals of bacteria and then reports them by expressing reporter genes. The yeast system is chosen generally based on four reasons: 1) Living organisms such as yeast are easy to modify genetically. 2) Yeast is eukaryote, different from target bacteria. It sets proper orthogonality between the detector and the target. 3) Being widely used in food and drink manufacturing, yeast is fundamentally friendlier to human health and easier to be accepted than other microorganisms. 4) Yeast has the dramatic mechanism of mating. We took advantage of this mechanism to separate the sensor and reporter in our detector, increasing the diversity and effectiveness of the system.

The next question is - how will the yeast on our test paper detect pathogens? Quorum sensing, a conserved mechanism in bacteria to sense the population around, is used to suggest the existence of the pathogen. In gram-negative bacteria, a small molecule important in quorum sensing called N-Acyl homoserine lactone (AHL) is released by bacteria and conveys communicating messages among the entire group. Moreover, each species has its own specific AHL molecule. Therefore, AHL is an ideal candidate to identify the existence as well as the size of a specific pathogen population. If the AHL sensing system of one pathogen is introduced into the yeast, the yeast will work as the detector and sense the pathogen. Downstream reporter genes will then be expressed and change the color of the test paper.

Figure 3. The principle of quorum sensing system
(Picture by Tsinghua iGEM team 2013)

Additionally, one feature of yeast unique from other microorganisms is that it has “sex”. Haploid yeasts of type a and type α can mate with each other to form a diploid. We used this feature of yeast to revise our detecting system. Yeasts of type a are utilized as sensors, while yeasts of type α act as reporters. The sensor is targeted to the pathogen of interest and its AHL specifically, while one of the type α yeasts with different reporting colors is selected as the output. There are at least four main advantages of this design: 1) the whole system is divided into two subparts, reducing the difficulty and time to complete each part. 2) The flexibility of the system is increased. Different combinations of sensors and reporters increase the number of possible detectors. 3) During the two-step process, the signal is amplified and easier to be observed. 4) New subparts are convenient to be added to the system, as modification is needed for only one part.

Figure 4. The principle of our portable pathogen detector
(Picture by Tsinghua iGEM team 2013)

To summarize our idea, a portable detector in the form of test paper is designed with genetically modified yeasts on it. The sensing yeast is able to sense the signal AHL from the pathogen and then the reporting yeast will reveal the signal with color change.

Figure 5. Portable test paper is designed for pathogen detection
(Picture by Tsinghua iGEM team 2013)

Reference

[1] Lazcka O, Campo F, Munoz F X. Pathogen detection: a perspective of traditional methods and biosensors[J]. Biosensors and Bioelectronics, 2007, 22(7): 1205-1217.

[2] Nassif X. A revolution in the identification of pathogens in clinical laboratories[J]. Clinical infectious diseases, 2009, 49(4): 552-553.