Introduction

The Rise of Paper Diagnostics

Paper diagnostics is a fast-growing area of bioengineering. It is especially appealing for detecting pathogens, such as markers for infectious diseases, in low-resource settings. However, paper test strips can be engineered for a wide range of uses, including to detect for heavy metals, environmental toxins, hormones, and ions. Paper-based technologies are generally affordable, fast-acting, easy to use, and easy to produce. Because of these benefits, they have recently begun to be explored as low-cost alternatives to more complex diagnostic technologies, such as PCR and ELISA.

Paper as a Platform for Cell Culture

While many have functionalized paper with proteins, fluorescent tags, and other molecules, few have tried to grow cells or use cellular detection pathways on paper. There are many benefits to using a paper platform in conjunction with synthetic biology; biological sensing systems are cheap when produced en masse and can be engineered for high specificity. They can often sense for long periods of time, and can form more complex signaling and receiving systems than can be achieved through chemistry. We took inspiration from the work of the Derda Research Group at the University of Alberta, which has performed some work in growing bacteria and mammalian cells on paper [1,2]. We especially drew from a 20XX article in which the group developed a low-cost test strip mainly using products that could be found in the home [2]. This makes our device very cheap and accessible to make.

S. Cerivisiae as a Biosensor

Saccharomyces Cerivisiae, commonly known as baker’s yeast, was chosen as the biosensing organism in this platform. This species has an activity within a pH range of 2.1 to 7 and within a temperature 12°C to 36°C [3], making it a relatively durable organism for use in a portable sensor. Another crucial feature of the test strip is longevity; S. Cerivisiae can live for approximately 20 to 120 hours [4]. In a dehydrated dormant state, however, the yeast can survive for years [5]. Additionally, the wild type S. Cerivisiae is not mutagenic, making it a more stable organism for use in the environment. This species is also well-studied; the genome of this organism has been sequenced, it is easy to obtain in the lab, and the genomic structure is relatively easy to modify. Thus, it seemed to be an ideal organism for the purpose of this project.

Methods

Making the Paper Device

The device has three basic layers - the filter paper platform, the yeast with media, and the PDMS (Polydimethyl Siloxane) window. Filter paper was chosen because of its predictable and even wicking properties, it’s durability when wet, and the fact that yeast cannot travel through the tight mesh of paper fibers. The PDMS provides a barrier from the outside that allows the yeast to be contained in a sterile environment. It also helps prevent evaporation of media from within a loaded device; while oxygen and other gases can diffuse through the PDMS, water and contaminates cannot. Squares of PDMS were secured to each side of the paper strip using heat-resistant cloth electrical tape, and the partially-built devices were autoclaved.

After they had been sterilized, media and yeast could be added. Media was added first by pipetting liquid or gel between the PDMS and the paper on one side. Using a pipette tip, yeast were then placed in the center of the window, and the device was sealed.

Performing Assays

Assays were performed using two different methods for the application of the target molecule; First, a small volume of target molecule was pipetted directly into the cell growth area behind the PDMS window. Secondly, the paper device was allowed to stand upright in a beaker containing 3mL of target solution, so that the target molecule could travel up the paper to the cells by wicking.

Our device could be easily modified to accommodate a wide range of biological systems. For different strains of cells, different media was added depending on the yeast marker. Additives, such as inducers, could also be pipetted into the media or wicked up the paper, making systems with inducible promoters viable. Cells could also be lysed on paper with the addition of 0.2% SDS and gentle massaging of the PDMS window.

https://static.igem.org/mediawiki/2015/8/81/Cell_Lysis_Experiments.jpeg

Results

Initial Testing

When first creating the paper device, experiments were done in which PDMS was not present. Instead, the cells were covered by a 1mm-thick layer of 1% agarose. However, this dried out quickly and cells were not able stay active.

During initial tests in which a color or fluorescence output was not being produced, it was difficult to tell whether yeast cells had survived on paper, and whether they could be reactivated after drying. To determine whether they could be revived, the paper strips were placed in selected media overnight cultures and growth was observed. However, when these cultures were plated on selective media with added ampicillin, contamination was observed. It was clear that keeping the samples sterile was critical to success of the device.

From that point, more rigorous measures were taken to ensure some degree of sterility in the fabrication process. Heat-resistant tape was used to assemble the device so that assembled devices could be autoclaved. Equipment like scissors and the benchtop were washed with ethanol before use.

Cell Lysis on Paper

In order to test whether the production of beta-galalactosidase in yeast could be viewed on paper, an assay was performed in which liquid c-ura media was pipetted into the cell window along with cells constitutively producing beta-galactosidase. These cells were then lysed by the addition of 0.2% SDS. Two controls were performed simultaneously; one with a yeast strain that does not produce beta-gal, and one without any yeast. X-gal was added to all three sample sets. As seen in the figure below, cells were lysed and the cells that constitutively produced beta-gal turned blue with the addition of X-gal, while the controls did not. This change could be seen within a half-hour of lysis, and was a deep blue within 2 hours.

Theophylline Detection on Paper

Yeast with the theophylline detection pathway were placed onto liquid media containing theophylline and allowed to incubate for 6 hours. While the positive control, in which the contained yeast strain constitutively produced YFP, showed fluorescence under a blue light, the theophylline-sensitive strain did not show any visible fluorescence. It was hypothesized that this was because the culture had dried out within that 6 hour period.

In order to prevent this, gel media was used instead. This time, when theophylline was added, cells began to show visible fluorescence within three hours.

Because the aptazyme system was leaky and it was difficult to visibly discern the difference in fluorescence between the repressed system (without theophylline) and the activated system (with theophylline), a Gal1 promoter was used to decrease baseline fluorescence. This modified system was tested on paper. While this system took longer to display a fluorescence signal and was left overnight in order to see expression, the media did not dry out and fluorescence was very bright the next morning. This can be seen in the figure below.

Conclusions

From these initial experiments, we can see that our paper device is capable of housing a yeast culture that not only survives but can also perform as a biosensor for theophylline. While this set of experiments is limited in scope, they suggest that a yeast biosensor on a paper test strip could be a viable detection method for a wide range of small molecules, including shellfish toxins, heavy metals, and disease pathogens.

There are a number of directions that development can take from this point. The device design can be made more streamlined for consistency of results and ease of use. As it is used to perform assays for other detection pathways, it can also be modified to make addition of inducers, lysis buffers, and other additives. For example, with a one-way valve sticking out of the top of the device, solutions or media could be added separately from wicking without exposing the cells to a non-sterile environment. Quantifiable results may also be desired; for example, ImageJ could be used to quantify the color or fluorescence on the paper.

References

[1] Derda R et al. Multizone Paper Platform for 3D Cell Cultures. PLOS One, May 2011. 0.1371/journal.pone.0018940.

[2] Derda R et al. Portable self-contained cultures for phage and bacteria made of paper and tape. Lab on a Chip, Jul 2012. 12:4269-4278.

[3] Arroyo J et al. Genomics in the detection of damage in microbial systems: cell wall stress in yeast. Environmental changes, microbial systems, and infections. Jan 2009, 15(s1): 44-46.

[4] Minois N et al. Chornological aging-independent replicative life span regulation by Msn2/Msn4 and Sod2 in S. cerevisiae. FEBS Letters, Jan 2004. 557(1-3): 136-142.

[5] Fabrizio P, Longo VD. The chronological life span of S. cerevisiae. Aging cell, Mar 2003. 2(2): 73-81.