Utilization of biofilm can be greatly advantageous in the application of biosensors. Traditional biosensors, usually composed of planktonic cells, have two shortcomings.
Firstly, the upper limit of detection is confined by cell’s physiological states. When concentrations of target chemicals are so high that they interfere with cell’s physiological activities, the resulting biosensor would probably malfunction or even be completely unresponsive. Besides high target chemical concentrations, environments (such as polluted water wastes containing various toxics) in which bacteria are working might be even more daunting and deleterious. Therefore, traditional biosensors require a moderate sensing range and a friendly working environment. Yet, these two requirements are hard to meet, as it’s laborious and impractical to prepare such environment before having any idea what the level of chemical concentration might be.
Secondly, detection is usually qualitative instead of being quantitative. Though there’re approaches to enlarge the linearity by rewiring and improving circuit design for quantification, most traditional biosensors are of an all-or-none or switch behavior. It reports only when a certain level of chemical is exceeded, yet does not tell of what specific concentration of level the target chemical is at.
Fortunately, the unique characteristics of biofilm can help to solve such problem and advance the development of biosensors. Biofilm can stand unfavorable growth conditions and is highly resistant to deadly chemicals such as antibiotics and heavy metal. To use biofilm as the basic biosensing module can cope with bad working conditions (real scenario). Moreover, chemical gradients are spontaneously formed from the outside to the inside of biofilm, which resembles an automatic diffusion device. Combined with modeling, each layer of bioflm can well indicate different chemical concentration level detected, which advances quantitative biosensing into a new era.
Here, we put forward two designs using our biofilm system to make biosensing stronger, quantitative and more applicable.
Quantitative Lab Biosensor Device
In the gradient along biofilm’s depth, each layer of biofilm detects different outside chemical levels. Thus, we come up with a quantitative biosensor using biofilm to make quantification possible in lab.
How it works
We designed a simple quantitative detection device. When expressed in normal planktonic cells, our system only displays an all-or-none response. However, when expressed in a biofilm, the yellow layer (expressing YFP and repressing CFP) increases its depth with an increase of Mercury concentration. Therefore, the quantification of Mercury concentration can be realized in lab simply by measuring the depth ratio of yellow layer and blue layer.
Stronger Field Biosensor Device
Besides producing powerful biosensors in lab, we’re more concerned with real world application. How can we use the merits of biofilm to benefit the world in real practice? We then produce stronger field biosensor device by directly applying traditional biosensor design into the biofilm system. The approach is simple and direct, with the circuit design as follows.
How it works
We used quorum sensing in our system to produce easily detectable signal across the whole biofilm. A positive feedback is also incorporated in quorum sensing to amplify the signal. This device is for directly detecting whether Mercury concentration exceeds a certain level and for producing easily detectable warning signals for naked eyes. Expressing such system in biofilm renders it greater vitality in unfavorable growth conditions. Furthermore, because of its natural gradient across the biofilm, when the environment becomes too harsh for normal cells to survive, cells in the inner layer of the biofilm could still manage to function well. Utilization of biofilm makes biosensors more robust in detection.