BACTRICITY: Bacterial Biosensors with Electrical Output
The metabolically versatile bacterium Shewanella oneidensis adapts to anaerobic environments by transporting electrons to its exterior, reducing a variety of environmental substrates. When grown anaerobically and provided with lactate as a carbon source, S. oneidensis transfers electrons to an electrode of a microbial fuel cell. We sought to engineer S. oneidensis to report variations in environmental conditions through changes in current production. A previous study has shown that S. oneidensis mutants deficient in the mtrB gene produce less current than the wildtype strain, and that current production in these mutants can be restored by the addition of exogenous mtrB. We attempted to control current production in mtrB knockouts by introducing mtrB on lactose, tetracycline, and heat inducible systems. These novel biosensors integrate directly with electrical circuits, paving the way for the development of automated, biological measurement and reporter systems.
Experimental overview
We attempted to develope three inducible systems for electrical current production in S. oneidensis. The first is a chemically inducible system, where LacI or TetR repression of the current-production gene expression in mtrB knock-out (KO) S. oneidensis can be alleviated by the addition of IPTG or anhydrotetracycline, respectively. Our second approach uses temperature-senstive cI which would allow for an increase in electrical output in response to heat. The third is a light-inducible system based on the 2005 UT Austin biological camera.
Using the LacI system, we attempted to interact with engineered S. oneidensis in multiple anaerobic microbial fuel cells which we built, measuring the effects of IPTG on current production in wild type and mtrB KO S. oneidensis containing plasmids expressing inducible or constitutive mtrB.
Future directions
Our work with creating a system of inducible electrical output in S. oneidensis has laid the foundations for many different exciting avenues of further inquiry which look to take advantage of a bacteria-computer interface that combines the amazing sensitivity and adaptability of bacteria with the speed and analytical abilities of electricity and computers.
Using the same principles underlying the lac system, the [http://parts.mit.edu/wiki/index.php/University_of_Edinburgh_2006 arsenic biosensor] developed by the University of Edinburgh iGEM 2006 team could be introduced into S. oneidensis, allowing for the coupling of arsenic sensing to an electrical output, a form of a data which is easier to automate and transmit. This could be further extended to other chemical sensing systems, resulting ultimately in an array of different strains S. oneidensis which all respond to the presence of different chemicals with an electrical output that can be monitored by a computer. This could theoretically allow for the remote sensing and analysis of the chemical composition of an environment over time.
Another interesting direction would be the linking of the light-sensing system developed by the UT Austin iGEM team with electrical output in S. oneidensis. In response to variations in light, the amount of electricity produced by S. oneidensis would change. This would allow for the intriguing possibility of not only S. oneidensis conveying information to the computer, but also the computer responding to the S. oneidensis. A simple example would be that in response to a chemical input, S. oneidensis may increase its electrical output. Sensing this increase, the computer could turn on or off a light directed at the S. oneidensis, modifying S. oneidensis's output, creating interesting feedback loops. This could ultimately be developed into more complex communications systems between bacteria and computers. We tried constructing this system over summer, but as the process requires making an EnvZ knockout strain of S. oneidensis, we could not finish it. We did, however, make a few parts to facilitate future attempts.
The possibilities are further broadened by our observations of co-cultures of E. coli and S. oneidensis. Either of the systems described above could be pursued through an alternative alternative strategy of co-cultures. For instance, an array of E. coli which respond to different chemicals by breaking down lactose into lactate could be cultured with S. oneidensis. In response to an increase in lactate, S. oneidensis would begin to produce higher levels of electricity. Co-cultures could also allow for more complex bacteria-computer interactions. This strategy could enable the coupling of almost any E. coli ability to electrical output.
These future directions in which our research can be taken demonstrate some of the exciting possibilities of BACTRICITY!
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