Ultrasensitive Indicator Gas Reporting Enables Biosensing in Native Soil Conditions

Gas-producing enzymes can be used to noninvasively monitor the output of genetic circuits expressed in cells that are embedded in hard-to-image soils and sediments. For example, when expressed in environmental samples, methyl halide transferase (MHT) synthesizes volatile CH₃X that diffuses through the matrix and can be quantified in the sample headspace, enabling nondestructive monitoring of microbial behavior in opaque environments. To date, gas-reporting biosensors have been limited in use to autoclaved soils amended with halide salts and containing high titers of engineered microbes. However, an ideal biosensor for environmental applications could be added in small titers directly to a living (unautoclaved) soil without adding additional nutrients or halides. Here, we report on progress towards using gas-producing biosensors under more environmentally-reflective conditions. We apply gas concentration techniques to amply the signal of MHT-expressing microbes added to an autoclaved forest soil sample, and we explore the effects of halide concentration on CH₃X signal. We also report on progress towards detecting biosensor-produced gas signal above the background of a living soil. These studies advance the application of living sensors towards non-destructive studies of native environment samples and pave the way for understanding and controlling microbial behaviors in ecologically-important environments.