In situ detection of aromatic compounds with biosensor Pseudomonas putida cells preserved and delivered to soil in water-soluble gelatin capsules.

While many types of bacteria have been engineered to produce an optical output in response to given analytes in a culture, their use for extensive, in situ monitoring of distinct chemical species in soil is hampered by a dearth of practicable spreading schemes. In this work, we report and validate a comprehensive system for the long-term preservation of Pseudomonas putida cells genetically designed for biosensing benzene, toluene, ethylbenzene, and xylenes (BTEX) in soil, along with a procedure to formulate, spread, and vigorously activate such bacteria at the desired site and occasion. To this end, various known lyoprotectants were tested for promoting the long-term maintenance of biosensor cells with quite variable outcomes. While a formulation of inositol and maltodextrines was optimal for preservation of freeze-dried BTEX-sensing bacteria, adsorption of P. putida cells to corncob powder (an abundant residue of the corn industry) endowed the resulting material with a lasting viability at ambient conditions. In any case, the thereby preserved bacterial biomass acquired physical and mechanical properties adequate for formulating the biosensor agent in water-soluble but otherwise hard dry gelatine capsules with a long shelf life. When such capsules were spread in a soil microcosm and subsequently liquefied with water or high humidity, the released microorganisms formed spots that gave an intense luminiscent signal upon exposure to effectors of the sensor circuit implanted in the chromosome of the P. putida strain. We argue that the procedures described here can facilitate implementation of wide-area biological detection strategies for revealing the location of toxic or perilous chemicals.