Microbial assay for tryptophan using silicon-based transducer

A potentiometric microbial assay for tryptophan was devised using silicon-based transducer and auxotrophic bacteria, and the chemical performances are discussed. Escherichia coli WP2, an auxotrophic mutant requiring tryptophan for its growth, was used as tryptophan recognition element, and extracellular pH changes resulting from metabolic activity was monitored by the light-addressable potentiometric sensor (LAPS). The potentiometric sensor detected the acidification of the medium by E. coli WP2, and the acidification rate was dependent on tryptophan concentration. The acidification rate was approximately linear between 0 and 15 μM of tryptophan. Furthermore, the cells were immobilized on the electrode, and the tryptophan sensor was characterized. The sensor responded to the addition of tryptophan, and the sensor could measure tryptophan in the range of 0–12 μM.