Illuminating Histidine-Deficient Intracellular Environments: A Novel Whole-Cell Microbial Fluorescence Sensor

Histidine is an essential amino acid with significant implications for human growth and neuromodulation. Its intracellular concentration, whether increased or decreased, can indicate different diseases. While various methods exist for measuring elevated histidine levels, there remains a significant lack of sensors capable of actively responding to histidine deficiency within cells and releasing strong signals. In this study, we exploited the high induction levels of the his operon in S. Typhimurium SL1344, a histidine auxotroph, within a histidine-deficient environment, to develop a specific bacterial sensor with sensitivity towards low histidine concentrations. By employing plasmid vectors with differing copy numbers, we developed two distinct bacterial fluorescence sensors, both capable of actively responding to histidine deficiency and emitting detectable fluorescence signals within either culture mediums or live cells. The SL1344-pGEX sensor, with a high copy number, exhibited remarkable sensitivity and selectivity to histidine in the range of 0 to 50 mu M. Notably, even a minimal addition of histidine (approximately 2.5 mu M) to the M9 medium led to observable fluorescence reduction, rendering it highly suitable for monitoring histidine-deficient cellular environments. In contrast, the low-copy-number SL1344-pSB3313 sensor exhibits a broader response range, capable of tracking more extensive shifts in histidine concentrations. These sensors allow for sensitive in situ detection of intracellular histidine concentrations in various live cells, particularly responding to real-time changes in cellular histidine levels. This provides a powerful tool for investigating histidine deficiency-related biological processes, the mechanisms of associated diseases, and the assessment and optimization of therapeutic strategies.