Development of a Pigment-Based Whole-Cell Zinc Biosensor for Human Serum

Deficiencies in vitamins and minerals (micronutrients) are a critical global health concern, in part due to logistical difficulties in assessing population micronutrient status. Whole-cell biosensors offer a unique opportunity to address this issue, with the potential to move sample analysis from centralized, resource-intensive clinics to minimal-resource, on-site measurement. Here, we present a proof-of-concept whole-cell biosensor in Escherichia coli for detecting zinc, a micronutrient for which deficiencies are a significant public health burden. Importantly, the whole-cell biosensor produces readouts (pigments) that are visible to the naked eye, mitigating the need for measurement equipment and thus increasing feasibility for sensor field-friendliness and affordability at a global scale. Two zinc-responsive promoter/transcription factor systems are used to differentially control production of three distinctly colored pigments in response to zinc levels in culture. We demonstrate strategies for tuning each zinc-responsive system to turn production of the different pigments on and off at different zinc levels, and we demonstrate production of three distinct color regimes over a concentration range relevant to human health. We also demonstrate the ability of the sensor cells to grow and produce pigment when cultured in human serum, the ultimate target matrix for assessing zinc nutritional status. Specifically, we present approaches to overcome innate immune responses that would otherwise hinder bacterial sensor survival, and we demonstrate production of multiple pigment regimes in human serum with different zinc levels. This work provides proof of principle for the development of low-cost, minimal-equipment, field-deployable biosensors for nutritional epidemiology applications.